Patent Publication Number: US-7711928-B2

Title: Method and structure for explicit software control using scoreboard status information

Description:
RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/558,391 filed Mar. 31, 2004 entitled “Method And Structure For Explicit Software Control Using Scoreboard Status Information” and naming Marc Tremblay, Shailender Chaudhry and Quinn A. Jacobson as inventors, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to enhancing performance of processors, and more particularly to methods for enhancing memory level parallelism. 
   2. Description of Related Art 
   Many modern processors attempt to exploit instruction-level parallelism to enhance performance. One common approach is to use dynamic scheduling for out-of-order execution and out-of-order completion for non data-dependent operations. 
   Typically, a processor  170  used a scoreboard  173  to monitor instructions in flight and to provide status information for each instruction waiting to be dispatched. Typically, once all the source operands were available in a register file  171  or directly from a functional unit and the required functional unit(s) of functional units  172 A to  172 D were available, as indicated by scoreboard  173 , the instruction was dispatched for execution. This centralized the decision-making. 
   Out-of-order execution exposes more instruction level parallelism to reduce the execution time of source program  130 . In out-of-order execution, a number of sequential instructions are fetched into a window where the instructions are executed according only to data dependencies, potentially out-of-order with respect to sequential order. 
   Exploiting instruction-level parallelism via out-of-order execution facilitated rapid processor performance improvements during the past decade. Continuing this performance growth requires larger and wider instruction windows. However, processor performance is outstripping memory performance and so greater instruction-level parallelism may not result in the expected performance benefits. 
   Typically, for memory intensive workloads with heavy pointer chasing, for example, sequential cache-misses dominate the overall execution time and enhanced instruction-level parallelism would not result in much, if any, improvement in processor performance in such situations. It has been recognized that memory-level parallelism is needed to significantly reduce execution times for such memory intensive workloads. In fact, memory-level parallelism is currently the number one target for a variety of techniques such as on-chip multiprocessor (CMP), coarse-grained multithreading (CMT), hardware scout, which uses multithreaded capability to scan ahead in the instruction stream to look for opportunities to prefetch data and speculatively traverse an instruction path, deep instruction windows, execute ahead, etc. 
   Unfortunately, all of these techniques are implemented in hardware and so are typically not accessible to software. Consequently, the software programmer cannot control the memory hierarchy or access to information concerning the memory hierarchy. This makes it difficult to enhance memory-level parallelism via software. 
   SUMMARY OF THE INVENTION 
   According to one embodiment of the present invention, the user is provided with means to sample the memory hierarchy via software. This allows a user to enhance memory-level parallelism via software 
   In one embodiment, a status of information needed for execution of a second computer program instruction is read in response to execution of a first computer program instruction. Execution continues with execution of the second computer program instruction upon the status being a first status. Alternatively, a third computer program instruction is executed upon the status being a second status different from the first status. Thus, execution of the first computer program instruction allows control of the memory hierarchy. 
   In one embodiment, the first computer instruction is a branch on status instruction. In another embodiment, the first computer instruction is a branch on ready instruction and an example of the branch on ready instruction is a branch on integer register ready instruction. In still another embodiment, the first computer instruction is a branch on not ready instruction and an example of the branch on not ready instruction is a branch on integer register not ready instruction. 
   A structure, for this embodiment, includes means for reading a status of information needed for execution of a second computer program instruction in response to execution of a first computer program instruction. The structure also includes means for continuing with execution of the second computer program instruction upon the status being a first status, and means for continuing, alternatively, with execution of a third computer program instruction upon the status being a second status different from the first status. 
   These means can be implemented, for example, by using stored computer executable instructions and a processor in a computer system to execute these instructions. The computer system can be a workstation, a portable computer, a client-server system, or a combination of networked computers, storage media, etc. 
   A computer system, for this embodiment, includes a processor and a memory coupled to the processor and having stored therein instructions wherein upon execution of the instructions on the processor, a method includes:
         reading a status of information needed for execution of a second computer program instruction in response to execution of a first computer program instruction;   continuing with execution of the second computer program instruction upon the status being a first status; and   continuing, alternatively, with execution of a third computer program instruction upon the status being a second status different from the first status.       

