# Patent

## Title
Compiler optimizations for vector instructions

## Abstract

An optimizing compiler includes a vector optimization mechanism that optimizes vector instructions by eliminating one or more vector element reverse operations. The compiler can generate code that includes multiple vector element reverse operations that are inserted by the compiler to account for a mismatch between the endian bias of the instruction and the endian preference indicated by the programmer or programming environment. The compiler then analyzes the code and reduces the number of vector element reverse operations to improve the run-time performance of the code.

## Background
RELATED APPLICATIONS
This patent application is a non-provisional of provisional application U.S. Ser. No. 62/036,741 filed Aug. 13, 2014 entitled “OPTIMIZING VECTOR ACCESSES ON AN ENDIAN-BASED MULTI-ENDIAN INSTRUCTION SET ARCHITECTURE”, and is also related to U.S. Ser. No. 14/576,391 filed on Dec. 19,2014 entitled “COMPILER METHOD FOR GENERATING INSTRUCTIONS FOR VECTOR OPERATIONS ON A MULTI-ENDIAN PROCESSOR” and is also related to U.S. Ser. No. 14/576,710 filed on Dec. 19,2014 entitled “COMPILER METHOD FOR GENERATING INSTRUCTIONS FOR VECTOR OPERATIONS IN A MULTI-ENDIAN INSTRUCTION SET”, all of which are incorporated herein by reference.
BACKGROUND
1. Technical Field
This disclosure generally relates to computer systems, and more specifically relates to compilers that generate executable code for computer systems.
2. Background Art
Computer systems have evolved into extremely sophisticated devices, and computer systems may be found in many different settings. Dramatic advances in both hardware and software (e.g., computer programs) have drastically improved the performance of computer systems. Modern software has become very complex when compared to early computer programs. Many modern computer programs have from tens of thousands to many millions of instructions. The execution time (and hence, performance) of a computer program is very closely related to the number and complexity of instructions that are executed as the computer program runs. Thus, as the size and complexity of computer programs increase, the execution time of the computer program increases as well.
Unlike early computer programs, modern computer programs are typically written in a high-level language that is easy to understand by a human programmer. Special software tools known as compilers take the human-readable form of a computer program, known as “source code”, and convert it into “machine code” or “object code” instructions that may be executed by a computer system. Because a compiler generates the stream of machine code instructions that are eventually executed on a computer system, the manner in which the compiler converts the source code to object code affects the execution time of the computer program.
The execution time of a computer program, especially complex computer programs, is a function of the number, arrangement and type of instructions within the computer program. Many different optimizations have been developed so the code produced by compilers has better run-time performance.
BRIEF SUMMARY
An optimizing compiler includes a vector optimization mechanism that optimizes vector instructions by eliminating one or more vector element reverse operations. The compiler can generate code that includes multiple vector element reverse operations that are inserted by the compiler to account for a mismatch between the endian bias of the instruction and the endian preference indicated by the programmer or programming environment. The compiler then analyzes the code and reduces the number of vector element reverse operations to improve the run-time performance of the code.
The foregoing and other features and advantages will be apparent from the following more particular description, as illustrated in the accompanying drawings.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
The disclosure will be described in conjunction with the appended drawings, where like designations denote like elements, and:
FIG. 1 is a block diagram of an apparatus that includes a vector optimization mechanism in a compiler;
FIG. 2 is a block diagram illustrating how a byte array is stored in little endian and big endian systems;
FIG. 3 is a block diagram illustrating how a word array is stored in little endian and big endian systems;
FIG. 4 is a flow diagram of a method for defining vector optimization rules for a compiler to optimize vector instructions;
FIG. 5 is a flow diagram of a method for a compiler to optimize vector instructions according to defined vector instruction processing rules;
FIG. 6 is a diagram of a table that shows an example of the vector instruction processing rules shown in FIG. 1;
FIG. 7 is a diagram showing a sample snippet of code processed by a compiler;
FIG. 8 is a diagram showing instructions generated by the compiler as a result of processing the instructions in FIG. 7;
FIG. 9 is a block diagram illustrating a first possible compiler optimization for the instructions in FIG. 8;
FIG. 10 is a block diagram illustrating a second possible compiler optimization for the instructions in FIG. 9;
FIG. 11 shows the table 600 in FIG. 6 with an added vector optimization rule;
FIG. 12 is a diagram showing a sample snippet of code processed by a compiler;
FIG. 13 is a diagram showing instructions generated by the compiler as a result of processing the instructions in FIG. 12;
FIG. 14 is a block diagram illustrating a compiler optimization for the instructions in FIG. 13;
FIG. 15 shows the table 600 in FIGS. 6 and 11 with localized vector optimization rules;
FIG. 16 is a diagram showing a sample snippet of code processed by a compiler;
FIG. 17 is a diagram showing instructions generated by the compiler as a result of processing the instructions in FIG. 16;
FIG. 18 is a block diagram showing a sample snippet of code processed by a compiler;
FIG. 19 is a diagram showing instructions generated by the compiler as a result of processing the instructions in FIG. 18;
FIG. 20 is a block diagram showing endian-biased instructions with vector element reverse operations;
FIG. 21 is a diagram showing conversion of the multiple instructions in FIG. 20 to a single instruction;
FIG. 22 is a block diagram showing a sample snippet of code processed by a compiler;
FIG. 23 is a diagram showing instructions generated by the compiler as a result of processing the instructions in FIG. 22; and
FIG. 24 is a flow diagram of a specific method for the optimizing compiler shown in FIG. 1 to perform vector optimizations based on vector optimization rules.



