Abstract:
A method, apparatus, and computer program product dynamically select compiled instructions for execution. Static instructions for execution on a first execution and dynamic instructions for execution on a second execution unit are received. The throughput performance of the static instructions and the dynamic instructions is evaluated based on current states of the execution units. The static instructions or the dynamic instructions are selected for execution at runtime on the first execution unit or the second execution unit, respectively, based on the throughput performance of the instructions.

Description:
BACKGROUND 
   The present invention relates generally to compiled instructions, and, more particularly, to dynamically selecting compiled instructions for execution. 
   A compiler is a specialized computer program that converts source code written in one programming language into another language, usually machine language (also called machine code), so that it can be understood by processors (i.e., logic chips). Source code is the version of software (usually an application program or an operating system) as it is originally written (i.e., typed into a computer) by a human in plain text (i.e., human readable alphanumeric characters). Source code can be written in any of numerous programming languages, some of the most popular of which are C, C++, Java, Perl, PHP, Python and Tcl/Tk. The output of a compiler is referred to as object code. 
   Compilers create programs that are optimized to target the processors and the fixed functions of their architecture. However, the architecture may be tuned very well for one application type, but will penalize others. Current architectures are optimized around the most typical coding sequences, or worst, towards benchmarks used in market comparisons. As a result, the ability to create optimum performance for multiple instruction sequence types is too broad an endeavor for current architecture and compiler methods. 
   Previous architectures had a fixed structure. The performance of fixed architectures can be very restrictive with their static execution units. It is nearly impossible for a generalized fixed architecture to be ideal for all problems. Custom execution units are not ideal due to their limited usefulness, chip area and power consumption. 
   Dynamic compilation is a process used by some programming language implementations to gain performance during program execution. The best known language that uses this technique is Java. Dynamic compiling originated in Self. It allows optimizations to be made that can only be known at runtime. Runtime environments using dynamic compilation typically have programs run slowly for the first few minutes, and then after that, most of the compilation and recompilation are done and the program runs quickly. However, due to this initial performance lag, dynamic compilation is undesirable in certain cases. In most implementations of dynamic compilation, some optimizations that could be done at the initial compile time are delayed until further compilation at runtime, causing further unnecessary slowdowns. 
   Attempts have been made to improve dynamic compilation. For example, just in time compilers have been developed that compile from architecture independent code (Java Byte code) into architecture dependent application based solely on the history of execution at runtime. This type of compiling performs optimization based on the target architecture. 
   Tensilica compilers create custom logic based on application specific needs to solve a particular problem, like an Application Specific Integrated Circuit (ASIC). Software routines are mapped to hardware macros through a tool. This optimization yields higher performance but only for a fixed problem domain. 
   Transmeta compilers convert Intel x86 code into internal VLIW instruction architecture format, recompiling often used parts of the code for the best optimization. It then replaces the translated code with optimized translated code based on historical usage patterns. Since the code is replaced, the previous code cannot be used when circumstances change, and the optimized code is no longer optimal. 
   Field Programmable Gate Arrays (FPGAs) have been used historically by hardware design engineers to design, validate, and test circuitry as an intermediate step, ultimately targeting the design for use in an ASIC, such as a custom digital signal processor (DSP) or other special purpose chips. ASICs are fast and highly specialized, and thus very efficient. However, they are very costly to bring to market, and thus are usually used in mass-market applications. For the past twenty years, text based hardware design languages (HDLs), such as VHDL and Verilog, have been used for designing, or programming such custom circuitry. FPGAs have had much slower clock speeds than processors, and thus were never originally intended for use as processing elements themselves. 
   Over the years, FPGAs have been catching up to processors, and have outstripped Moore&#39;s law, becoming much denser, faster, and cheaper at a much faster rate than microprocessors. In fact, today the majority of designs for custom circuitry can now remain on an FPGA for execution instead of going through the long and expensive process of bringing a custom ASIC to market. 
   Although the C based FPGA programming environments today can facilitate an application programmer inputting circuitry (Cores) into FPGAs by making simple redirected function calls to place these cores in the FPGA based hardware, they were never really designed as parallel hardware design languages for creating optimal cores. In particular, current C based FPGA programming techniques are not suited for creating complex designs. C and C++ were never actually designed to do parallel programming in reconfigurable FPGA hardware and mix them with hardware design languages, such as VHDL. Using current C based FPGA programming techniques to accomplish tasks that they were never designed to do can produce an awkward and challenging experience for programmers. These solutions are sufficient for placing cores and simple single chip designs into a single FPGA, but in order to maximize performance with larger parallelized applications in a deep-scaling environment (including, e.g., multiple FPGA designs, and multiple FPGA board to board designs), these tools will need to greatly evolve their capabilities. 
   With current compiler architecture, it is nearly impossible for a generalized fixed architecture to be ideal for all problems. Custom execution units are not ideal due to their limited usefulness, chip area, and power consumption. The choice of execution units would ideally be a dynamic choice that is based on the current state of the execution unit at the time of execution. 