   A computer-program product, for this embodiment, includes a medium configured to store or transport computer readable code for a method including:
         reading a status of information needed for execution of a second computer program instruction in response to execution of a first computer program instruction;   continuing with execution of the second computer program instruction upon the status being a first status; and   continuing, alternatively, with execution of a third computer program instruction upon the status being a second status different from the first status       

   In another embodiment, a branch on status software instruction is used to expose a scoreboard of a processor. Status information in the scoreboard is used upon execution of the branch on status software instruction to explicitly control which software instruction of a multiplicity of software instructions in a computer source program is executed after execution of the branch on status software instruction. 
   For this embodiment, a system includes a processor and a memory, coupled to the processor, storing a branch on status instruction. The system also includes a scoreboard wherein execution of the branch of status instruction on the processor exposes the scoreboard. 
   For this embodiment, a structure includes means for using a branch on status software instruction to expose a scoreboard of a processor. The structure also includes means for using status information in the scoreboard upon execution of the branch on status software instruction to explicitly control which software instruction of a multiplicity of software instructions in a computer source program is executed after execution of the branch on status software instruction. 
   Again, these means can be implemented, for example, by using stored computer executable instructions and a processor in a computer system to execute these instructions. The computer system can be a workstation, a portable computer, a client-server system, or a combination of networked computers, storage media, etc. 
   For this embodiment, a computer-program product includes a medium configured to store or transport computer readable code for a method including:
         using a branch on status software instruction to expose a scoreboard of a processor; and   using status information in the scoreboard upon execution of the branch on status software instruction to explicitly control which software instruction of a multiplicity of software instructions in a computer source program is executed after execution of the branch on status software instruction.       

   In still another embodiment, a structure includes means for explicit software control of memory-level parallelism by exposing a scoreboard of a processor. The structure also includes means for using status information from the scoreboard in the explicit software control of memory-level parallelism. 
   These means can be implemented, for example, by using stored computer executable instructions and a processor in a computer system to execute these instructions. The computer system can be a workstation, a portable computer, a client-server system, or a combination of networked computers, storage media, etc. 
   In still yet another embodiment, memory-level parallelism is placed under explicit software control by exposing a scoreboard of a processor. Status information from the scoreboard is used in the explicit software control of memory-level parallelism. 
   For this embodiment, a computer-program product includes a medium configured to store or transport computer readable code for a method including:
         placing memory-level parallelism under explicit software control by exposing a scoreboard of a processor; and   using status information from the scoreboard in the explicit software control of memory-level parallelism.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a prior art system that included a scoreboard that was visible only to the processor. 
       FIG. 2  is a block diagram of a system that includes an instruction that upon execution on a processor provides software control of memory-level parallelism, according to an embodiment of the present invention. 
       FIG. 3  is a process flow diagram for a branch on status method that results from executing an embodiment of the branch on status instruction, according to an embodiment of the present invention. 
       FIG. 4  is a process flow diagram for a branch on status method that results from executing another embodiment of the branch on status instruction, according to an embodiment of the present invention. 
       FIG. 5  is an illustration of a specific embodiment of the branch on status instruction, according to an embodiment of the present invention. 
       FIG. 6  is a high-level network system diagram that illustrates several alternative embodiments for using a source program including a branch on status instruction that provides explicit software control of the memory-level parallelism. 
   