DETAILED DESCRIPTION
Exploiting opportunities for parallel computation is very important for modern computing systems. One of the most common opportunities for parallel computation arises when the same operation is to be performed on an array (or “vector”) of homogeneous data elements. Today's processor instruction set architectures (ISAs) usually include a set of single-instruction, multiple-data (SIMD) instructions that can operate on 2, 4, 8, 16, or 32 values simultaneously. SIMD instructions are examples of what are more broadly termed vector instructions as used herein. For example, the Power ISA currently defines the Vector Add Floating-Point (vaddfp) instruction. This instruction operates on 128-bit vector registers, whose contents are interpreted as four 32-bit floating-point values. The corresponding values in each input register are added together and placed in the corresponding position in the output register. Thus four additions are performed using a single instruction.
Another aspect of today's processors is the concept of “endianness.” This refers to how the processor stores bytes of a multi-byte value in memory. For example, a 64-bit integer in a machine register contains 8 bytes, arranged from most-significant byte (MSB) containing the bits representing the largest portions of the integer, to the least-significant byte (LSB) containing the bits representing the smallest portions of the integer. On a so-called Big Endian (BE) architecture, the same value is stored in memory with byte 0 containing the MSB, and byte 7 containing the LSB. On a so-called Little Endian (LE) architecture, the value is stored in memory with byte 0 containing the LSB, and byte 7 containing the MSB. These two methods arose for historical performance reasons that no longer matter a great deal, except that in today's world we must deal with these two incompatible views of memory.
BE and LE systems typically view values differently in vector registers as well. When an array of four 32-bit values is loaded into a 128-bit BE vector register, the zeroth element of the array occupies the most significant bytes, while the third element of the array (zero-based indexing) occupies the least significant bytes. When the same array is loaded into a 128-bit LE vector register, the zeroth element of the array occupies the least significant bytes, while the third element of the array occupies the most significant bytes. These are considered to be the “natural element order” for BE and LE memory models. The contents of each 4-byte element are represented in the same fashion on both BE and LE architectures, with the sign bit of the floating-point value placed in the most significant bit of the element.
Some ISAs (Power and ARM, for example) are designed to operate either in Big Endian mode or in Little Endian mode. In the Power ISA, the endian mode is selected with a control bit in the machine state register (MSR). Thus the same instructions are available to carry out computation regardless of endianness. This is of no concern for instructions such as “vaddfp” described above, where the computation is performed uniformly on all elements of the instruction's input and output registers. However, when an instruction implicitly or explicitly refers to the element numbering within a vector register, the numbering that is natural for one endianness is unnatural for the other.
In some cases, an ISA may provide instructions to facilitate maintaining elements in vectors using a particular element order, regardless of the endian mode specified by the programmer. For example, the Load VSX Vector Doubleword*2 Indexed (lxvd2x) instruction in the Power ISA specifically loads elements into a vector register using the BE natural element order, whether or not the machine is using the BE memory model or the LE memory model. Similarly, the Store VSX Vector Doubleword*2 Indexed (stxvd2x) reverses this transformation, storing to memory as though the elements in the vector register use the BE natural element order. Using these instructions allows a programmer to ignore the actual endian memory model in use, for a subset of data types and instructions. Data within each element are formatted in memory based on the selected endian model, e.g., in accordance with the mode selected by MSR[LE].
An instruction that regards vector elements in vector registers using BE natural element order is said to have a BE vector element endian bias. Conversely, an instruction that regards vector elements in vector registers using LE natural element order is said to have an LE vector element endian bias. When the preponderance of vector instructions in an ISA have the same endian bias, this is referred to as the inherent endian bias of the ISA.
FIGS. 2 and 3 show examples of how data arrays may be represented in memory, using big-endian and little-endian data representation, and how they may be represented in vector registers, using l→r and r→l element ordering. In FIG. 2, an array of bytes containing {0, 1, . . . , 15} is shown. Since each vector element is one byte long, the little-endian (LE) and big-endian (BE) representations in memory are identical, with the first element stored in the lowest address. Using r→l element ordering (LE), the first element is stored in the least significant byte (LSB) of the vector register; while using l→r element ordering (BE), the first element is stored in the most significant byte (MSB).
FIG. 3 shows representations for an array of 4-byte integers, containing {0x00010203, 0x04050607, 0x08090a0b, 0x0c0d0e0f}. In memory, elements appear in ascending address order for both BE and LE, but the byte order within each element differs. In registers, the opposite is true: the elements appear from right to left using r→l order (LE) and from left to right using l→r order (BE), but within each element the byte ordering is identical. Thus, we see the 4-byte integer stored in the least significant four bytes of the LE register are the same order, namely 00, 01, 02, 03 as the 4-byte integer stored in the most significant four bytes of the BE register. The order that changes is the order of elements, not the order of bytes within the elements.
