# Patent

## Title
Just in time compiler in spatially aware emulation of a guest computer instruction set

## Abstract

A selected group of Guest machine instructions in an emulation environment are translated to a semantic routine of Host machine instructions, wherein Guest cells corresponding to an opcode portion of a Guest instruction are mapped to corresponding Host cells, wherein the semantic routine of Host machine instructions are patched into a Host cell corresponding to the first Guest cell of the group of Guest machine instructions, wherein other Host cells of the corresponding Host cells are patched with semantic routines for emulating single instructions associated with the corresponding Guest cell.

## Background
FIELD OF THE INVENTION
The present invention is related to computer systems and more particularly to emulation of a guest computer system processor by a host computer system processor having an incompatible instruction set architecture.
BACKGROUND OF THE INVENTION
Trademarks: IBM® may be a registered trademark of International Business Machines Corporation, Armonk, N.Y., U.S.A. S/390®, z900 and z990 and other product names are registered trademarks or product names of International Business Machines Corporation or other companies.
Before our invention IBM has created through the work of many highly talented engineers beginning with machines known as the IBM® System 360 in the 1960s to the present, a special architecture which, because of its essential nature to a computing system, became known as “the mainframe” whose principles of operation state the architecture of the machine by describing the instructions which may be executed upon the “mainframe” implementation of the instructions which had been invented by IBM inventors and adopted, because of their significant contribution to improving the state of the computing machine represented by “the mainframe”, as significant contributions by inclusion in IBM's Principles of Operation as stated over the years. The Seventh Edition of the IBM® z/Architecture® Principles of Operation which was published February, 2008 has become the standard published reference as SA22-7832-06 and are incorporated in IBM's z9® mainframe servers. The IBM Z/Architecture® Principles of Operation, Publication SA22-7832-06 is incorporated by reference in its entirety herein.
Referring to FIG. 1A, representative components of a prior art Host Computer system 50 are portrayed. Other arrangements of components may also be employed in a computer system, which are well known in the art. The representative Host Computer 50 comprises one or more CPUs 1 in communication with main store (Computer Memory 2) as well as I/O interfaces to storage devices 11 and networks 10 for communicating with other computers or SANs and the like. The CPU 1 is compliant with an architecture having an architected instruction set and architected functionality. The CPU 1 may have Dynamic Address Translation (DAT) 3 for transforming program addresses (virtual addresses) into real address of memory. A DAT typically includes a Translation Lookaside Buffer (TLB) 7 for caching translations so that later accesses to the block of computer memory 2 do not require the delay of address translation. Typically a cache 9 may be employed between Computer Memory 2 and the Processor 1. The cache 9 may be hierarchical having a large cache available to more than one CPU and smaller, faster (lower level) caches between the large cache and each CPU. In some implementations the lower level caches are split to provide separate low level caches for instruction fetching and data accesses. In an embodiment, an instruction may be fetched from memory 2 by an instruction fetch unit 4 via a cache 9. The instruction may be decoded in an instruction decode unit (6) and dispatched (with other instructions in some embodiments) to instruction execution units 8. Typically several execution units 8 are employed, for example an arithmetic execution unit, a floating point execution unit and a branch instruction execution unit. The instruction may be executed by the execution unit, accessing operands from instruction specified registers or memory as needed. If an operand is to be accessed (loaded or stored) from memory 2, a load store unit 5 typically handles the access under control of the instruction being executed. Instructions may be executed in hardware circuits or in internal microcode (firmware) or by a combination of both.
U.S. Pat. No. 5,560,013 “METHOD OF USING A TARGET PROCESSOR TO EXECUTE PROGRAMS OF A SOURCE ARCHITECTURE” to Scalzi et al., filed Dec. 6, 1994, assigned to IBM, incorporated by reference herein teaches a method of utilizing large virtual addressing in a target computer to implement an instruction set translator (1ST) for dynamically translating the machine language instructions of an alien source computer into a set of functionally equivalent target computer machine language instructions, providing in the target machine, an execution environment for source machine operating systems, application subsystems, and applications. The target system provides a unique pointer table in target virtual address space that connects each source program instruction in the multiple source virtual address spaces to a target instruction translation which emulates the function of that source instruction in the target system. The target system efficiently stores the translated executable source programs by actually storing only one copy of any source program, regardless of the number of source address spaces in which the source program exists. The target system efficiently manages dynamic changes in the source machine storage, accommodating the nature of a preemptive, multitasking source operating system. The target system preserves the security and data integrity for the source programs on a par with their security and data integrity obtainable when executing in source processors (i.e. having the source architecture as their native architecture). The target computer execution maintains source-architected logical separations between programs and data executing in different source address spaces—without a need for the target system to be aware of the source virtual address spaces.
“Dynamic Native Optimization of Interpreters” published 2000 by Hewlett-Packard Labs 1 Main St. Cambridge, Ma 02142 incorporated herein by reference teaches “There is a long history of approaches to removing interpretive overhead from programming language implementations. In practice, what often happens is that, once an interpreted language becomes popular, pressure builds to improve performance until eventually a project is undertaken to implement a native Just In Time (JIT) compiler for the language. Implementing a JIT is usually a large effort, affects a significant part of the existing language implementation, and adds a significant amount of code and complexity to the overall code base.”
U.S. Pat. No. 6,332,216 “Hybrid just-in-time compiler that consumes minimal resource” filed Mar. 9, 1999 and incorporated herein by reference teaches a virtual machine with a hybrid just-in-time compiler that enables execution of a set of cross-platform code on a hardware platform. The hybrid just-in-time compiler consumes relatively few resources of the hardware platform by compiling only selected basic blocks of the cross-platform code and by making use of existing elements in the virtual machine that are otherwise used to interpret the cross-platform code. The just-in-time compiler is hybrid in that only selected basic blocks of the cross-platform code are compiled while the remaining portions are interpreted.
In FIG. 1B, an example of a prior art emulated Host Computer system 21 may be provided that emulates a Host computer system 50 of a Host architecture. In the emulated Host Computer system 21, the Host processor (CPU) 1 may be an emulated Host processor (or virtual Host processor) and comprises an emulation processor 27 having a different native instruction set architecture than that of the processor 1 of the Host Computer 50. The emulated Host Computer system 21 has memory 22 accessible to the emulation processor 27. In the example embodiment, the Memory 27 may be partitioned into a Host Computer Memory 2 portion and an Emulation Routines 23 portion. The Host Computer Memory 2 may be available to programs of the emulated Host Computer 21 according to Host Computer Architecture. The emulation Processor 27 executes native instructions of an architected instruction set of an architecture other than that of the emulated processor 1, the native instructions obtained from Emulation Routines memory 23, and may access a Host instruction for execution from a program in Host Computer Memory 2 by employing one or more instruction(s) obtained in a Sequence & Access/Decode routine which may decode the Host instruction(s) accessed to determine a native instruction execution routine for emulating the function of the Host instruction accessed. Other facilities that are defined for the Host Computer System 50 architecture may be emulated by Architected Facilities Routines, including such facilities as General Purpose Registers, Control Registers, Dynamic Address Translation and I/O Subsystem support and processor cache for example. The Emulation Routines may also take advantage of function available in the emulation Processor 27 (such as general registers and dynamic translation of virtual addresses) to improve performance of the Emulation Routines. Special Hardware and Off-Load Engines may also be provided to assist the processor 27 in emulating the function of the Host Computer 50.
In a mainframe, architected machine instructions are used by programmers, usually today “C” programmers often by way of a compiler application. These instructions stored in the storage medium may be executed natively in a z/Architecture IBM Server, or alternatively in machines executing other architectures. They can be emulated in the existing and in future IBM mainframe servers and on other machines of IBM (e.g. pSeries® Servers and xSeries® Servers). They can be executed in machines running Linux on a wide variety of machines using hardware manufactured by IBM®, Intel®, AMDTM, Sun Microsystems and others. Besides execution on that hardware under a Z/Architecture®, Linux can be used as well as machines which use emulation by Hercules, UMX, FSI (Fundamental Software, Inc) or Platform Solutions, Inc. (PSI), where generally execution may be in an emulation mode. In emulation mode, emulation software may be executed by a native processor to emulate the architecture of an emulated processor.
The native processor 27 typically executes emulation software 23 comprising either firmware or a native operating system to perform emulation of the emulated processor. The emulation software 23 may be responsible for fetching and executing instructions of the emulated processor architecture. The emulation software 23 maintains an emulated program counter to keep track of instruction boundaries. The emulation software 23 may fetch one or more emulated machine instructions at a time and convert the one or more emulated machine instructions to a corresponding group of native machine instructions for execution by the native processor 27. These converted instructions may be cached such that a faster conversion can be accomplished. Not withstanding, the emulation software must maintain the architecture rules of the emulated processor architecture so as to assure operating systems and applications written for the emulated processor operate correctly. Furthermore the emulation software must provide resources identified by the emulated processor 1 architecture including, but not limited to control registers, general purpose registers, floating point registers, dynamic address translation function including segment tables and page tables for example, interrupt mechanisms, context switch mechanisms, Time of Day (TOD) clocks and architected interfaces to I/O subsystems such that an operating system or an application program designed to run on the emulated processor, can be run on the native processor having the emulation software.
A specific instruction being emulated may be decoded, and a subroutine called to perform the function of the individual instruction. An emulation software function 23 emulating a function of an emulated processor 1 may be implemented, for example, in a “C” subroutine or driver, or some other method of providing a driver for the specific hardware as will be within the skill of those in the art after understanding the description of the preferred embodiment. Various software and hardware emulation patents including, but not limited to U.S. Pat. No. 5,551,013 for a “MULTIPROCESSOR FOR HARDWARE EMULATION” of Beausoleil et al.; and U.S. Pat. No. 6,009,261 “PREPROCESSING OF STORED TARGET ROUTINES FOR EMULATING INCOMPATIBLE INSTRUCTIONS ON A TARGET PROCESSOR” of Scalzi et al; and U.S. Pat. No. 5,574,873 “DECODING GUEST INSTRUCTION TO DIRECTLY ACCESS EMULATION ROUTINES THAT EMULATE THE GUEST INSTRUCTIONS” of Davidian et al; and U.S. Pat. No. 6,308,255 “SYMMETRICAL MULTIPROCESSING BUS AND CHIPSET USED FOR COPROCESSOR SUPPORT ALLOWING NON-NATIVE CODE TO RUN IN A SYSTEM” of Gorishek et al; and U.S. Pat. No. 6,463,582 “DYNAMIC OPTIMIZING OBJECT CODE TRANSLATOR FOR ARCHITECTURE EMULATION AND DYNAMIC OPTIMIZING OBJECT CODE TRANSLATION METHOD” of Lethin et al; and U.S. Pat. No. 5,790,825 “METHOD FOR EMULATING GUEST INSTRUCTIONS ON A HOST COMPUTER THROUGH DYNAMIC RECOMPILATION OF HOST INSTRUCTIONS” of Eric Traut, each of which are incorporated by reference herein, and many others, illustrate the a variety of known ways to achieve emulation of an instruction format architected for a different machine for a target machine available to those skilled in the art, as well as those commercial software techniques used by those referenced above.
U.S. Pat. No. 5,953,520 “Address Translation Buffer for Data Processing System Emulation Mode”, (Mallick) assigned to IBM, Filed Sep. 22, 1997 and incorporated herein by reference, teaches a processor and method of operating a processor which has a native instruction set and emulates instructions in a guest instruction set are described. According to the method, a series of guest instructions from the guest instruction set are stored in memory. The series includes a guest memory access instruction that indicates a guest logical address in guest address space. For each guest instruction in the series, a semantic routine of native instructions from the native instruction set may be stored in memory. The semantic routines, which utilize native addresses in native address space, can be executed in order to emulate the guest instructions. In response to receipt of the guest memory access instruction for emulation, the guest logical address may be translated into a guest real address, which may be thereafter translated into a native physical address. A semantic routine that emulates the guest memory access instruction may be then executed utilizing the native physical address.
SUMMARY OF THE INVENTION
A Host processor of a Host computer architecture includes emulation software, the emulation software executing on the Host computer processor makes the Host computer system appear to be equivalent to a Guest processor of a Guest computer architecture that employs a Guest instruction set and Guest facilities that are otherwise incompatible with those of the Host processor.
Programs such as an Operating System (OS) or application programs written to be run on a Guest processor, are run-able on the Host processor having the emulation software as if the Host processor were a Guest processor. The method of the invention may be implemented in tangible storage media for use on a computer system or in a computer system comprising a processor and memory.
In an embodiment, the invention provides emulated execution of a group of Guest instructions of a Guest processor architecture with a single semantic routine of Host instructions of a Host processor architecture, designating a plurality of Host cells in Host memory, each designated Host cell corresponding to a separate Guest cell of Guest memory, each Host cell addressable based on a Guest program counter value, then dynamically selecting a first plurality of Guest instructions, the selected first plurality of Guest instructions beginning with a first Guest instruction and ending with a second Guest instruction. A selected first plurality of Guest instructions are Just-In-Time (JIT) compiled the into a second plurality of first Host instructions, a first Host cell of Host memory is patched with Host instructions for executing the compiled second plurality of first Host instructions, the first Host cell corresponding to a first Guest cell of Guest memory, the first Guest cell corresponding to a beginning portion of said first Guest instruction, then responsive to a Guest program counter indicating the first Guest cell corresponding to the first Guest instruction to be executed, executing the compiled second plurality of first Host instructions of the patched first Host cell corresponding to the first Guest cell, and responsive to the Guest program counter indicating a second Guest cell corresponding to a third Guest instruction to be executed, executing one or more second Host instructions of a second Host cell corresponding to the second Guest cell to emulate execution of the second Guest instruction, the second Guest cell corresponding to a beginning portion of said second Guest instruction.
In another embodiment, the invention further comprises monitoring execution of emulated Guest instructions, and responsive to a group of monitored Guest instructions meeting predetermined criteria for group emulation, making the group a candidate for the dynamically selecting as the first plurality of Guest instructions.
In another embodiment, the predetermined criteria comprises branch Guest instruction criteria, the method further comprising encountering a branch Guest instruction, the branch Guest instruction, responsive to being executed, branching back to a target Guest instruction, wherein the target Guest instruction is the first Guest instruction, wherein the branch Guest instruction is the second Guest instruction.
In another embodiment, the predetermined criteria comprises a threshold value, the method further comprising determining that the group of monitored Guest instructions have been executed as a group a number of times, and responsive to the number of times meeting the threshold value criteria, making the group of monitored Guest instructions the first plurality of Guest instructions.
In another embodiment, the predetermined criteria comprises a predetermined sequence of instructions, the method further comprising based on encountering a group of monitored Guest instructions equivalent to the predetermined sequence of instructions, making the group of monitored Guest instructions the first plurality of Guest instructions.
In another embodiment, prior to the dynamically selecting the first plurality of Guest instructions, populating said first Host cell with one or more second Host instructions for emulating the first Guest instruction corresponding to the first Guest cell, and prior to the dynamically selecting the first plurality of Guest instructions, populating said second Host cell with the one or more second Host instructions for emulating the second Guest instruction corresponding to the second Guest cell.
In an embodiment, third Host cells are populated with one or more third Host instructions for performing a wild branch operation according to the Guest processor architecture, each third Host cell corresponding to any one of a portion other than a beginning portion of said a corresponding Guest instruction, or a Data portion of Guest memory, and responsive to the Guest program counter indicating a third Host cell, executing the one or more third Host instructions to perform a wild branch function according to the Guest processor architecture.
In an embodiment, the JIT compiling comprises, searching a predetermined table for a table entry corresponding the first plurality of Guest instructions, and responsive to the table entry being found, based on the table entry, obtaining the second plurality of first Host instructions as the compiled first plurality of first Host instructions.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following written description.
Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter which may be regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1A is a diagram depicting an example Host computer system (processor (CPU)) of the prior art;
FIG. 1B is a diagram depicted an example Guest (Emulated) computer system of the prior art;
FIG. 1C is a diagram depicting prior art components of a computer system;
FIG. 2 is a diagram depicting a prior art network of computer systems;
FIG. 3 is a diagram depicting an example Guest computer system to be emulated;
FIG. 4 is a diagram depicting an example Host computer system for emulating a Guest computer system;
FIG. 5 depicts components of an example Host computer system memory system;
FIG. 6 depicts a mapping of Guest program instructions to Emulation cells of the invention;
FIG. 7 depicts an example mapping of Guest instructions to Host (native) memory execution cells (ECs);
FIG. 8 depicts initialized execution cells;
FIG. 9 depicts populated execution cells;
FIGS. 10-20 depict embodiments of execution cells;
FIG. 21 depicts mapping of Guest cells to Host cells;
FIGS. 22-26 example flows of a method for practicing the invention; and
FIG. 27 depicts an example of JIT compiled group of Host instructions for emulating a loop of Guest instructions.