   In the drawings, elements with the same reference numeral are the same or similar elements. Also, the first digit of a reference numeral indicates the figure number in which the element associated with that reference numeral first appears. 
   DETAILED DESCRIPTION 
   According to one embodiment of the present invention, scoreboard  273  ( FIG. 2 ) is made visible to the programmer via execution of an instruction  235  on processor  170 . Consequently, the programmer can use information in scoreboard  273 , e.g., status information, to explicitly control execution of source program  230 . This allows the programmer to use software to enhance memory-level parallelism because instruction  235  gives the programmer explicit control to go somewhere else in program  230  when status information in scoreboard  273  shows that needed data, or perhaps resources, are unavailable. 
   For example, code segment  231  includes instruction  235  that determines the status, e.g., available or unavailable, of a source operand, for example, and then takes an appropriate action depending upon whether the source operand is available. If instruction  235  determines that the source operand is available, execution of code segment  231  continues. Conversely, if instruction  235  determines that the source operand is unavailable, code in second code segment  232  of source program  230  is executed. 
   For example, if code segment  231  is a linked list, instruction  235  determines that a load instruction will miss based upon information in scoreboard  273 , instruction  235  jumps to code segment  232  that is another linked list. 
   Thus, instead of stalling on dependent load misses and not achieving high memory-level parallelism, source program  230  detects the miss and jumps to another linked list  232  and tries to traverse it. If there is another miss in linked list  232 , the memory-level parallelism is higher. If there is not a miss, great progress is achieved. 
   Functional units  272 A to  272 D, register file  271 , and scoreboard  273  are illustrative only and are not intended to limit the invention to the specific layout illustrated in  FIG. 2 . A processor  270  may include multiple processors on a single chip. Each of the multiple processors may have an independent register file and scoreboard or the register file and scoreboard may, in some manner, be shared or coupled. Similarly, register file  271  may be made of one or more register files. In view of this disclosure, those of skill in the art can implement instruction  235  for a particular processor architecture and operating system of interest. Also, when it is stated herein that an instruction, software, source program, or program code takes an action or actions, those of skill in the art appreciate that the action or actions are the result of execution of an appropriate computer program instruction or instructions. 
   Instruction  235 , which makes scoreboard  273  visible to the programmer, provides a new level of control and information to the programmer and allows the programmer to explicitly control program flow to achieve greater memory-level parallelism. This new functionality also is particularly useful for explicit programmer control of garbage collection and data speculation, for example. See commonly assigned and commonly filed, U.S. patent application Ser. No. 11/082,281, entitled “Method and Structure for Explicit Software Control of Data Speculation,” of Christof Braun, Quinn A. Jacobson, Shailender Chaudhry and Marc Tremblay, filed on Mar. 16, 2005 and U.S. patent application Ser. No. 11/083,163, entitled “Method and Structure for Explicit Software Control of Execution of A Thread Including a Helper Subthread,” of Christof Braun, Quinn A. Jacobson, Shailender Chaudhry and Marc Tremblay, filed on Mar. 16, 2005, each of which is incorporated herein by reference in its entirety. 
   In one embodiment, instruction  235  is a branch on status instruction. An embodiment of the branch on status instruction is a branch on register status instruction that in turn is, in one embodiment, a branch if not ready instruction, e.g., the register status is “not ready.”  FIG. 3  is a process flow diagram of one embodiment of a method  300  that results from execution of the branch if not ready instruction. The format for one embodiment of the branch if not ready instruction is:
 
Branch_if_not_ready % reg label
 
   where
         % reg is a register in scoreboard  273 , which in this embodiment is a hardware instruction scoreboard, and   label is a label in code segment  232 .       //   //   //   //   //   //   //   

   A pseudo code example using this instruction is: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             load 
                             ⁢ 
                             
                                 
                             
                             [ 
                             
                                 
                             
                             ] 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           % 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           reg 
                         
                       
                     
                     
                       
                         … 
                       
                     
                     
                       
                         
                           
                             Branch_if 
                             ⁢ 
                             _not 
                             ⁢ 
                             _ready 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             % 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             reg 
                           
                           , 
                           label 
                         
                       
                     
                     
                       
                         
                           use 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           % 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           reg 
                         
                       
                     
                     
                       
                         … 
                       
                     
                   
                   } 
                 
                 ⁢ 
                 
                   
                     
                       
                         First 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Code 
                       
                     
                   
                   
                     
                       Segment 
                     
                   
                 
               
               ⁢ 
               
                 
 
               
               ⁢ 
               
                 
                   
                     
                       label 
                       ⁢ 
                       
                         : 
                       
                     
                   
                 
                 
                   
                     … 
                   
                 
               
             
             } 
           
           ⁢ 
           
             
               
                 
                   Second 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Code 
                 
               
             
             
               
                 Segment 
               
             
           
         
       
     
   
   In operation  301 , processor  270  reads the status, and not the value, of register % reg from hardware instruction scoreboard  273 . Operation  301  transfers processing to status check operation  302 . 
   Status check operation  302  determines the status of register % reg, as read from scoreboard  273 . If the status of register % reg is “not ready,” status check operation  302  transfers processing to branch to label operation  303 . Branch to label operation  303  in turn transfers processing to label in the second code segment. If the status of register % reg is “ready,” status check operation  302  transfers processing to continue execution operation  304 . Continue execution operation  304  continues execution of the first code segment because the value in register % reg is ready for use. 
   The above example of instruction  235  was illustrative only. Another example of a branch on register status instruction is a branch if ready instruction.  FIG. 4  is a process flow diagram of one embodiment of a method  400  that results from execution of the branch if ready instruction. The format for one embodiment of the branch if ready instruction is:
 
Branch_if_ready % reg label
 
   where
         % reg is a register in scoreboard  273 , which in this embodiment is a hardware instruction scoreboard, and   label is a label at the start of code segment  232 .       