The two related applications cited herein disclose ways for a compiler to insert one or more vector element reverse operations into a computer program to account for a mismatch in endian bias. The addition of vector element reverse operations reduces performance of the code, but assures correct operation. The disclosure herein is directed to a compiler that analyzes the code that includes the added vector element reverse operations and eliminates one or more of the vector element reverse operations to improve performance of the code.
Referring to FIG. 1, a computer system 100 is one suitable implementation of a server computer system that includes a compiler with a vector optimization mechanism as described in more detail below. Server computer system 100 is an IBM POWER8 computer system. However, those skilled in the art will appreciate that the disclosure herein applies equally to any computer system, regardless of whether the computer system is a complicated multi-user computing apparatus, a single user workstation, or an embedded control system. As shown in FIG. 1, computer system 100 comprises one or more processors 110, a main memory 120, a mass storage interface 130, a display interface 140, and a network interface 150. These system components are interconnected through the use of a system bus 160. Mass storage interface 130 is used to connect mass storage devices, such as local mass storage device 155, to computer system 100. One specific type of local mass storage device 155 is a readable and writable CD-RW drive, which may store data to and read data from a CD-RW 195.
Main memory 120 preferably contains data 121, an operating system 122, source code 123, an intermediate representation 124, a compiler 125, and machine code 128. Data 121 represents any data that serves as input to or output from any program in computer system 100. Operating system 122 is a multitasking operating system. There are three different representations of a computer program in FIG. 1, namely the high-level source code 123, the intermediate representation 124 that is generated by a front-end compiler from the source code 123, and the machine code 128 that is generated by a back-end compiler from the intermediate representation 124. The vector optimization mechanism 126 disclosed herein could be implemented within a front-end compiler, within a back-end compiler, or different aspects of the vector optimization mechanism 126 could be implemented in both a front-end compiler and a back-end compiler. The compiler 125 thus could represent either a front-end compiler, a back-end compiler, or a full compiler that includes a front-end and a back-end. The compiler 125 includes the vector optimization mechanism 126 that optimizes vector instructions according to defined vector optimization rules 127.
Note the source code 123, intermediate representation 124, compiler 125, and machine code 128 are all shown residing in memory 120 for the convenience of showing all of these elements in one drawing. One skilled in the art will appreciate that this is not the normal mode of operation for most compilers. A front-end compiler processes source code 123 and generates therefrom intermediate representation 124. This processing may occur on a computer system separate from computer system 100. A back-end compiler processes intermediate representation 124 and generates therefrom machine code 128, which may also occur on a separate computer system. Compiler 125 could be a front-end compiler, or could be a compiler that includes both a front end and a back end. In the extreme, source code 123 could reside on a first computer system and a front-end compiler could reside on a second computer system. The front-end compiler could read the source code 123 from the first computer system, generate the intermediate representation 124, and store the intermediate representation 124 on a third computer system. A back-end compiler could be executed on a fourth computer system, which reads the intermediate representation 124 from the third computer system, and generates therefrom machine code 128, which could be written to a fifth computer system. This simple example shows that the preferred embodiments expressly extend to any suitable configuration and number of computer systems to accomplish the front-end and back-end compiling. The “apparatus” described herein and in the claims expressly extends to a multiple computer configuration, as described by the example above. In addition, the vector optimization mechanism 126 can include different portions implemented in different parts of the compiler.
Computer system 100 utilizes well known virtual addressing mechanisms that allow the programs of computer system 100 to behave as if they only have access to a large, contiguous address space instead of access to multiple, smaller storage entities such as main memory 120 and local mass storage device 155. Therefore, while data 121, operating system 122, source code 123, intermediate representation 124, compiler 125, and machine code 128 are shown to reside in main memory 120, those skilled in the art will recognize that these items are not necessarily all completely contained in main memory 120 at the same time. It should also be noted that the term “memory” is used herein generically to refer to the entire virtual memory of computer system 100, and may include the virtual memory of other computer systems coupled to computer system 100.
Processor 110 may be constructed from one or more microprocessors and/or integrated circuits. Processor 110 executes program instructions stored in main memory 120. Main memory 120 stores programs and data that processor 110 may access. When computer system 100 starts up, processor 110 initially executes the program instructions that make up operating system 122. Processor 110 also executes the compiler 125. Processor 110 preferably includes a vector processing unit (VPU) 112 and multiple vector registers 114. The VPU 112 and vector registers 114 allow the processor to execute Single Instruction Multiple Data (SIMD) instructions, which are examples of vector instructions discussed herein.