   SUMMARY 
   The foregoing discussed drawbacks and deficiencies are overcome or alleviated by a method, system, and computer program product for dynamically selecting compiled instructions for execution. 
   According to one embodiment, a method for dynamically selecting compiled instructions for execution includes receiving static instructions for execution on a first execution unit and receiving dynamic instructions for execution on a second execution unit. The method further includes evaluating throughput performance of the static instructions and the dynamic instructions based on current states of the execution units and selecting the static instructions or the dynamic instructions for execution at runtime on the first execution unit or the second execution unit, respectively, based on the throughput performance of the instructions. 
   According to another embodiment, an apparatus for dynamically selecting compiled instructions for execution includes an input for receiving static instructions for execution on a first execution unit and dynamic instructions for execution on a second execution unit. The apparatus further includes an instruction selection element adapted to evaluate throughput performance ofthe static instructions and dynamic instructions based on current states of the execution units and select the static instructions or the dynamic instructions for execution at runtime on the first execution unit or the second execution unit, respectively, based on the throughput performance of the instructions. 
   According to another embodiment, a computer program product for dynamically selecting compiled instructions for execution comprises a computer usable medium having a computer readable program. The computer readable program, when executed on a computer, causes the computer to receive static instructions for execution on a first execution unit, receive dynamic instructions for execution on a second execution unit, evaluate throughput performance of the static instructions and the dynamic instructions based on current states of the execution units, and select the static instructions or the dynamic instructions for execution at runtime on the first execution unit or the second execution unit, respectively, based on the throughput performance of the instructions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
       FIG. 1  is a flow diagram depicting a method for dynamic compilation according to an exemplary embodiment. 
       FIG. 2  illustrates dynamic instruction architecture according to an exemplary embodiment. 
       FIG. 3  illustrates an exemplary code sequence for an instruction that may be performed by a dynamic execution unit according to an exemplary embodiment. 
       FIG. 4  illustrates a dynamic string matching example according to an exemplary embodiment. 
       FIG. 5  illustrates a method for runtime instruction selection according to an exemplary embodiment. 
       FIG. 6  illustrates an example of how special instructions within an instruction sequence may be handled according to an exemplary embodiment. 
       FIG. 7  illustrates how an incorrect branch prediction may be handled according to an exemplary embodiment. 
   

   DETAILED DESCRIPTION 
   According to exemplary embodiments, execution units are chosen at runtime, depending on the state of the execution units. Thus, for example, a traditional processor may be chosen to execute traditional architecture-dependent instructions or another execution unit, e.g., an FPGA, a graphic processor, physics processor, etc., may be chosen to execute architecture-independent instructions. For ease of explanation, FPGAs are described below as alternatives for executing instructions. It should be understood, however, that other types of execution units, programmed with other architecture-independent instructions, may be used. 
   According to an exemplary embodiment, upon compiling, a set of instruction sequences is evaluated for the number of occurrences in the code, potential parallelization, and other criteria to determine a partioning of an integrated set of programmable execution units. A set of chosen code sequences is then converted to dedicated FPGA index pointers. A set of FPGA instructions are created to replace the substitute in-line execution instructions. A single merge program is the output with the set of standard instruction sequences and FPGA instructions. Upon execution, the prefetcher can detect the pre-compiled FPGA unit to use or execute the standard instructions. 
     FIG. 1  is a flow diagram illustrating a method for dynamic compilation according to an exemplary embodiment.  FIG. 1  shows a method of converting a sequence of instructions of a high level language into a partitioned set of instructions. This set is converted into a new hybrid FPGA marker instruction and FPGA personality, along with traditional in-line architecture instructions that are seamlessly integrated into a single instruction stream. 
   Referring to  FIG. 1 , the method begins at step  110  at which the source code is input in a high-level programming language. At step  120 , architecture independent root instructions are generated. At step  130 , FPGA instructions and FPGA personalities are generated. The FPGA “personalities” correspond to the configuration or the function of an FPGA, while the FPGA instructions indicate how to use the FPGA functions. For example, an FPGA function may be to multiply two integers, and an FPGA instruction may indicate what two integers to multiply. At step  140 , combined FPGA personalities are created. These combined FPGA personalities may be stored for retrieval later, if needed. At step  150 , architecture specific instructions are generated. At step  160 , the FPGA instructions and FGPA personalities are combined with the architecture specific instructions, and dynamic selections are inserted. At step  170 , an instruction is generated with a reconfiguration stream with dynamic selections. 
     FIG. 2  illustrates exemplary dynamic instruction architecture. An instruction sequence stored in a cache  200  includes traditional, architecture-dependent instructions and non-traditional architecture-independent instructions, e.g., FPGA instructions. The instructions are fetched by a pre-fetch unit  210  and fed into an FPGA mapping unit  220  in which the FPGA instructions are mapped to FPGA programs for executing those instructions. The FPGA mapping unit  220  communicates with an FPGA programmer  230  to determine whether an FPGA program is stored in an SRAM for executing the function corresponding to the program. If the FPGA program is not stored in an SRAM, it may be retrieved from a file of combined FPGA personalities, as described above. Once the FPGA program is retrieved, it is loaded into an available FPGA in FPGA unit  260 . 