DESCRIPTION OF THE INVENTION
In an embodiment, the invention may be practiced by software (sometimes referred to Licensed Internal Code, Firmware, Micro-code, Milli-code, Pico-code and the like, any of which would be consistent with the present invention). Referring to FIG. 1A, software program code which embodies the present invention may be typically accessed by the processor also known as a CPU (Central Processing Unit) 1 of the system 50 from long-term storage media 7, such as a CD-ROM drive, tape drive or hard drive. The software program code may be embodied on any of a variety of known media for use with a data processing system, such as a diskette, hard drive, or CD-ROM. The code may be distributed on such media, or may be distributed to users from the computer memory 2 or storage of one computer system over a network 10 to other computer systems for use by users of such other systems.
Alternatively, the program code may be embodied in the memory 2, and accessed by the processor 1 using the processor bus. Such program code includes an operating system which controls the function and interaction of the various computer components and one or more application programs. Program code may be normally paged from dense storage media 11 to high-speed memory 2 where it may be available for processing by the processor 1. The techniques and methods for embodying software program code in memory, on physical media, and/or distributing software code via networks are well known and will not be further discussed herein. Program code, when created and stored on a tangible medium (including but not limited to electronic memory modules (RAM), flash memory, Compact Discs (CDs), DVDs, Magnetic Tape and the like is often referred to as a “computer program product”. The computer program product medium is typically readable by a processing circuit preferably in a computer system for execution by the processing circuit.
FIG. 1C illustrates a representative workstation or server hardware system in which the present invention is practiced. The system 100 of FIG. 1C comprises a representative computer system 101, such as a personal computer, a workstation or a server, including optional peripheral devices. The workstation 101 includes one or more processors 106 and a bus employed to connect and enable communication between the processor(s) 106 and the other components of the system 101 in accordance with known techniques. The bus connects the processor 106 to memory 105 and long-term storage 107 which can include a hard drive (including any of magnetic media, CD, DVD and Flash Memory for example) or a tape drive for example. The system 101 might also include a user interface adapter, which connects the microprocessor 106 via the bus to one or more interface devices, such as a keyboard 104, mouse 103, a Printer/scanner 110 and/or other interface devices, which can be any user interface device, such as a touch sensitive screen, digitized entry pad, etc. The bus also connects a display device 102, such as an LCD screen or monitor, to the microprocessor 106 via a display adapter.
The system 101 may communicate with other computers or networks of computers by way of a network adapter capable of communicating 108 with a network 109. Example network adapters are communications channels, token ring, Ethernet or modems. Alternatively, the workstation 101 may communicate using a wireless interface, such as a CDPD (cellular digital packet data) card. The workstation 101 may be associated with such other computers in a Local Area Network (LAN) or a Wide Area Network (WAN), or the workstation 101 can be a client in a client/server arrangement with another computer, etc. All of these configurations, as well as the appropriate communications hardware and software, are known in the art.
FIG. 2 illustrates a data processing network 200 in which the present invention may be practiced. The data processing network 200 may include a plurality of individual networks, such as a wireless network and a wired network, each of which may include a plurality of individual workstations 101 201 202 204. Additionally, as those skilled in the art will appreciate, one or more LANs may be included, where a LAN may comprise a plurality of intelligent workstations coupled to a host processor.
Still referring to FIG. 2, the networks may also include mainframe computers or servers, such as a gateway computer (client server 206) or application server (remote server 208 which may access a data repository and may also be accessed directly from a workstation 205). A gateway computer 206 serves as a point of entry into each network 207. A gateway may be needed when connecting one networking protocol to another. The gateway 206 may be preferably coupled to another network (the Internet 207 for example) by means of a communications link. The gateway 206 may also be directly coupled to one or more workstations 101 201 202 204 using a communications link. The gateway computer may be implemented utilizing an IBM eServerTM zSeries® z9® Server available from IBM Corp.
Software programming code which embodies the present invention may be typically accessed by the processor 106 of the system 101 from long-term storage media 107, such as a CD-ROM drive or hard drive. The software programming code may be embodied on any of a variety of known media for use with a data processing system, such as a diskette, hard drive, or CD-ROM. The code may be distributed on such media, or may be distributed to users 210 211 from the memory or storage of one computer system over a network to other computer systems for use by users of such other systems.
Alternatively, the programming code 111 may be embodied in the memory 105, and accessed by the processor 106 using the processor bus. Such programming code includes an operating system which controls the function and interaction of the various computer components and one or more application programs 112. Program code may be normally paged from dense storage media 107 to high-speed memory 105 where it may be available for processing by the processor 106. The techniques and methods for embodying software programming code in memory, on physical media, and/or distributing software code via networks are well known and will not be further discussed herein. Program code, when created and stored on a tangible medium (including but not limited to electronic memory modules (RAM), flash memory, Compact Discs (CDs), DVDs, Magnetic Tape and the like are often referred to as a “computer program product”. The computer program product medium may be typically readable by a processing circuit preferably in a computer system for execution by the processing circuit.
In an emulation environment, a Host or Native processor may be provided with emulation code 23. Emulation code is software that runs on the Host processor to provide an interface to Guest programs (Guest Operating systems, Guest Application Programs and the like) such that the Guest programs (written for an architecture other than that of the Host processor architecture) can execute on the Host processor as if it were a Guest processor having the Guest architecture. In an example, a Host processor such as a PowerPC® 27 from IBM® may be provided with Emulation software (code) 23 that interprets programs written for a s/390® from IBM into PowerPC routines (semantic routines) 23 that “emulate” the s/390 instructions of a Guest s/390 computer system 50 and functions on the Host Pentium processor 27. The emulation code may provide a variety of functions, including managing the Host partition (image) that may be provided to the guest (sometime called a Hypervisor function), performing Guest architecture functions such as Guest dynamic address translation, providing Guest architecture facilities such as control registers, TOD clocks etc., providing Guest virtual to Host real address translation or providing Guest to Host instruction interpretive functions. Typically, a semantic routine is provided by the emulation code that executes the function of a corresponding Guest machine instruction in Host instructions on the Host processor (Host Central Processing Unit (CPU)). The emulation code further supports fetching of Guest instructions to be emulated and interpreting the fetched instruction to determine the corresponding semantic routine as well as calling the corresponding semantic routine to be executed to “emulate” the execution of the Guest instruction. Various techniques have been proposed to perform the emulation function. A goal may be to reduce the emulation code overhead in order to improve performance. One performance advantage can be achieved by re-using the translated code rather than translating each time the Guest instruction is encountered.
The present invention, in a preferred embodiment, dynamically interprets a Guest instruction(s) in one or more “cells” of Guest real memory and utilizes a corresponding cell(s) location in Host real memory to hold at least a portion of the semantic routine that emulates the Guest instruction(s). This may be done by, for example providing a Host cell location for each halfword (2 bytes or 16 bits) of a Guest instruction (where the Guest instructions may be a variable length instruction). When the Guest instruction is first executed, the emulation code (semantic routine) in a Host cell may be executed, wherein the Host cell corresponds to the Guest cell having the first halfword of the Guest instruction. The Host cell may be preferably initialized to have code that branches to a common handler to interpret the Guest instruction and over-write the Host cell with the semantic routine customized to emulate the Guest instruction. The semantic routine loaded in the Host cell may comprise a branch instruction for branching to a specific handler for handling any one of the specific Guest instruction, the type of Guest instruction, or the whole semantic routine could reside in the Host cell(s) corresponding to the Guest instruction.
In another embodiment, the Host cells corresponding to a Guest instruction can be loaded with emulation routines to handle wild branches for the case where a branch may be taken to a portion of a Guest instruction other than the beginning portion.
In an embodiment, when a semantic routine emulating a Guest instruction may be executed, the semantic routine preferably includes a return instruction (a branch instruction) whereby the return instruction, either branches back to the emulation code to obtain a next instruction, branches directly to a target Host semantic routine corresponding to a target Guest instruction or “falls thru” directly to a Host cell having a next semantic routine corresponding to a next sequential Guest instruction. Thus, in certain cases, several Guest instructions might be emulated by executing several semantic routines without any intervening emulation code translation of the Guest instructions.
In an embodiment, a plurality of Guest instructions are implemented in a common semantic routine. Each of the Guest instructions are mapped to corresponding Host cells, however when the first Host cell is executed, it performs the function of the plurality of Guest instructions in a single semantic routine. Preferably, if any Host cell other than the first Host cell is executed (such as the case where there is a branch to one of the Guest instructions in the plurality other than the first Guest instruction), the corresponding Host cells other than the first Host cell implement semantic routines of the individual Guest instruction of the plurality of Guest instructions.
The portion of the emulation activity involved in interpreting the Guest instruction(s) to emulate the Guest processor may in one embodiment emulate the function of an instruction fetch unit, a Dynamic Address Translation storage unit, a Translation Lookaside Buffer unit and dispatch unit of a hardware processor.
Prior art Emulation interpreters may use an interpreter-loop to step through units of work (byte-code, assembler instructions etc). The interpreter loop fetches the next unit of work (such as a Guest instruction to be executed), decodes it, and then calls a handler routine that implements a specific instance of work (emulation semantic routine). Typical hardware pipelines of a Host processor do not behave very well for such an environment, as the dispatching branch in the loop may be indirect and cannot be well predicted due to the large set of targets that implement the many different work units. For Example:















for(;;) {



wu = fetch_next_work_unit(address);



address += size_of(wu);



address = call_work_unit_handler(wu); // poorly predicted branch



}










The present invention provides a methodology for improving the performance of interpretation by removing the unpredictable branch behavior.
Context Threaded Interpreters
The present invention reduces branch-penalties for virtual-machine (VM) based languages like OCam1 and Java®. In an embodiment of the invention a binary translation environment may be used. As a result, this embodiment provides methodologies for:
1. building a self-modifying execution environment based on the logical mapping of emulate-to-native instructions,
2. handling emulated branches into the middle of instructions, and
3. handling concurrency of execution.
Additionally, the present invention provides a spatially aware interpreter capable of interacting with a traditional trace-based JIT compiler. An example JIT compiler compiles a selected group of guest machine instructions into a host semantic routine that is directly executable by a host processor The selected group of guest instructions may be referred to as a “trace”, although the term “trace” may also include the corresponding host semantic routine. The trace is to be compiled by the JIT compiler into a semantic routine to be executed by the host processor to emulate the corresponding trace. In an embodiment the JIT compiler may merely detect familiar sequences (traces) of guest instructions and map them to pre-designed semantic routines to be executed by the Host processor, where the pre-designed semantic routines have been created in Host instructions and kept in a table (cached) with an association with the corresponding Guest trace along with implementation dependent metadata providing parameters for the semantic routine.
In one embodiment (FIG. 6) an emulated Guest processor comprising a Virtual memory, presents “logical” or “virtual” addresses to programs. These addresses may be translated on a page basis to pages of memory (“real” addresses of real memory). Some of the pages may be present in the memory but others are not. The Operating System (OS) “pages” fixed sized blocks of data between a peripheral mass storage and processor memory to make it appear as if the whole amount of virtual memory is resident. Thus, the processor memory acts as a cache for addressable virtual storage. The present invention (FIG. 21) works through a page-mapped set of Guest execution-cells 2101. A single (a, b etc) or a set of Guest execution cells maps 2103 2104 to the absolute location of work unit being interpreted (A, B etc). When a new Guest page 2101 (4 Kbytes in an embodiment) of Guest work-units (a-l) is first loaded into Guest processor memory for execution, an associated Host logical page 2102 of execution cells (A-L) is created. Each Guest page is made up of a number of Guest cells (in an example, each Guest cell (a-l) is 2 halfwords of the Guest page). Each Host cell of the associated Host logical page is a predetermined size and is associated with a Guest cell. The interpreter maintains a look-aside table for the page mapping from Guest work unit to Host execution unit pages. In an embodiment a Host execution cell and Host page are both larger than one or both of the Guest cell and the Guest page. A Host execution cell preferably is large enough to hold a simple dispatch sequence. Additionally, there must be at least one Host execution cell for each possible location of a Guest work-unit. Hence the smallest Guest work-unit dictates the number or execution units per Guest page. In the mainframe environment, instructions may be 2, 4 or 6 bytes long, therefore the Guest work-unit is 2 bytes. z/Architecture from IBM provides various levels of addressing. A “Real address” of a page may subject to relocation to an “Absolute address” of the page in memory based on a relocation value. This permits the OS in a partition to provide each processor with a real address range that has a processor unique portion (containing the processor's program counter) allocated to each processor transparent to the processor. The Absolute address may further be relocated in order to map multiple partitions (images) into physical memory being the actual hardware address. Other architectures my implement address relocation equivalent to more of these addresses or only 1 or 2 of them. The present specification will refer to Real addresses to differentiate between Virtual address (logical address) and physical address (Real or Absolute), not differentiating between Absolute and Real.
Preferably the mapping can be described as:












 ec_address = wu_to_ec_page_table(page(wu)) + size_of_cell *


(wu − page(wu)) / min_wu_size;


 wu_address = ec_to_wu_page_table(page(ec)) + min_wu_size *


(ec − page(ec)) / size_of_cell;