Although computer system 100 is shown to contain only a single processor and a single system bus, those skilled in the art will appreciate that a compiler as described herein may be practiced using a computer system that has multiple processors and/or multiple buses. In addition, the interfaces that are used preferably each include separate, fully programmed microprocessors that are used to off-load compute-intensive processing from processor 110. However, those skilled in the art will appreciate that these functions may be performed using I/O adapters as well.
Display interface 140 is used to directly connect one or more displays 165 to computer system 100. These displays 165, which may be non-intelligent (i.e., dumb) terminals or fully programmable workstations, are used to provide system administrators and users the ability to communicate with computer system 100. Note, however, that while display interface 140 is provided to support communication with one or more displays 165, computer system 100 does not necessarily require a display 165, because all needed interaction with users and other processes may occur via network interface 150.
Network interface 150 is used to connect computer system 100 to other computer systems or workstations 175 via network 170. Network interface 150 broadly represents any suitable way to interconnect electronic devices, regardless of whether the network 170 comprises present-day analog and/or digital techniques or via some networking mechanism of the future. Network interface 150 preferably includes a combination of hardware and software that allows communicating on the network 170. Software in the network interface 150 preferably includes a communication manager that manages communication with other computer systems 175 via network 170 using a suitable network protocol. Many different network protocols can be used to implement a network. These protocols are specialized computer programs that allow computers to communicate across a network. TCP/IP (Transmission Control Protocol/Internet Protocol) is an example of a suitable network protocol that may be used by the communication manager within the network interface 150.
The present invention may be a system, a method, and/or a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.
The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.
Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.
Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention.
Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.
These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.
The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.
The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.
A compiler as disclosed and claimed herein includes a vector optimization mechanism that optimizes vector instructions by eliminating one or more vector element reverse operations. The compiler can generate code that includes multiple vector element reverse operations that are inserted by the compiler to account for a mismatch between the endian bias of the instruction and the endian preference indicated by the programmer or programming environment. The compiler then analyzes the code and reduces the number of vector element reverse operations to improve the run-time performance of the code.
Referring to FIG. 4, method 400 defines one or more vector optimization rules for processing an intermediate representation of the code (step 410). The vector optimization rules defined in step 410 correspond to vector optimization rules 127 shown in FIG. 1. A method 500 shown in FIG. 5 analyzes the intermediate representation generated from the source code (step 510). When the intermediate representation has no vector instructions (step 520=NO), method 500 is done. When the intermediate representation has one or more vector instructions (step 520=YES), but the vector instruction(s) do not satisfy the vector optimization rules (step 530=NO), method 500 is done. When the vector instruction(s) satisfy the vector optimization rules (step 530=YES), the vector instruction(s) are optimized according to the vector optimization rules (step 540). The optimization in step 540 can include, for example, eliminating one or more vector element reverse operations. Various examples are now presented to illustrate different optimizations that could be performed in step 540.
An example of a vector optimization rule 127 shown in FIG. 1 is shown in FIG. 6. For all vector copy operations, the unneeded vector element reverse (vreverse) operations in the intermediate representation are eliminated, as shown at 610.
The disclosure and claims herein provide a way for a compiler to mitigate the performance cost of added vector element reverse operations, such as vector element reverse operations added by the compiler after vector load instructions and before vector store instructions. In this disclosure, we refer to any vector load instruction (whether biased-endian like “lxvd2x”, or natural-endian like “lvx”) as a vload, and similarly refer to any vector store instruction as a vstore. We will also refer to any operation that reverses the elements of a vector register as a vreverse, and to an instruction that copies the contents of one register into another as a vcopy. However, the methods described herein are equally applicable regardless of the kinds of loads, stores, and reverse operations employed.
A “vreverse operation” generally refers to a series of one or more instructions that reverses the order of elements in a vector register. There are different vreverse operations for each element size (1 byte, 2 bytes, 4 bytes, 8 bytes, 16 bytes, etc.). An ISA may, but need not, include machine instructions that map directly to vreverse operations of every size. Alternatively, more general instructions (such as permutes or shuffles) may be used instead.