   The program counter  240  fetches instructions to be performed by the processor core/execution units  250  or by the FPGA if the processor determines that this is appropriate. The program counter  240  keeps track of the instruction that is currently being executed, while the prefetcher unit fetches instructions before they are to be executed. The prefetcher unit  210  also feeds into the processor core/execution units  250 . At runtime, the processor  250  makes a selection whether to use the FPGA unit to execute FPGA instructions or to execute the traditional instructions fed directly from the prefetcher  210 . This selection may be made by determining whether an FPGA is available, whether an FPGA is free, whether the instruction would be executed faster using an FPGA, etc. If the processor core  250  determines that an FPGA should execute the instructions, it directs an FPGA from FPGA unit  260  to execute the instructions and return the results. If the processor core  250  determines that it would be more optimal to execute the traditional instructions, the instructions are executed in the processor  250 . Details ofthe steps involved in this selection process are provided below with reference to  FIG. 5 . 
     FIG. 3  illustrates an example of a code sequence for an instruction that may be suitable for execution by a dynamic execution unit, such an FPGA. 
     FIG. 4  illustrates a dynamic string matching example. This process includes a set of string compare functions  410 ,  420 ,  430  that can be re-used for other similar string sequences in other code segments. It should be appreciated that although only three comparison units  410 ,  420 , and  430  are shown, any number of comparison units may be used, depending upon the number of strings. According to this embodiment, multiple instruction sequences are read in parallel without any hits in bandwidth or performance, and the common instructions that can be referenced from the first time they are seen are pulled out via matching unit  440 . 
     FIG. 5  illustrates a method for selecting instructions at runtime according to an exemplary embodiment. The steps of this method may be performed, e.g., by software running in the processor  250  shown in  FIG. 2 . The selection of dynamic instructions versus standard (static) instructions takes into account the performance of each execution method. In the example shown in  FIG. 5 , selection is based upon string length and whether or not a dynamic execution has been built in time execute the instructions. A dispatcher in the processor  250  may distribute the instructions to the appropriate execution unit, based on upon the selection. 
   Referring to  FIG. 5 , the method begins with state-based instruction selection at step  510 . During this step, a run-time instruction flow is analyzed, and a determination is made as to which instructions have the best throughput performance. In the example shown, this involves determining whether the length of the instruction, e.g., (length (my string)) is greater than or equal to a predetermined value X and a loaded FPGA personality. If so, the throughput performance is determined to be optimal for an FPGA instruction, and an FPGA instruction is selected at step  550 . The execution results are provided at step  570 . Otherwise, traditional (static) instructions are chosen to execute at run-time at step  520 . A determination is made at step  530  whether an FPGA is loaded. This may occur when an FPGA is not loaded (built) in time for the instruction to be executed, but is finished being built before the traditional instructions are completed. In this rare case, the execution of the traditional instructions may be aborted at step  560 , and the FPGA instruction may be chosen for execution instead at step  550 . Otherwise, a determination is made at step  540  whether an instruction loop has been completed. If so, the execution results are produced at step  570 . Otherwise, the process returns to step  520 . 
     FIG. 6  illustrates an example how special instructions within an instruction sequence may be handled according to an exemplary embodiment. As shown in  FIG. 6 , the instruction sequence includes a branch condition. When a “branch loop” instruction is seen by the pre-fetcher, the branch might be predicted as taken, so FPGAs may be preprogrammed. If the branch prediction is incorrect, re-programming an FPGA to accommodate instructions might take several thousand cycles, degrading performance. However, this is not a problem as the FPGA program logic informs the XC instruction to interpret “FPGA1” as a skip to the original TRAD instructions. 
     FIG. 7  graphically illustrates a comparison of traditional versus FPGA performance in a string matching example. In  FIG. 7 , the “x” axis represents time, the “y” axis represents how much slower the FPGA is at executing instructions compared to execution of traditional instructions, and the “z” axis represents string length or width. The graph  710  string length comparison by a processor using traditional instructions, and graph  720  represent strength length comparison by an FPGA. As explained above, an FPGA compares entire strings, while a processor uses traditional instructions to compare one portion of the string at a time. Thus, as shown in  FIG. 7 , the longer the string length, the longer the time it takes for the processor to compare strings. Conversely, the longer the string length, the shorter the amount of time it takes for the FPGA to compare strings. The FPGA takes constant time for any string length. But the slower the FPGA, the longer it takes. For example, an FPGA might take 20 units of time for any length string, while a traditional processor might take a number of units of time equal to the length of the string. The processor is superior for strings shorter than 20 characters, while the FPGA is superior for strings longer than 20 characters. The dashed line  705  represents the string length at which the FPGA performs matching faster. 
   While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.