Preferably when a Guest page is paged into processor memory, a corresponding Host page of Host execution cells is created, each of the Host execution cells FIG. 8 801 of the Host page may be initialized with a simple assembly sequence that dispatches to a common handler. This common handler, when executed, decodes the work unit (the Guest instruction) that maps to the given Host execution cell and then patches this execution-cell 801 to provide the Host semantic routine for executing the function of the Guest instruction, in an example the Host execution cell may be patched to provide a call to the given work unit handler (specific semantic routine).















 common_handler(context, ec_address, ...) {



 // Decode the wu that maps to ec_address



 wu = decode_work_unit(ec_address);



// Find the wu_handler for the given wu



 wu_handler = lookup_handler(wu);



// patch the dispatch sequence to call wu_handler



// and have the return address jump to the next execution-cell



// (based on the size of the associated work-unit).



 patch_execution_cell(wu_handler, ec_address);



// invoke the updated version of the cell



 dispatch_to_ec(context, ec_address, ...);



}










A preferred work-unit handler will manipulate the interpreter's context (registers, status and memory will be updated according to the work-unit), and will then return to the execution cell to the instruction following the handler dispatch. In an embodiment, the instruction following the handler dispatch may be a branch that jumps to the next expected Guest execution-cell. The common-handler may update this branch based on the decoded work-unit. As the length of the work-unit is known, the next execution-unit is also known.
Example execution cell at initialization:















...



label_cell_n:



branch-and-link common_handler(context, ec_address);



ec_address = label_cell_n+1



branch label_cell_n+1



label_cell_n+1:



...










Example execution cell after its initialized code is executed:
















...



label_cell_n:



branch-and-link ADD_handler(context);



ec_address = label_cell_n+4



branch label_cell_n+4 // the ADD work unit is 4 cells in length



label_cell_n+1:



 ...










In-Lining Work Units into Execution Cell

For simpler work units, it may be possible to avoid calling the work unit handler if the implementation of the work unit can be completely represented in the respective Guest execution cell. For instance, an execution cell implementation of the add handler for an add work unit (result=operand1+operand2) could look like this after the initialization code has been executed . . .















label_cell_n:



*context->add_result_location =



 *context->add_operand_one_location +



 *context->add_operand_two_location



ec_address = label_cell_n+4



branch label_cell_n+4



label_cell_n+1:



...










In one embodiment, Host execution cells are 32 Bytes and Guest cells are 2 Bytes, the Host execution cells may be made even larger for the purpose of allowing more aggressive in-lining of work units (permitting semantic routines for some Guest instructions to fit entirely in a single Host execution cell).
In another embodiment an emulator could use a plurality of the Host execution cells allotted to a Guest variable length instruction to hold an entire Host semantic routine in order to provide better performance.
When populating an execution cell the handler could look ahead at subsequent instructions and then either encode a call to a native routine that emulates the behavior of these multiple guest instructions (or even inline the native instructions in the cell—dependent on size constraints). This native routine could either be based on a pre-compiled template or constructed from scratch each time based on the guest instructions.
As the handler would know how many instructions it has grouped together a branch to the next guest instruction after the group could be encoded following the grouped instruction execution.
Execution cells for guest instructions included in the group compile would be left with the generic instruction sequence that includes a branch to the common handler. Therefore if these instructions are encountered they could either be encoded as a stand-alone instruction or form the start of a new instruction group to be sent to the compiler.
In an embodiment, a spatially aware interpreter may be a jumping off point to a more traditional JIT compiler where multiple instructions are sent to the compiler as a group. A difference here is that there wouldn't necessarily have to be a ‘recording phase’ concurrent with interpretation to form this trace. Instead the handler itself could figure out the instructions to compile before they have been executed once.
In the context of branches, the common_handler provides a unique opportunity for managing the interaction of the page-mapped execution environment with either a more traditional interpreter or a Just-In-Time (JIT) compiler. The common_handler provides an interface for implementing new trace discovery policy. In combination with code re-use/hotness mechanisms, this interface that can be used for directing the execution back to a traditional looped interpreter, more page-mapped translation, or potentially could invoke a JIT to perform and execute more aggressive compilation of the trace.
In an embodiment, the situation exists where a Guest branch target lands in the middle of a set of Host cells that represent a specific instruction. If, for example, a branch were to target the address that maps to cell C in the example in the FIG. 20, incorrect behavior could ensue as the cell could hold the initial un-modified generic handler sequence or a partial set representation of the instruction a ‘b’.
Referring to FIG. 21, Cell ‘a’ holds a 2-byte inst. in emulated memory. Cell ‘a’ maps to execution cell ‘A’ in native memory which holds self-modifying code for dispatching to the handler of the instruction represented in ‘a’.
Cells ‘b, c, d’ is a 6-byte inst. Cells ‘b, c, d’ map to execution cells ‘B C D’ in native memory which hold self-modifying code for dispatching to the handler of the instruction represented in ‘b, c, d’. Cells ‘C’ and ‘D’ hold code to return to the interpreter to correctly emulate a mid-instruction branch target.
In an embodiment, multiple Host cells (super-cells) are used to perform a function where the Guest instruction occupies multiple Guest cells that map to multiple Host cells.
In an embodiment where super-cells are not used, Cells ‘b, c, d’ represents a 6-byte Guest instruction. Cells ‘b, c, d’ map to Host execution cells ‘B, C, D’ in native memory which hold self-modifying code for dispatching to the handler of the instruction represented in ‘b, c, d’. Cells ‘C’ and ‘D’ hold code to return to the interpreter to correctly emulate a mid-instruction branch target. Cells C and D hold a special dispatch sequence that dispatches to a handler for mid-instruction targets. This handler returns execution to the interpreter.
In an embodiment where super-cells are used, a bit map is used for example:
Bit map: <(A:0), (B:0), (C:1), (D:1), (E:0), (F:0), (G:0), (H:0)>
Cells ‘b, c, d’ represent a 6-byte Guest inst. Cells ‘b, c, d’ map to execution cells ‘B C D’ in native memory which hold self-modifying code for dispatching to the handler of the instruction represented in ‘b, c, d’. Cells ‘C’ and ‘D’ hold code to return to the interpreter to correctly emulate a mid-instruction branch target. Cells C and D hold super-cell instructions representing the function for instruction ‘b’. A bit map representing cells A through H shows that cells C and D are not legal targets and hence any branch that targets these cells should revert to interpretation.
In an embodiment, if super-cells are not used, only the first cell is used to dispatch to the appropriate handle. Subsequent cells belonging to the same instruction can be filled with a special handler that inherently handles mid-instruction branch targets. This handler, by virtue of its context, recognizes that the branch target's cell should not be patched and hence dispatches to traditional interpretation of the partial instruction stream.
If super-cells are used, a bit-map may be maintained for each paged-mapped set of cells. The bit map holds information identifying the set of cells that could not be safely branched to as a result of being part of a super-cell and not being the first cell representing the instruction. Preferably, each bit of the bit-map represents a Host cell of the Host page. When a branch instruction targets a given page-mapped set, the bit map may be inspected to see if the target may be safe. If the target is not safe, the branch reverts to the traditional interpreter loop.
In another embodiment, it would be possible to group a set of consecutive cells that map back to a single instruction to create super-execution cells. To correctly handle situations where the application branches into the middle of such an instruction a bit map of super-cells per logical page would be maintained and checked when branching to a given page. Should the application jump into the middle of the super-cell, the jump would have to be handled specially to correctly represent the expected behaviour of the hardware being emulated.
Calls and Jumps
Branch instructions may be handled in a different way. For a call type branch instruction instead of changing the branch-and-link common handler to branch-and-link CALL_handler it could be changed to branch-and-link to the target of the call. For a jump type branch instruction (no return) the patched code will simply branch to the target (or the branch-and-link portion could be no-op'd and the existing fall through branch could be modified):
For a jump type branch instruction (branch relative to the current program counter address), an example Guest execution cell after it has been executed for a jump type branch instruction (that branches ahead x bytes) is as follows:















...



label_cell_n:



nop



branch label_cell_n+x



label_cell_n+1:



...










Example Execution cell after it is executed for a call type branch instruction (that branches ahead x bytes):















...



lable_cell_n:



link_register = current_wu_address (constant)



branch label_cell_n+x



label_cell_n+1:










In an embodiment for emulating Guest branch instructions, a preferred common_handler provides a unique opportunity for managing the interaction of the page-mapped execution environment with either a more traditional interpreter or a Just-In-Time (JIT) compiler. A JIT compiler in an embodiment, dynamically “compiles” guest machine instructions into host machine instructions while executing the guest program. In one embodiment the input for such a JIT compiler may be a series of guest machine instructions that represent a particular path or ‘trace’ taken through the corresponding guest program. As a trace may be a particular runtime path, there would advantageously be some common point that will decide if and when to record a new trace and to subsequently initiate a JIT compile. The common_handler can serve as this decision point and therefore provide an interface for implementing this new trace discovery policy. In combination with code re-use/hotness mechanisms, this interface can be used for directing the execution back to either a traditional looped interpreter, continue with more page-mapped translation, or potentially invoke a JIT compiler to perform and execute more aggressive compilation of the trace.
In an example using execution cells as described, a call to an interrupt handler would not signal any sort of architected system interruption but would deal with the ‘interruption” in building and executing the execution cells. This interrupt handler's main purpose would be to revert back to an interpreter loop to execute the guest instructions and deal with any subsequent architected interruptions or exceptions from there.
Thread Safety and Concurrency of Execution
In a preferred implementation, multiple Host processors may be performing emulation activity executing instructions in a common Host page. Each Host processor may be executing a thread of a Guest processor. This presents a race condition if several processors are attempting to access the same initialized Host execution cell at the same time.
When multiple Host threads on different Host processors concurrently access the same initialized page mapped execution cell there may be a danger of inconsistent updates to the execution cell. For example, the first processor may be executing the execution cell initialization code and preparing to update the cell with semantic routine code while the second processor is also executing the initialization code. As a result, the second processor may be corrupting the first processor's execution sequence by overlaying new semantic code on the initialization code. As different Host processors are reading and writing the shared data that is in the execution cell, then extra care must be taken so only one of a few safe consistent states will be observed by any one Host thread. One approach to handling this could be for the logical page mapped execution units to be thread specific, thus avoiding concurrency all together.
Alternatively, to save space and reduce redundancy in cell patching phase, it may be preferable to have a common set of execution cells for all threads. In this case, the first thread through a given execution cell will drive invocation of the common_handler and be responsible for subsequent patching of the cell. If another thread tries to execute the same cell, this thread should either be held back on a lock while the first thread completes its patching operation, redirected back to a looped interpreter for execution until the first thread has completed its update to the execution cell, or allowed to fully repeat the first thread's update to the execution cell. In an embodiment, a bit significant table holds a lock bit for each cell. When a processor accesses a Host cell, it checks the bit table to see if the bit corresponding to the cell (or alternatively, a group of cells or an entire page of cells) is ‘0’. The processor modifies the bit to ‘1’ and begins using the cell if it was ‘0’. If the cell was ‘1’, the processor spins until the cell is ‘0’ before setting the bit to ‘1’ for its own use.
The choice for how concurrent execution may be managed may be very much dependent on characteristics of the implementation of this disclosure with respect to the hardware (and respective memory model) and the characteristics and constraints on the emulated environment.
In an example, the lead thread patches the call to common_handler with a branch back to the interpreter loop making it safe for the thread to patch the code below the branch-and-link. If on the other hand, the thread were to right away patch in the branch to the wu_handler then the code below the branch-and-link would not be guaranteed to have been updated yet and a subsequent thread might take the branch-and-link to the wu_handler but then encounter inconsistent or incorrect code when it returned.
It may be safe for two or more threads to race and both reach the common_handler and both start patching as the patched data by each thread should be the same.
Each thread that reaches the common_handler would preferably perform the following steps:

    1) Change “branch-and-link common_handler” to “branch interpreter_loop_entry”.
    2) Update the necessary code below the branch-and-link such as the branch to the next cell and the ec_address. Also, if the cell may be going to inline the work-unit this inlined code would be patched in at this point.
    3) Use platform dependent instruction cache invalidate or flush instructions and any necessary memory fence or sync instructions on the patched instructions so they are visible to all threads and processors.
    4) Change “branch interpreter_loop_entry” to “branch-and-link <wu>_handler”. If inlining the work unit then changes “branch interpreter_loop_entry” to the first inlined instruction or to a no-op instruction to replace the guarding branch instruction.


As long as the “branch-and-link common_handler” may be patched in such a way that it may be always observable as a valid branch instruction to either the common_handler or the interpreter_loop_entry then no thread can see any inconsistent updates to the code below the branch-and-link. On a number of platforms guaranteeing that the branch can be safely patched in this way may be mostly a matter of getting the alignment of the instruction correct. There are two cases, described below for the two threads Thread1 and Thread2, but applicable in general for any number of competing threads.

    1) Thread1 gets to the cell first and starts patching but its change to the branch-and-link instruction may be not yet visible. In this case Thread2 will also take the branch-and-link (bl) to the common_handler and start patching—this may be safe as the to-be-patched code or data may be the same as what Thread1 may be already patching (i.e. the change may be id-empotent).
    2) Thread1 gets to the cell first and starts patching and the change to bl may be visible. In this case Thread2 will take the newly patched in branch back to the interpreter loop and Thread1 can continue to safely patch.


An example emulation environment according to the present invention may be presented in order to provide a basis for understanding elements of the invention. The invention may be not limited to the example and one of average skill can appreciate that the invention could be advantageously practiced in a variety of environments.
Referring to FIG. 3, a representative Guest Computer System 300 to be Emulated by a Host computer system is presented. The Guest Computer System 300 includes for example one or more Guest Processors 304 to be emulated. Functions of the Guest Processor 304 that may be emulated include an instruction fetch unit and one or more execution units, a Program Status Word (PSW) including a program counter for keeping track of the location of Guest Instructions in Guest Virtual memory 301. Other components of the Guest Achitecture of the Guest Processor 304 may be emulated including architected general purpose registers, architected floating point registers, architected control registers and the like. The Guest Computer System 300 includes a Guest Real Memory 303, Dynamic Address Translation (DAT) facility 302 (which includes mechanisms for translating the addresses of the program (virtual addresses) into real addresses of Guest Real Memory 303. The DAT may include Segment Tables and Page tables as are well known in the art. The page tables may have entries related to 4 Kbyte blocks of storage called “pages”. The Guest Virtual Memory 301 may be typically much larger than Guest Real Memory 303 and therefore, only portions of Guest Virtual Memory 301 are held in Guest Real Memory 303 at any time. When a virtual page needs to be accessed, the Operating System (OS) may bring the page from Mass Storage 305 into Guest Real Memory 303. This operation may be commonly referred to as “paging” in. Paging may also occur when a page in Guest Real Memory 303 needs to be evicted in order to make room for a new page, this may be known as paging out.
The Guest Computer System 300 typically may be implemented in a Guest Architecture such as the z/Architecture from IBM® Corp. having an guest instruction set that may be a different instruction set than the host instruction set of the Host Architecture (such as X86 from INTEL® Corp.).
Referring to FIG. 4, an example Host Computer System is shown comprised of similar functions of the Guest Computer System 300 including one or more Host Processors 404 for executing instructions of the native processor architecture, a Mass Storage 405 for paging pages of virtual memory into Host Real Memory 403, Host DAT 402 and Host Virtual Memory 401 having Host Operating System (OS) and Host Applications. In an embodiment, emulation of the Guest Computer System 300 may be provided by one or more Host Applications.
Referring to FIG. 5, an example of Host Virtual Memory 401 shows partitions having Guest Virtual Memory 301, Guest Real Memory 303, Host Emulation Programs, the Host OS and Host support programs. The Host Support programs may provide an I/O Subsystem, Logical Partitioning (Hypervisor), and operator console support for example. The Guest Real Memory 303 mapped to Host Virtual Memory 401 space, could be implemented in another embodiment in Host Real memory 403. The Host Emulation Program(s) are executed by the Host Processor 404 to emulate (simulate) operation of a Guest Processor 304. Thus, for example, the Host Emulation program will provide emulated registers and emulated Program Counter (PC) functions (logical program counter) of an emulated guest processor 304 for fetching and executing Guest Programs comprising guest instructions. In an implementation Host Emulation Programs translate Guest instructions into semantic routines emulating the function of the Guest instruction using Native instructions of the Host Processor 404.
Referring to FIG. 6, an embodiment of the present invention is shown wherein memory locations of Guest Programs 601 in Guest Real Memory 303 are mapped to corresponding addresses in Host Memory containing “Cells” 602. In a RISC Guest architecture having a fixed length instruction architecture (such as 32 bits or Word instruction length), each guest instruction location (Guest Cell) has a corresponding Host cell location. In a CISC Guest architecture, guest instructions can be different lengths (16 bits or Half Word, 32 bits or Word or 48 bits for example) so Emulator Cells in Host Memory 602 correspond to a minimum size instruction (one halfword for example) such that one cell corresponds to each Half Word for example. Thus the Guest Real Address 603 of a logical program counter increments according to the Guest instruction size (in the example 603, the guest real address may be a 64 bit address and bit 62 may be the lowest Guest PC address since it may be incremented with a Half Word granularity) and the corresponding Host Memory address 604, also 64 bits in this example, may be incremented with bit N being the lowest Host PC address corresponding to the size of a Host Cell in the Host Memory Cells 602.
In the present invention, referring to FIG. 7, each page (4 Kbytes for example) of Guest Real Memory 701 may be initially mapped to a corresponding block (page) of Host Execution Cells (Host ECs). Initially, the corresponding EC's are un-initialized Host (Native) Memory 702. In an embodiment where the Guest instructions may be variable length, there may be one EC for each two bytes of guest memory page. Thus, in an example, the first HW of Guest page x corresponding to the logical Program Counter (PC) of the Guest contains a Guest (add register) AR instruction (with the hexadecimal encoding 0x1A12) for adding contents of Guest register R1 to R2 and storing the result in R1. The AR instruction at page_x+0 has a corresponding EC (ec—1). The next instruction may be a Full Word instruction (Subtract) S (0x5B30∥0x1004), which has two corresponding EC's (ec—2, ec—3).
Referring to FIG. 8, after an execution cell (EC) is initially allocated (referring to FIG. 8), it may be initialized with a generic native instruction sequence 801 802 803. In an embodiment, all ECs corresponding to a Guest page are initialized when the Guest page is “paged into Guest memory”. The generic native instruction sequence 801 802 803 may contain a branch to a common routine to patch the current EC (or a group of ECs). Thus, when the generic native instruction sequence is first executed, it branches to a common routine (common_handler) that over-writes the cell with a specific routine for emulating the Guest instruction.
Referring to FIG. 9, the host processor uses a Native (Host) Program Counter (Native PC) to increment through the Native (Host) instructions. When the Guest program is to execute the Guest instruction beginning at a Guest logical PC address, the corresponding Native PC points to the Native memory Host cell to be executed. Preferably, the portion of the Host PC 604 that locates the Host cell of the Host page, is a shifted version of a portion of the Guest PC 603 that indexes into the Guest page.
The mapping of a guest memory work unit (wu) address (such as a Guest instruction address) to a native (Host) execution cell (EC) address is, for example:















 ec_address = wu_to_ec_page_table(page(wu)) + size-of-cell *



((wu − page(wu)) /min_wu_size);