In the most preferred implementation, there are preferably different vector element reverse instructions for each different element size that can be specified in an instruction set. Thus, if a system defines vectors with element sizes of bytes, halfwords (2 bytes), words (4 bytes) and doublewords (8 bytes), there will be a different vector element reverse instruction for each of these. For example, a byte vector element reverse instruction could be vreverse. A halfword vector element reverse instruction could be vreverse_hw. A word vector element reverse instruction could be vreverse w. A double word vector element reverse instruction could be vreverse_dw. Of course, any suitable syntax could be used, and any suitable number of vector element reverse instructions could be defined, depending on the element sizes defined by the instruction set, whether currently known or developed in the future. For example, a quadword vector element reverse instruction could be defined and vector element reverse instructions for elements larger than quadwords could also be defined. For the simplicity of the examples herein, the size of the vector element reverse instruction is not specified, realizing that the size could vary as described above.
The compiler optimizations disclosed herein may be performed during any appropriate stage of the compilation process in order to eliminate one or more vreverse operations in the code. A compiler operates on one or more intermediate representations of code, which may be organized in various ways that may be more or less appropriate to a particular optimization. For example, an intermediate representation may represent expressions in a tree or directed-acyclic graph (DAG) form, or may use a variant of three-address code. Of course, many more variations are possible, whether currently known or developed in the future. This disclosure applies regardless of the chosen intermediate representation and without regard to specific unimportant details of how the optimizations are implemented.
In the simplest case, it is common for a vector to be copied from one memory location to another, such as shown in FIG. 7. Using various methods, including the methods disclosed in the two related application, the compiler could generate for the code in FIG. 7 the instructions shown in FIG. 8, where t1, t2, and t3 are vector registers. The effect of each vreverse is to reverse the order of the elements in the vector register. We assume for this example the vreverse t2=t1 instruction was added by the compiler to reverse the order of the vector elements after the vload t1=a instruction, and the vreverse t3=t2 instruction was added by the compiler to reverse the order of the vector elements before the vstore b=t3 instruction. Thus the first vreverse will reverse the elements, and the second vreverse will restore them to their original locations. If the value of t2 is not used anywhere else, the compiler may replace the instructions shown in FIG. 8 with the instructions shown in FIG. 9. Then, standard compiler optimizations known as copy propagation and/or value numbering can reduce the instructions in FIG. 9 to the instructions shown in FIG. 10. Note that all vreverse operations have been removed, so there is now no performance penalty.
More specifically, an exemplary compiler performing an exemplary translation of the code of FIG. 7 described in conjunction with these rules may generate assembly code corresponding to FIG. 8 for a little-endian environment in accordance with the instructions for POWER8 as follows:
lxvd2x 0,0,4
xxpermdi 0,0,0,2
xxpermdi 0,0,0,2
stxvd2x 0,0,3
In accordance with one exemplary implementation of the present invention, when the optimizations described herein are performed, an exemplary compiler may generate code corresponding to FIG. 10 for a little-endian environment in accordance with the POWER8 instruction set as follows:
lxvd2x 0,0,4
stxvd2x 0,0,3
Note that a code sequence where one vreverse operation feeds another vreverse operation for elements of the same size can arise in other contexts than a vector copy. For example, the optimization rules in rule 620 of FIG. 11 may introduce such a code sequence. At any time such a code sequence is introduced, every use of the result of the second vreverse operation may be replaced with the input to the first vreverse operation, and the second vreverse operation may be removed. If the result of the first vreverse operation is not used elsewhere, it may also be removed.
Recall that a vreverse operation may not be implemented as a single instruction. For example, the Power ISA implemented by the POWER8 processor does not include single instructions corresponding to vreverse operations for elements of size 1, 2, or 4 bytes. Consequently, a vector load of a data type such as vector char, vector short, vector int, or vector float is often implemented using a sequence of a vector load of two doublewords followed by a permute that reverses the elements, and a vector store of such a data type is often implemented using a sequence of a vector store of two doublewords preceded by a permute that reverses the elements.
Suppose a and b in FIG. 7 are of type vector signed int, rather than vector signed long. In accordance with one embodiment, this may be translated into a sequence similar to that of FIG. 8, but due to the lack of a suitable vreverse instruction for vector word elements, the generated sequence would be more expensive:
vload t1=a
vload rev_w=<control vector to reverse word elements>
vperm t2, t1, rev_w
vperm t3, t2, rev_w
vstore t3
As can be seen, this sequence requires an additional load of a vector permute control vector that instructs a permute instruction to reverse the word elements of a vector registers. Furthermore, using the POWER8 instruction set, only a subset of the vector registers may be used to perform a vector permute instruction, which may lead to additional register copy instructions, data spills to memory, and so forth.
In accordance with one optimized embodiment, the vector word copy sequence is translated into an intermediate representation which may load words with an element order of element 1, element 0, element 3, element 2 (denoted hereafter by vload_w1032) and use a suitable big-endian permutation (2,3,0,1) to create the “normal” little-endian order (3,2,1,0), and the converse operations for a vector store. This provides the following initial translation to an intermediate representation:
vload_w1032 t1=a
vperm_w2301 t2=t1
vperm_w2301 t3=t2
vstore_w1032 b=t3
Advantageously, vload_w1032 may be implemented by an lxvd2x instruction, vperm_2301 may be implemented by an xxpermdi instruction, and vstore_w1032 may be implemented by an stxvd2x instruction. This yields the following improved copy sequence:
lxvd2x 0,0,4
xxpermdi 0,0,0,2
xxpermdi 0,0,0,2
stxvd2x 0,0,3
Note that, although we are using it on a vector of words, vperm_w2301 is actually a vreverse operation for doublewords. Thus in accordance with the teachings herein, since the first vperm_w2301 instruction feeds the second vperm_w2301 instruction, and the result of the first vperm_w2301 instruction is not used elsewhere, the two vperm_w2301 instructions may be removed as above, yielding:
vload_w1032 t1=a
vstore_w1032 b=t1
This final intermediate representation may then be translated to the following code to copy a vector of word elements:
lxvd2x 0,0,4
stxvd2x 0,0,3
FIG. 11 shows the table 600 of vector optimization rules that includes an additional rule 620 that states that for computations where all operations performed on input vectors are pure SIMD instructions, eliminate the unneeded vector element reverse operations in the intermediate representation. Suppose we have a computation where all operations performed on the input vectors are pure SIMD instructions; that is, the instructions perform parallel computations on the corresponding elements of all input vectors independently. In this case, the relative order of elements within the registers will not affect the final results as long as it is consistent, and we can avoid vreverse operations for the entire sequence. A sample source statement is shown in FIG. 12. Letting vadd represent vector addition and vand represent vector bit-wise AND, the compiler could generate the code shown in FIG. 13, where t1 through t9 are vector registers. If the compiler recognizes that none of the reversed values t2, t4, t5, t7, and t8 is used outside this sequence, then the entire sequence can be converted to the instructions shown in FIG. 14. One way to implement this is to first apply the rule from the example shown in FIGS. 18 and 19, below, to the vadd, then apply the same rule to the vand, and finally apply the rule from the example in FIGS. 7-10. Another possible implementation is to analyze webs of computation where all sources originate from loads after which vector element reverse operations have been added and all sinks flow into stores before which vector element reverse operations have been added. The former is simpler to implement and faster to optimize, while the latter is more general and can be performed on computations that contain loops. Any implementation in which the instructions producing intermediate results and other instructions consuming those intermediate results are identified can be used. Examples include the well known Value Numbering, Value Propagation, Reaching Analysis, Instruction Generation Folding, and Peephole optimizations.
In another aspect of the present invention, using the POWER8 instruction set, the compiler may be presented with a variant of FIG. 13 that operates on a vector of single precision floating-point elements of 4-byte size:
vload_w1032 t1=x
vperm_w2301 t2=t1
vload_w1032 t3=z
vperm_w2301 t4=t3
vadd_fp t5=t2, t4
vload_w1032 t6=w
vperm_w2301 t7=t6
vand_w t8=t5, t7
vperm_w2301 t9=t8
vstore_w1032 y=t9
In conjunction with the teachings herein, the compiler generates:
vload_w1032 t1=x
vload_w1032 t3=z
vadd_fp t5=t2, t4
vload_w1032 t6=w
vand_w t8=t5, t7
vstore_w1032 y=t9
The intermediate representation may then be translated into the following machine instruction sequence on a POWER8 processor:
lxvd2x 0,0,4
lxvd2x 1,0,5
xvaddsp 0,0,1
lxvd2x 12,0,6
xxland 0,0,12
stxvd2x 0,0,9
A variant on the example in FIGS. 11-14 occurs when the compiler automatically generates vector code from scalar code. This “auto-vectorization” optimization may be performed on loops over array elements or on straight-line sections of code that make references to adjacent array elements. The compiler can replace operations on individual array elements with SIMD instructions that operate on several elements in parallel.
When auto-vectorized code is generated, the same sort of transformations can be made as shown in FIGS. 13 and 14. However, it may be easier in some cases for the compiler to recognize these cases. The compiler may generate special symbols or flags associated with vector loads and stores created for auto-vectorization, and thereby know that the vloads and vstores that it generates need not have vreverses inserted.
Even when an entire expression may not be optimized as shown in FIGS. 13-14, subexpressions may be optimized with localized rules. Examples of localized rules are shown at 630 in FIG. 15 to include unary operations rules, binary operations rules, endian conversion rules, and loads of literals or named constants rules. Examples of each of these follow.
For some unary operations such as negate, the instructions in FIG. 16 can be changed to the instructions in FIG. 17, or vice versa. While this does not reduce the number of instructions, the changed order of instructions may result in making optimization possible that was not possible before. For example, if a vector reverse operation preceded the unaryop y=x instruction, switching the order of the instructions as shown in FIG. 17 could result in two consecutive vreverse operations, which could then be optimized by removing the vreverse operations as shown in FIGS. 8-10. Even when the reversal does not allow eliminating a vreverse operation, it may improve performance by allowing one or more other instructions to use the result of the unary operation sooner.
For pure SIMD binary operations, the instructions shown in FIG. 18 could be reduced to the instructions shown in FIG. 19. Conversions between natural-endian loads/stores and biased-endian loads/stores are possible given the correct vreverses for the type. For example, referring to FIGS. 20 and 21, an instruction lvx+vreverse4×4 can be converted to lvxw4x, where “vreverseNxS” represents reversal of a vector having N elements, each of which has size S bytes. An instruction lvx+vreverse2×8 can be converted to lxvd2x. An instruction vreverse4×4+stvx can be converted to stvxw4x. An instruction vreverse2×8+stvx can be converted to stvxd2x. Note that it may also be beneficial to make the reverse transformation, for example, converting a lxvd2x into a an lvx+vreverse2×8. If the lxvd2x is itself followed by a vreverse2×8, this will produce back-to-back vreverse2×8 instructions that can then be eliminated as shown in FIGS. 8-10.