   A pseudo code example using this instruction load [] % reg 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             load 
                             ⁢ 
                             
                                 
                             
                             [ 
                             
                                 
                             
                             ] 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           % 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           reg 
                         
                       
                     
                     
                       
                         … 
                       
                     
                     
                       
                         
                           
                             Branch_if 
                             ⁢ 
                             _ready 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             % 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             reg 
                           
                           , 
                           label 
                         
                       
                     
                     
                       
                         … 
                       
                     
                   
                   } 
                 
                 ⁢ 
                 
                   
                     
                       
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                         ⁢ 
                         
                             
                         
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               ⁢ 
               
                 
                   
                     
                       label 
                       ⁢ 
                       
                         : 
                       
                     
                   
                 
                 
                   
                     
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                       ⁢ 
                       
                           
                       
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             } 
           
           ⁢ 
           
             
               
                 
                   Second 
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                   Code 
                 
               
             
             
               
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   Method  400  is equivalent to method  300  described above except status check operation  402  transfers processing to branch to label operation  303  if the status is “ready” and to continue execution operation  304  if the status is “not ready.” The above descriptions of operations  301 ,  302  and  304  are incorporated herein by reference. 
   Thus, in these embodiments of the branch on status instruction, processor  170  includes two new branch instructions that enable software to test scoreboard  273  that also is used in hardware scheduling. These novel branch on status instructions check if an instruction that used the register as a source operand would stall waiting for a previous operation to complete or would be able to execute immediately. These instructions enable software to dynamically adapt to data dependencies. 
   A common use of these branch on status instructions is to check if a load instruction had completed. For a processor that has a non-blocking load architecture, a program can issue a load instruction and then wait an appropriate time and issue one of these new branch on status instructions to test if the load had completed or is still waiting on a cache miss. Depending if the load data were already available, the software could either issue a use for the load data or perform potentially independent work before using the load data. 
   These new branch on status instructions test the scoreboard at the time the branch on status instruction is dispatched. It is therefore important that code making use of these branch on status instructions understand the dispatch grouping rules and the expected latency of operations. If a branch on not ready instruction were issued immediately after a load instruction, the instruction would typically see the load as not ready because for example, the load has a three cycle minimum latency even for the case of level-one data cache hit. 
   The above examples are illustrative only and are not intended to limit the invention to the specific examples used. The branch on status instructions also could be based on the status of functional units since the status of functional units is typically included in the scoreboard. For example, the branch on status test could be used for variable latency operations such as a division operation. Using the branch instructions for results from functional units provides an enhancement of both instruction level parallelism and memory level parallelism. 
   In one embodiment, the two novel branch on status instructions are defined based on the SPARC Architecture. See for example, David L. Weaver/Tom Germond, Editors,  The SPARC Architecture Manual , Version 9, SPARC International, Inc., San Jose, Calif., SA-V09-R147-Jul2003, which is published by Prentice Hall, Englewood Cliffs, N.J. 07632, U.S.A. with ISBN 0-13-825001-4, which is incorporated herein by reference to show information known to those of skill-in-the-art. The two instructions are (i) a branch on integer register not ready with prediction instruction, and (ii) a branch on integer register ready with prediction instruction. 
   In this embodiment, these branch on status instructions are implemented by using experimental instructions that utilize opcodes that are otherwise illegal instructions in the SPARC Architecture. An Experimental Control Register enables experimental instructions. If an experimental instruction is not enabled, an attempt to execute that opcode causes an illegal instruction trap. 
   TABLE 1 defines opcodes for the two branch on integer register status with prediction instructions along with the values of field rcond that uniquely identifies the instructions. 
   
     
       
         
             
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Instruction 
               rcond 
               Operation 
               Register Test 
             
             
                 
             
           
          
             
               BRR 
               000 
               Branch on Register Ready 
               r[rs1] ready 
             
             
                 
                 
                 
               for immediate 
             
             
                 
                 
                 
               use 
             
             
               BRNR 
               100 
               Branch on Register Not Ready 
               r[rs1] not 
             
             
                 
                 
                 
               ready for 
             
             
                 
                 
                 
               immediate use 
             
             
                 
             
          
         
       
     
   
     FIG. 5  is an illustration of a format  500  for the instructions of TABLE 1. In  FIG. 5 , two-bit field d 16 hi and fourteen-bit field d 16 lo together are a word-aligned, sign-extended, program counter relative displacement for the branch. Five-bit field rs 1  is the address of the first integer register r source operand. One-bit branch prediction field p encodes a static prediction for the instructions as presented in 
                       TABLE 2               p   Branch Prediction                  0   Predict branch will not be taken       1   Predict branch will be taken                    
Three-bit field rcond selects the register-contents status to test for a branch on integer register status with prediction instruction. One-bit annul field a, upon being set, annuls the execution of the following instruction if the branch is conditional and untaken, or if the branch is unconditional and taken.
 