In this example FIG. 9, we find the ec_address “ec—4” (highlighted) for Move Character (MVC) guest instruction at page_x+6 (0xD207) as follows:












 wu = page_x+6 = 0x10002006, page (wu) = page_x = 0x10002000


 min_wu_size = 2 bytes


 size_of_cell = 4 byte/instruction * 080003000


 ec_address = ec_4 = 0x8000300 + 16 * (0x10002006−0x10002000)/


2 = 0x80003030









Execution of the common routine will preferably result in a call to the common_handler routine. This routine preferably performs the following steps in order to populate an EC with a specific routine for emulating the Guest instruction (wu).
1. Map the input ec_address to the corresponding guest work unit address (wu-address) and use this address to decode the instruction type into a wu_type.
2. Look-up the work unit handler routine based on the decoded wu_type.
3. Populate the generic execution cell instruction sequence so it handles the specific work unit type.
4. Branch to the input ec_address to execute the patched native instructions.
For this example, FIG. 10, program execution has reached the common_handler through the call in ec—2 for the logical address page_x+2 (containing the S guest instruction that may be 4 bytes in length occupying 2 ECs)
Step 1—Find wu_Address/wu_Type:
In an example in FIG. 10, mapping of a native execution cell address to a guest memory work unit address may be as follows:












 wu_address = ec_to_wu_page_table(page(ec)) + min_wu_size *


((ec − page(ec)) / size_of_cell):









In this case, for example:















 page_x+2 = 0x10002002



 ec = ec_2 = 0x80003010, page(ec) = 0x80003000



 min_wu_size = 2 bytes



 size_of_cell = 4 byte/instruction * 4 instructions = 16 bytes



 ec_to_wu_page_table(page(ec)) = 0x10002000



wu_address = 0x10002000 + 2 * (0x80003010−0x80003000)/16 =



0x10002002



wu_type =*(0x10002002) = 0x5b = opcode for S (subtract)










Step 2—Look-Up wu_Handler:

Use the wu_type of 0x5b to index into a table of function pointers. In this case the returned wu_handler routine would be the <subtract_handler>. The wu_size will also be derived from the wu_type. In this case wu_size=4.
Step 3—Populate Execution Cell:
Preferably, the common_handler performs the four following patching actions for patching a corresponding Host EC in step 3 (referring to FIG. 11):
1) The execution cell “ec—2” branch to the common_handler may be changed to a call to the subtract_handler “ec—2”.
2) As this wu_size>min_wu_size the ((wu_size−min_wu_size)/min_wu_size) execution cells after the current one must be patched with a call to an interrupt handler so that branches into the middle of a guest instruction are dealt with correctly (e.g. 4>2 so patch the (4−2)/2=1 execution cells)
3. The b #ec—3 in ec—2 may be updated to point to the execution handler for the next guest instruction.
4. The ec_address argument setup (move regB, #ec—2) may be replaced with a no-op (no operation) instruction as the ec_address may be not required by the wu_handler.
Step 4—Invoke Execution Cell:
Referring to FIG. 12, after the execution cell is fully patched, then the common_handler may invoke the newly patched instructions in “ec—2”:















void common_handler (void *context, void *ec_address)



{



// STEP1, STEP2, STEP3



goto *(ec_address+4); // at (FIG. 12 ec_2) (skip over 4_byte nop)



}










Then after the subtract handler is called in ec—2 the previously patched in branch in “ec—2” to “ec—4” will be taken and the common_handler called for the MVC guest instruction:















void common_handler (void *context, void *ec_address



{



// STEP1, STEP2, STEP3, STEP4



}










Referring to FIG. 13, the native memory execution cells 702 are shown after the example cells have been populated.
Other configurations of configured execution cells are possible within the scope of the invention. For example, execution cells may be padded with no-op instructions rather than branch instructions in order to perform better in machine pipelines when the next sequential guest instruction may be the next sequential Host execution cell.
Guest Branch Instructions:
Guest branch instructions may be handled differently than non-branching Guest instructions.
Referring to FIG. 14 in an embodiment, instead of encoding a call to a branch_handler in order to emulate the guest branch instruction, a native (Host) branch instruction branching directly to the destination Host execution cell (corresponding to the target guest instruction of the guest branch instruction) will be encoded in the execution cell.
As in the earlier example, as each new guest page is encountered then the corresponding execution cells on the newly allocated native page are initialized with a generic native instruction sequence to branch to a common handler routine as shown in FIG. 15.
Referring to FIG. 16, in an embodiment, execution of an initialized, but unpopulated, cell will result in a call to the common_handler routine. In an embodiment, the common-handler routine performs the following steps in order to populate an execution cell for a guest branch instruction.
1. Map the input ec_address to the corresponding guest work unit address, wu_address, and use this address to decode the instruction type into wu_type (here wu_type may be some type of branch)
2. Obtain the target execution cell address from the current guest target address and the delta (offset into the page) from the current branch instruction text (here the delta may be +200 half words=+400 bytes). In an embodiment, a page size is 4096 bytes (4 Kb).















ec_address = wu_to_ec_page_table(page(wu)) +



 size_of_cell * ((wu − page(wu)) / min_wu_size);










In this example the ec_target_address of ec_y—154 for the SR guest instruction of the z/Architecture from IBM may be found:












target_wu = page_3+4002 + 400 = page_4+306 = 0x10004132


page(target_wu) = page_4 = 0x10004000


min_wu_size = 2 bytes


size_of_cell = 4 byte/instruction * 4 instructions = 16 bytes


wu_to_ec_page_table(page(target_wu)) = 0x80009000


ec_target_address = ec_y_154 = 0x80009000 + 16 * (0x10004132−


 0x10004000)/2 = 0x80009990









3. Replace the ‘b common_handler’ instruction with a ‘b #ec_y—154’ native instruction
4. Replace the ‘b #ec_y—155’ with a no-op instruction as this branch may be now unreachable.
5. Replace both argument setup instructions with no-op instructions as no handler may be being called now.
As this wu_size>min_wu_size the ((wu_size−min_wu_size)/min_wu_size), execution cells after the current one may be patched with a call to an interrupt handler so that branches into the middle of a guest instruction are dealt with correctly. (e.g. 4>2 so patch the (4−2)/2=1 execution cells).
Execution Cell Patching:
Referring to FIG. 17. after the execution cell is fully patched then the common_handler may branch directly to the execution cell target address. Subsequent executions of the patched execution cell will reach the target cell by executing the patched in branch instruction.
There are several possible ways to patch the execution cell so that it will emulate the corresponding guest work unit.
In one embodiment (FIG. 18) the execution cell may be patched with a call to a handler that may be specific to a particular type work unit.
In this embodiment the only parameter required by the handler routine may be a pointer to a data structure that contains all of the required context for the CPU being emulated. Among other data, this context includes the current guest work unit address and all parts of the emulated CPU facilities such as the registers.
This embodiment has the advantage that it may be simple to implement across a range of instruction types as each type may be dealt with in the same way when patching the execution cell. Another advantage may be that a minimal amount of space may be required in each execution cell for the native instruction sequence. A possible disadvantage may be that the instruction must be decoded on every subsequent execution of the cell in the handler routine.
A second embodiment (FIG. 19) further specializes the execution cell by decoding, or ‘cracking’ the instruction text when populating the execution cell and then encoding native instructions to pass the decoded parts, such as the register numbers, to the handler routine. This removes the overhead of cracking the instruction text on subsequent executions of the patched cell. The cost may be that more space may be required in each execution cell to provide space for encoding the extra argument passing instructions.
In a third embodiment (FIG. 20), the execution cell may be specialized even more so that the call to the specific handler may be removed completely and the execution cell may be patched with the complete native instruction sequence required to emulate the corresponding work unit. To avoid excessive growth in the size of the execution cells this technique would only be applied to simpler work unit types that do not require too many native instructions to emulate. In this example, when populating the execution cell the guest instruction may be cracked and the offsets of the context pointer for the emulated registers and the wu_address are determined. With this information the exact native instruction sequence that will emulate the corresponding work unit can be encoded in the execution cell.
In a preferred embodiment (FIG. 22) responsive to paging a first Guest page 2201 comprising Guest instructions into a first Guest memory location of Host processor memory, a first Host page 2202 is created in a first Host memory location of Host processor memory, wherein the first Guest page consists of n Guest cell positions, wherein each Guest cell is x bytes, wherein the first Host page consists of n Host cell positions, wherein each Host cell is y bytes, wherein each on said n cell positions is positionally associated with a corresponding one of said n Host cell positions. Then, Host cells are Loaded 2203 with semantic routines for emulating execution of corresponding Guest instructions of first Guest cells, wherein each first Guest cell comprises at least an opcode portion (beginning portion that is intended to be addressable by a program counter) of a Guest instruction. The Guest program counter is used to identify Guest instructions at first Guest cells for execution. Preferably, the Guest program counter is also used to locate the Host cell corresponding to the first Guest cell. The low order portion of the program counter is shifted to index into the Host page to locate the Host Cell. The amount of shift is dependent on how large the Host Cell is. In an example, the Host cell is 32 bytes, while the corresponding Guest Cell is 2 bytes, therefore, the low order bits of the program counter are shifted left 4 bits to locate the Host cell.
In an embodiment (FIG. 23), when a host cell is created 2202, each Host cell associated with each of said first Guest cell of each Guest instruction is loaded 2301 with an initialization routine, wherein the initialization routine, when executed by the Host processor patches the initialized Host cell with a semantic routine for executing the corresponding Guest instruction of the first Guest cell. Responsive to an emulation program of the Host processor selecting a first Guest instruction for emulation, if the Host cell contains the initialization routine 2303 the emulation program causes the initialization routine of the Host cell corresponding to the first Guest cell of the first Guest instruction to be executed 2302, wherein the execution of the initialization routine of the Host cell causes the initialization routine of the Host cell to be over-written 2304 with a semantic routine for emulating the first Guest instruction. However if 2303 the Host cell semantic routine, the semantic routine is executed. Once the semantic routine has been executed, the emulator causes the next Guest instruction to be emulated.
Interaction Between a Spatially Aware Interpreter and a Trace-Based JIT Compiler:
Referring to FIGS. 24-25, execution of Guest instructions is monitored 2501 by a monitoring routine. A short series of Guest instructions as determined 2501 by a predetermined criteria 2401 is determined 2502 as being candidates to be emulated as a group. The candidate instructions may be determined 2502 to be likely to be repeated according to the predetermined criteria 2401, such as instructions executed in a loop for some portion of a runtime for example, or a short series of Guest instructions that are equivalent to a predetermined group of Guest instructions, or a short series of Guest instructions that are monitored and determined to be executed more frequently than a predetermined threshold number of times. The selected guest instructions will be translated (JIT compiled) to Host instructions for emulating the selected Guest instructions as a group rather than individually. Preferably the Host instructions are executed and then patched to a corresponding initialized Host cell for example by the methods previously described and shown in FIGS. 22 and 23.
To further improve performance of frequently executed instructions it is advantageous to compile such instructions as a group so that more aggressive optimization techniques, such as those found in trace based JIT compilers, can be used.
To enable the spatially aware interpreter to interact with a trace based JIT compiler then a separate table (perhaps a hash implementation indexed by guest instruction address) may be maintained. This table (not shown) will map guest instructions to a count value that indicates the frequency of execution or ‘hotness’ of a particular instruction. A higher count value will indicate the instruction may be more frequently used than a lower count value.
In an embodiment, the type of instruction entry in this table may be a target of a backwards branch. In many cases this branch target instruction starts a loop and is therefore an ideal place to start ‘recording’ instructions for a group compile.
The extra processing to enable this spatially aware interpreter to trace based JIT compiler interaction is preferably implemented as part of the work handler routines for branch type instructions.
In one example embodiment, on encountering a backwards branch the branch handler examines the target address and then takes one of two actions:
1. If the target address is already present in the table then a corresponding count value is incremented.
2. If the target address in not present in the table then an entry is created and its count value is initialized to a start value, likely zero or one.
Next, the count value is compared against some predetermined compilation threshold. When this threshold is reached then a ‘recording’ phase is initiated to begin collecting instructions. This recording phase starts at the current target address in hand and proceeds concurrent with program execution to collect instructions to enable a more aggressive group compile by a trace-based JIT compiler.
There are several ways this recording phase could be implemented as part of a spatially aware interpreter. In one embodiment, Each work unit handler contains the recording logic necessary to add its underlying guest instruction to the instruction list for compilation. This recording logic will only be executed after checking an emulator status bit set when the hotness threshold has been reached.
In another embodiment, if any of the work units have been in-lined then the embodiment reverts to a special interpreter recording loop for the duration of the recording phase. This has the advantage of not slowing down execution when no recording is necessary.
By carefully choosing the hotness thresholds the amount of recording can be minimized.
There are several possible conditions to determine when to terminate recording. In embodiments, the recording is terminated:
1. When a backwards branch is reached or
2. When the start of an already compiled trace is found
After recording finishes the collected instructions are sent as a group to a trace-based JIT compiler to be optimized and compiled as a whole.
After compilation the table of addresses is updated to indicate that the target address has a corresponding compiled trace, via an ‘is_Compiled’ bit for example, and a corresponding host memory address is set to indicate the start native address for the compiled host instructions.
The branch handlers' actions may further include the steps of:
1a. If the target address is already present in the table then examine its corresponding ‘is_Compiled’ bit:
1b. if is_Compiled is set to a true value then retrieve the corresponding host address for the start of the compiled instructions and dispatch to this location
1c. if is_Compiled is set to a false value then increment the corresponding count value
1d. If the target address in not present in the table then an entry is created and its count value is initialized to a start value, likely zero or one.
When the compiled trace reaches the end of its execution it dispatches to a common routine to compute the next Host execution cell address.
In certain cases, such as when the return address is on the same page as the entry address, the compiled trace can dispatch directly to a next Host execution cell.
Additionally, this method of interaction would also allow all of the branch handler lookup/retrieve actions to be performed on a trace exit point.
Therefore another trace recording and compile phase or a counter increment could be initiated at trace exit points and, when a compiled trace is available for the next address, then the trace exit point could be patched up to branch directly the next compiled trace. In effect, multiple compiled traces will be ‘linked’ together and the most frequently executed portions of the program will execute entirely in the compiled traces.
In an embodiment, referring to FIG. 27, 5 Guest memory instructions 2701 are to be emulated by semantic routines associated with corresponding Host (Native) memory cells 2702. A threshold is reached where it is determined that a subgroup of 3 of the 5 Guest memory instructions have looped for a threshold period of time. A JIT group compiled routine of Host (Native) instructions is created and executed to emulate the corresponding subgroup of Guest instructions. In the embodiment, assume an initial branch count value in R5 of 1000 and a compile threshold of 50. The first emulated execution of the guest instructions A,B,C and D in 2701 results in the initialized execution cells 2711, 2712, 2713, 2714 and 2715 and the initialized and populated execution cells 2711, 2712, 2713 and 2714 in 2702.
The first execution of the bct_handler routine in Host execution cell 2714 detects a backwards branch to the guest address of A.
The bct_handler routine then looks up the guest address of instruction A in the table of instruction addresses and initializes a new entry with a count of 1, is_Compiled=false and a host instruction address to some null value.
Execution continues through the populated execution cells and the entry for instruction A in the table of addresses is decremented each time the bct_handler routine is reached.
After 50 executions of the bct_handler then the count for instruction A reaches the compile threshold and a recording phase is initiated starting at instruction A.
Guest instructions A,B,C and D are added to the group of instructions to be compiled and recording is terminated when the backwards branch at instruction D is reached.
This recorded logical group of instructions, the trace A→B→C→D, is then sent to the trace-based JIT compiler. When compilation of the logical group (trace) is complete then the start address of the compiled instructions, indicated by point 2706 in 2703, is set in the table of addresses for instruction A and the corresponding isCompiled is set to true.
Assume, for this example, that execution continues concurrently with trace based compilation and that this compilation takes the time equivalent to 50 loops of A,B,C,D. Then, on the 101st execution of the bct_handler the lookup for instruction A returns is_Compiled=true and the bct_handler dispatches to point 2716.
Execution continues in the compiled trace for the remaining 899 iterations of the loop A,B,C,D. After the compiled trace exits at point 2717 then, assuming that the guest address on exit (for instruction E) is on the same page as the entry point (instruction A), then the next Host execution cell, point 2715, is looked up and invoked.
Execution now continues as normal in the populated execution cells.
Referring to FIG. 24, the emulation software dynamically 2402 selects a first plurality of Guest instructions based on predetermined criteria 2401. The selected sequence begins with a first Guest instruction and ends with a second Guest instruction. Until the first Guest instruction is dynamically selected, the first Host cell corresponding to the first Guest instruction contains an emulation routine for emulating only the first Guest instruction 2403. When the first Guest instruction is selected 2402, a translator or JIT compiler compiles the selected Guest instructions into first Host instructions 2404. These first Host instructions are patched 2405 into the first Host cell corresponding to the beginning of the Guest instruction. When the Guest program counter indicates 2406 the first Guest instruction is to be executed 2406, the Host instructions of the first Host cell are executed 2407. If the first Host cell Host instructions emulate a plurality of Guest instructions, the emulator updates the program counter 2406 after the plurality of Guest instructions have been executed to point to a Guest instruction following the second Guest instruction. In an embodiment, the Host cell of that instruction is executed executed directly, without intervention by the emulation software.
In an embodiment FIG. 26, dynamically selecting the first plurality of Guest instructions 2402 goes to a table lookup mechanism 2404 that searches 2601 a table for predetermined Guest instruction sequences. The search criteria can be the complete sequence of Guest instructions or an electronic signature formed from the Guest instructions. If a table entry table entry 2605 is found in the table, the corresponding first Host instructions 2606 are the compiled first plurality of first Host instructions 2604. If 2603 no Guest instruction sequences are found the Host cell is not changed. Either way, the resulting Host cell 2407 is executed.
The forgoing may be useful in understanding the terminology and structure of one computer system embodiment. The present invention may be not limited to the z/Architecture or to the description provided thereof. The present invention can be advantageously applied to other computer architectures of other computer manufacturers with the teaching herein.
While the preferred embodiment of the invention has been illustrated and described herein, it may be to be understood that the invention may be not limited to the precise construction herein disclosed, and the right may be reserved to all changes and modifications coming within the scope of the invention as defined in the appended claims.

## Claims

1. A computer program product for emulating execution of a group of Guest instructions of a Guest processor architecture with a single semantic routine of Host instructions of a Host processor architecture, the computer program product comprising non-transitory storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising:
monitoring execution of Guest instructions;
determining that a group of the monitored Guest instructions have been executed, as a group, a predefined number of times;
responsive to the predefined number of times meeting a threshold value criteria, dynamically selecting the group of Guest instructions, the selected group of Guest instructions beginning with a first Guest instruction and ending with a second Guest instruction;
Just-In-Time (JIT) compiling the selected group of Guest instructions into a plurality of Host instructions;
designating a plurality of Host cells in Host memory, each designated Host cell corresponding to a separate respective single Guest cell of Guest memory, each Host cell addressable based on a Guest program counter value, wherein each Guest cell has a size that is equal to a predetermined Guest cell size, each Host cell has a size that is equal to a predetermined Host cell size, and wherein the predetermined Host cell size is greater than the predetermined Guest cell size;
patching a first Host cell of the plurality of Host cells with Host instructions for executing the plurality of Host instructions, the first Host cell corresponding to a first Guest cell of Guest memory, the first Guest cell corresponding to a beginning portion of said first Guest instruction;
responsive to the Guest program counter value indicating the first Guest cell corresponding to the first Guest instruction is to be executed, executing the plurality of Host instructions of the patched first Host cell corresponding to the first Guest cell;
responsive to the Guest program counter value indicating a second Guest cell corresponding to another Guest instruction is to be executed, executing one or more second Host instructions of a second Host cell corresponding to the second Guest cell to emulate execution of the second Guest instruction, the second Guest cell corresponding to a beginning portion of said second Guest instruction;
wherein each of the plurality of Host cells corresponds to a respective lock bit in a bit significant table, and further comprising:
before accessing a particular Host cell by the Host processor, determining whether the lock bit corresponding to the particular Host cell in the bit significant table is set to locked;
based on the lock bit corresponding to the particular Host cell in the bit significant table being set to locked, waiting by the Host processor until the lock bit corresponding to the particular Host cell is set to unlocked; and
based on the lock bit corresponding to the particular Host cell in the bit significant table being set to unlocked, accessing the particular Host cell by the Host processor.