For loads of literals or named constants, the two instructions shown in FIG. 22 can be converted to the one instruction shown in FIG. 23, where “vreverse(literal)” represents the vector literal with its elements reversed in memory. In some cases the reverse transformation is also useful, to enable other transformations; for example, so that both inputs to an add are vreverse operations.
As a general rule, combinations of vreverse with binary or unary operations should be transformed first, and combinations of vreverse with loads and stores should be transformed last, in order to maximize other opportunities. These and other transformations may be used to reduce the number of vreverse operations added to the generated code. For instance, there are many rules specific to a particular instruction set architecture that may be applied to optimize combinations of vreverse and other instructions. In some cases another pass over the non-load or store instructions can find more opportunities.
Referring to FIG. 24, a method 2400 shows one possible method that could be performed by the compiler as described herein. The optimization requires pre-computation of def-use and use-def chains (step 2410). This is an industry-standard analysis to produce data structures that represent register relationships between instructions. If an instruction I modifies (defines) a register R, then the def-use chain for (I,R) lists all instructions that consume that definition of R. Also, if an instruction I makes use of a register R, then the use-def chain for (I,R) lists all instructions that could provide the definition of R that I uses. In many optimizers, def-use and use-def chains are maintained throughout the optimization phases. In this case, step 2410 requires no work. Otherwise, step 2410 performs this industry-standard analysis to make the chains available.
Next, step 2420 is performed as follows. The compiler walks through every instruction in the intermediate representation of the current function. For each instruction I, check whether I mentions a vector register. If not, continue to the next instruction. Otherwise, mark I as “relevant.” For each use of a register R in I, union {I} with each instruction in the use-def chain for (I,R). For each definition of a register R in I, union {I} with each instruction in the def-use chain for (I,R). If I is a load or store, record that information. If I is a vreverse operation, record that as well. If I is not a vreverse operation, determine whether it is swappable (is not sensitive to element order) or swappable with special handling (is sensitive to element order, but can be modified if its containing subgraph is optimized). Note that an instruction can be both a load or a store, and a vector reverse that indicates a load or store like lxvd2x. If I contains a register use that is live on entry to the function, record that information. If I defines a register that is live on exit from the function, record that information. When step 2420 is finished, the relevant instructions in the function have been partitioned into maximal subgraphs of vector computation. Each subgraph has a representative instruction, as determined by the classic set-union algorithm.
At this point step 2430 is performed as follows. Walk through each instruction a second time. If anything about the instruction makes the web not optimizable, record this fact in the representative instruction of its subgraph. This includes the instruction being marked live on entry or live on exit, a load or store that is not permuting (and is not swappable with special handling), a permuting load or store that is not accompanied by a register swap, or an instruction that is not marked as either a swap or swappable.
Next, step 2440 is performed as follows. Walk through each instruction a third time, looking for permuting loads and stores that are accompanied by a vector element reverse operation, and whose representative instruction has not been marked as not optimizable. Mark the accompanying vector element reverse operations for future removal. During the same pass, look for instructions that have been marked as swappable with special handling, and whose representative instruction has not been marked as not optimizable. Perform the special handling for these. Examples of special handling include changing the element number on the instruction, such as for a vector-splat, vector-insert or vector-extract operation, changing the element number selectors in a permute or shuffle instruction, changing an instruction operating on even elements, odd elements, upper elements or lower elements to the opposite, converting a non-permuting load or store into a permuting one, and swapping the elements in a constant vector.
Next, step 2450 is performed as follows. Walk through the instructions a fourth time, looking for vector element reverse operations that have been marked for deletion, and replace each of them with a register copy. It is necessary to do this separately from step 2440 because a single swap can be associated with more than one load or store due to other optimizations.
The claims and disclosure herein provide a compiler that includes a vector optimization mechanism that optimizes vector instructions by eliminating one or more vector element reverse operations. The compiler can generate code that includes multiple vector element reverse operations that are inserted by the compiler to account for a mismatch between the endian bias of the instruction and the endian preference indicated by the programmer or programming environment. The compiler then analyzes the code and reduces the number of vector element reverse operations to improve the run-time performance of the code.
One skilled in the art will appreciate that many variations are possible within the scope of the claims. Thus, while the disclosure is particularly shown and described above, it will be understood by those skilled in the art that these and other changes in form and details may be made therein without departing from the spirit and scope of the claims.