   The assembly level syntax for the two branch on integer register status with prediction instructions is: 
   
     
       
         
             
             
           
             
                 
               TABLE 3 
             
             
                 
                 
             
           
          
             
                 
               brr{,a}{,pt|,pn} regrs1, label 
             
             
                 
               brnr{,a}{,pt|,pn} regrs1, label 
             
             
                 
                 
             
          
         
       
     
   
   To set annul bit a for the two instructions, an “,a” is appended to the opcode mnemonic. For example, use “brr,a % i3, label.” In TABLE 3, braces signify that the “,a” is optional. To set branch prediction bit p, append either “,pt” for predict taken or “,pn” for predict not taken to the opcode mnemonic. If neither “,pt” nor “,pn” is specified, the assembler defaults to “,pt” in this embodiment. 
   Those skilled in the art readily recognize that in this embodiment the individual operations mentioned before in connection with the branch on status embodiment of instruction  235  ( FIG. 6 ), are performed by executing a computer program instruction on processor  270  of computer system  200 . In one embodiment, a storage medium  611  has thereon installed computer-readable program code that includes instruction  235 , and execution of the computer-readable program code causes processor  270  to perform the individual operations explained above for methods  300  and/or method  400 . 
   In one embodiment, computer system  200  is a hardware configuration like a personal computer or workstation. However, in another embodiment, computer system  200  is a client-server computer system  600 . For either a client-server computer system or a stand-alone computer system, memory  220  typically includes both volatile memory  610 , such as main memory, and non-volatile memory  611 , such as hard disk drives. In still another embodiment, computer system  200  is contained in a single package. 
   While memory  220  ( FIG. 2 ) is illustrated as a unified structure, this should not be interpreted as requiring that all memory in memory  220  be at the same physical location. All or part of memory  220  can be in a different physical location than processor  270 . For example, instruction  235  may be stored in memory that is physically located in a location different from processor  270 . 
   Processor  270  should be coupled to the memory containing instruction  235 . This could be accomplished in a client-server system  600 , or alternatively via a connection to another computer via modems and analog lines, or digital interfaces and a digital carrier line. For example, all of part of memory  220  could be in a World Wide Web portal, while processor  270  is in a personal computer, for example. 
   More specifically, computer system  200 , in one embodiment, can be a portable computer, a workstation, a server computer, or any other device that can execute instruction  235 . Similarly, in another embodiment, computer system  200  can be comprised of multiple different computers, wireless devices, server computers, or any desired combination of these devices that are interconnected to perform, method  300  and/or method  400 , as described herein. 
   Herein, a computer program product comprises a medium configured to store computer readable code for instruction  235  or in which computer readable code for instruction  235  is stored. Some examples of computer program products are CD-ROM discs, ROM cards, floppy discs, magnetic tapes, computer hard drives, and servers on a network. 
   Herein, a computer memory refers to a volatile memory, a non-volatile memory, or a combination of the two. Similarly, a computer input unit, e.g., keyboard  615  and/or mouse  618 , and a display unit  616  refer to the features providing the required functionality to input the information described herein, and to display the information described herein, respectively, in any one of the aforementioned or equivalent devices. 
   In view of this disclosure, instruction  235  can be implemented in a wide variety of computer system configurations using an operating system and computer programming language of interest to the user. In addition, instruction  235  could be stored on a first device and executed on a different device. For example, instruction  235  could initially be stored in a server computer, and then as necessary, a module of containing instruction  235  could be transferred to a client device and executed on the client device. 
   In yet another embodiment, the module containing instruction  235  is stored in a memory of another computer system. The stored module is transferred, over a network to memory  220  in system  200 . 
   The computer program including instruction  235  may be stored on any common data carrier like, for example, a floppy disk or a compact disc (CD), as well as on any common computer system&#39;s storage facilities like hard disks. Therefore, one embodiment of the present invention also relates to a data carrier for storing a computer program for carrying out the inventive method. Another embodiment of the present invention also relates to a method for using a computer system for carrying out method  300  and/or method  400 . Still another embodiment of the present invention relates to a computer system with a storage medium on which a computer program including instruction  235  is stored. 
   While instruction  235  hereinbefore has been explained in connection with various embodiments thereof, those skilled in the art will readily recognize that modifications can be made to these embodiments without departing from the spirit and scope of the present invention. For example, in the above examples, the status information was obtained from a scoreboard. However, as is known to those of skill in the art, status information can be obtained in a variety of ways, for example, ready bits. Thus, the use of a scoreboard is illustrative of a mechanism for providing status information that can be sampled, and so scoreboard should be interpreted as including other techniques used to represent status information within a processor that can be sampled using a software instruction.