2. The computer program product according to claim 1, wherein the patching the first Host cell further comprises:
saving the plurality of Host instructions at a Host memory location, wherein Host instructions for executing the plurality of Host instructions of the patched first Host cell comprise a Host branch instruction for branching to the Host memory location having the first Host instructions.

3. The computer program product according to claim 2, the method further comprising encountering a branch Guest instruction, the branch Guest instruction, responsive to being executed, branching back to a target Guest instruction, wherein the target Guest instruction is the first Guest instruction, wherein the branch Guest instruction is the second Guest instruction.

4. The computer program product according to claim 2, the method further comprising:
based on encountering another group of the monitored Guest instructions that is equivalent to a predetermined sequence of instructions, JIT compiling the another group of the monitored Guest instructions into the plurality of Host instructions.

5. The computer program product according to claim 1, further comprising:
prior to the dynamically selecting the group of Guest instructions, populating said first Host cell with one or more second Host instructions for emulating the first Guest instruction corresponding to the first Guest cell; and
prior to the dynamically selecting the group of Guest instructions, populating said second Host cell with the one or more second Host instructions for emulating the second Guest instruction corresponding to the second Guest cell.

6. The computer program product according to claim 1, further comprising:
populating third Host cells with one or more third Host instructions for performing a wild branch operation according to the Guest processor architecture, each third Host cell corresponding to any one of a portion other than a beginning portion of said a corresponding Guest instruction, or a Data portion of Guest memory; and
responsive to the Guest program counter indicating a third Host cell, executing the one or more third Host instructions.

7. The computer program product of claim 1, the method further comprising:
mapping the monitored Guest instructions to a plurality of count values, each count value indicating a frequency of execution of a corresponding Guest instruction;
responsive to a determination that one of the monitored Guest instructions has a corresponding count value that meets a predetermined threshold, recording the monitored Guest instructions starting a current target address;
terminating the recording in response to reaching a backwards branch or in response to finding a start of an already compiled trace; and
sending the recorded Guest instructions for use during the JIT compiling.

8. A computer system for emulating execution of a group of Guest instructions of a Guest processor architecture with a single semantic routine of Host instructions of a Host processor architecture comprising:
a memory; and
a processor in communication with the memory, the computer system capable of performing a method comprising:
monitoring execution of Guest instructions;
determining that a group of the monitored Guest instructions have been executed as a group a predefined number of times;
responsive to the predefined number of times meeting a threshold value criteria, dynamically selecting the group of Guest instructions, the selected group of Guest instructions beginning with a starting Guest instruction and ending with an ending Guest instruction;
Just-In-Time (JIT) compiling the selected group of Guest instructions into a plurality of Host instructions;
designating a plurality of Host cells in Host memory, each designated Host cell corresponding to a separate respective single Guest cell of Guest memory, each Host cell addressable based on a Guest program counter value, wherein each Guest cell has a size that is equal to a predetermined Guest cell size, each Host cell has a size that is equal to a predetermined Host cell size, and wherein the predetermined Host cell size is greater than the predetermined Guest cell size;
patching a first Host cell of the plurality of Host cells with Host instructions for executing the plurality of Host instructions, the first Host cell corresponding to a first Guest cell of Guest memory, the first Guest cell corresponding to a beginning portion of said starting Guest instruction;
responsive to the Guest program counter value indicating the first Guest cell corresponding to the starting Guest instruction is to be executed, executing the plurality of Host instructions of the patched first Host cell corresponding to the first Guest cell;
responsive to the Guest program counter value indicating a second Guest cell corresponding to another Guest instruction is to be executed, executing one or more second Host instructions of a second Host cell corresponding to the second Guest cell to emulate execution of the ending Guest instruction, the second Guest cell corresponding to a beginning portion of said ending Guest instruction;
wherein each of the plurality of Host cells corresponds to a respective lock bit in a bit significant table, and further comprising:
before accessing a particular Host cell by the Host processor, determining whether the lock bit corresponding to the particular Host cell in the bit significant table is set to locked;
based on the lock bit corresponding to the particular Host cell in the bit significant table being set to locked, waiting by the Host processor until the lock bit corresponding to the particular Host cell is set to unlocked; and
based on the lock bit corresponding to the particular Host cell in the bit significant table being set to unlocked, accessing the particular Host cell by the Host processor.

9. The computer system according to claim 8, wherein the patching the first Host cell further comprises:
saving the plurality of Host instructions at a Host memory location, wherein Host instructions for executing the plurality of Host instructions of the patched first Host cell comprise a Host branch instruction for branching to the Host memory location having the starting Host instructions.

10. The computer system according to claim 9, the method further comprising encountering a branch Guest instruction, the branch Guest instruction, responsive to being executed, branching back to a target Guest instruction, wherein the target Guest instruction is the starting Guest instruction, wherein the branch Guest instruction is the ending Guest instruction.

11. The computer system according to claim 9, further comprising:
prior to the JIT compiling the group of Guest instructions, populating said first Host cell with one or more second Host instructions for emulating the starting Guest instruction corresponding to the first Guest cell; and
prior to the JIT compiling the group of Guest instructions, populating said second Host cell with the one or more second Host instructions for emulating the ending Guest instruction corresponding to the second Guest cell;
populating third Host cells with one or more third Host instructions for performing a wild branch operation according to the Guest processor architecture, each third Host cell corresponding to any one of a portion other than a beginning portion of said a corresponding Guest instruction, or a Data portion of Guest memory; and
responsive to the Guest program counter indicating a third Host cell, executing the one or more third Host instructions.

12. The computer system of claim 8, the method further comprising:
mapping the monitored Guest instructions to a plurality of count values, each count value indicating a frequency of execution of a corresponding Guest instruction;
responsive to a determination that one of the monitored Guest instructions has a corresponding count value that meets a predetermined threshold, recording the monitored Guest instructions starting a current target address;
terminating the recording in response to reaching a backwards branch or in response to finding a start of an already compiled trace; and
sending the recorded Guest instructions for use during the JIT compiling.

13. The method according to claim 12, wherein the patching the first Host cell further comprises:
saving the plurality of Host instructions at a Host memory location, wherein Host instructions for executing the plurality of Host instructions of the patched first Host cell comprise a Host branch instruction for branching to the Host memory location having the first Host instructions.

14. The method according to claim 13,
the method further comprising encountering a branch guest instruction, the branch Guest instruction, responsive to being executed, branching back to a target Guest instruction, wherein the target Guest instruction is the starting Guest instruction, wherein the branch Guest instruction is the ending Guest instruction.

15. The method according to claim 13, further comprising:
prior to the dynamically selecting the first plurality of Guest instructions, populating said first Host cell with one or more second Host instructions for emulating the first Guest instruction corresponding to the first Guest cell; and
prior to the dynamically selecting the first plurality of Guest instructions, populating said second Host cell with the one or more second Host instructions for emulating the second Guest instruction corresponding to the second Guest cell.

16. The method according to claim 15, further comprising:
populating third Host cells with one or more third Host instructions for performing a wild branch operation according to the Guest processor architecture, each third Host cell corresponding to any one of a portion other than a beginning portion of said a corresponding Guest instruction, or a Data portion of Guest memory; and
responsive to the Guest program counter indicating a third Host cell, executing the one or more third Host instructions.

17. A computer implemented method for emulating execution of a group of Guest instructions of a Guest processor architecture with a single semantic routine of Host instructions of a Host processor architecture, the method comprising:
monitoring execution of Guest instructions;
determining that a group of the monitored Guest instructions have been executed as a group a predefined number of times;
responsive to the predefined number of times meeting a threshold value criteria, dynamically selecting the group of Guest instructions, the selected group of Guest instructions beginning with a starting Guest instruction and ending with an ending Guest instruction;
Just-In-Time (JIT) compiling the selected group of Guest instructions into a plurality of Host instructions;
designating a plurality of Host cells in Host memory, each designated Host cell corresponding to a respective single Guest cell of Guest memory, each Host cell addressable based on a Guest program counter value, wherein each Guest cell has a size that is equal to a predetermined Guest cell size, each Host cell has a size that is equal to a predetermined Host cell size, and wherein the predetermined Host cell size is greater than the predetermined Guest cell size;
patching a first Host cell of the plurality of Host cells with Host instructions for executing the plurality of Host instructions, the first Host cell corresponding to a first Guest cell of guest memory, the first Guest cell corresponding to a beginning portion of a starting Guest instruction in the series of Guest instructions;
responsive to the Guest program counter value indicating the first Guest cell corresponding to the beginning portion of the starting Guest instruction is to be executed, executing the plurality of Host instructions of the patched first Host cell corresponding to the first Guest cell;
responsive to the Guest program counter value indicating a second Guest cell corresponding to a third Guest instruction is to be executed, executing one or more second Host instructions of a second Host cell corresponding to the second Guest cell to emulate execution of an ending Guest instruction in the series of Guest instructions, the second Guest cell corresponding to a beginning portion of the ending Guest instruction;
wherein each of the plurality of Host cells corresponds to a respective lock bit in a bit significant table, and further comprising:
before accessing a particular Host cell by the Host processor, determining whether the lock bit corresponding to the particular Host cell in the bit significant table is set to locked;
based on the lock bit corresponding to the particular Host cell in the bit significant table being set to locked, waiting by the Host processor until the lock bit corresponding to the particular Host cell is set to unlocked; and
based on the lock bit corresponding to the particular Host cell in the bit significant table being set to unlocked, accessing the particular Host cell by the Host processor.

18. The computer-implemented method of claim 17, the method further comprising:
mapping the monitored Guest instructions to a plurality of count values, each count value indicating a frequency of execution of a corresponding Guest instruction;
responsive to a determination that one of the monitored Guest instructions has a corresponding count value that meets a predetermined threshold, recording the monitored Guest instructions starting a current target address;
terminating the recording in response to reaching a backwards branch or in response to finding a start of an already compiled trace; and
sending the recorded Guest instructions for use during the JIT compiling.