## Claims

1. A computer-implemented method executed by at least one processor for processing a plurality of instructions in a computer program, the method comprising:
providing a computer program including a plurality of instructions that includes at least one vector operation; and
processing the plurality of instructions to eliminate at least one vector element reverse operation from the computer program to enhance run-time performance of the computer program by:
recording characteristics of vector instructions and forming subgraphs of related instructions by analyzing def-use and use-def chains for the computer program in a first pass;
determining whether any of the subgraphs cannot be optimized in a second pass;
identifying a computation in the computer program where all operations performed on input vectors are single instruction multiple data (SIMD) instructions and marking for removal at least one vector element reverse operation that corresponds to the computation in a third pass; and
deleting in a fourth pass the at least one vector element reverse operation marked for removal in the third pass.

2. The method of claim 1 further comprising:
identifying in the third pass a first vector element reverse operation and a second vector element reverse operation in the computer program, such that the result of the first vector element reverse operation is the source of the second vector element reverse operation; and
eliminating in the fourth pass at least one of the first and second vector element reverse operations.

3. The method of claim 1 further comprising:
identifying a unary operation accompanied by at least one vector element reverse operation; and
changing order of instructions for the unary operation and the at least one vector element reverse operation.

4. The method of claim 1 further comprising:
identifying in the third pass a binary operation accompanied by at least one vector element reverse operation; and
eliminating in the fourth pass the at least one vector element reverse operation that accompanies the binary operation.

5. The method of claim 1 further comprising:
identifying in the third pass a first instruction that specifies an endian load followed by a second instruction that performs a vector element reverse operation; and
eliminating in the fourth pass the second instruction by converting the first instruction into a third instruction that specifies an endian load that does not require the second instruction.

6. The method of claim 1 further comprising:
identifying in the third pass a first instruction that is a vector element reverse operation that precedes a second instruction that is an endian store; and
eliminating in the fourth pass the first instruction by converting the second instruction into a third instruction that specifies an endian store that does not require the first instruction.

7. The method of claim 1 further comprising:
identifying in the third pass a first instruction that specifies a vector load of a literal value followed by a second instruction that is a vector element reverse operation; and
eliminating in the fourth pass the second instruction by reversing order of the elements in the literal value in the first instruction.

8. A computer-implemented method executed by at least one processor for processing a plurality of instructions in a computer program, the method comprising:
providing a computer program including a plurality of instructions that includes at least one vector instruction;
in a first pass, recording characteristics of vector instructions and forming subgraphs of related instructions by analyzing def-use and use-def chains for the computer program;
in a second pass, determining whether any of the subgraphs cannot be optimized;
in a third pass:
identifying a first vector element reverse operation and a second vector element reverse operation in the computer program, such that the result of the first vector element reverse operation is the source of the second vector element reverse operation, and marking the first and second vector element reverse operations for removal;
identifying a computation in the computer program where all operations performed on input vectors are single instruction multiple data (SIMD) instructions and marking at least one vector element reverse operation that corresponds to the computation for removal;
identifying a unary operation accompanied by at least one vector element reverse operation and changing order of instructions for the unary operation and the at least one vector element reverse operation;
identifying a first instruction that specifies an endian load followed by a second instruction that performs a vector element reverse operation, and marking the second instruction for removal by converting the first instruction into a third instruction that specifies an endian load that does not require the second instruction;
identifying a binary operation accompanied by at least one vector element reverse operation and marking the at least one vector element reverse operation that accompanies the binary operation for removal;
identifying a fourth instruction that is a vector element reverse operation that precedes a fifth instruction that is an endian store, and marking the fourth instruction for removal by converting the fifth instruction into a sixth instruction that specifies an endian store that does not require the fourth instruction; and
identifying a seventh instruction that specifies a vector load of a literal value followed by an eighth instruction that is a vector element reverse operation, and marking the eighth instruction for removal by reversing order of the elements in the literal value in the seventh instruction; and

in a fourth pass, deleting the vector element reverse operations marked for removal in the third pass.