Abstract:
Data processing time is enhanced in a system in which the executable code has inserted therein certain instructions, by a system and method which anticipates which switch will occur when multipath decision points are reached. The code is profiled using test data and a record is made as to the history of the possible switch states. This record is used to optimize the revised executable code, based upon probability of selection.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to computer program systems in which performance is increased and more particularly to a system and method for using or exploiting case correlation to increase the program performance. 
     BACKGROUND OF THE INVENTION 
     Currently, computer program source code often contains constructs called &#34;switch statements&#34; which are multi-way branches. When such statements, in the form of object code, run in real-time, the branch that is taken depends on the value of the data that is present at that time. When a switch statement is inside a program loop, there may be a pattern to which branch is selected branch as the loop is executed. Such patterns can be used, if predictable, to optimize system performance, particularly as the code is being compiled from the source program. 
     However, a branch statement is expensive (in terms of machine cycles) on modern computer systems. This is particularly so when a branch is an indirect branch, meaning that the branch is unpredictable as to where it might go. Most instructions today can execute in a cycle or less so that often several instructions could execute during the same cycle. But when a branch statement is encountered, it might take perhaps six cycles to figure out where the target of the branch is, i.e., which branch is being followed and to obtain the necessary data from memory. 
     Another problem with branches, other than the fact that they are expensive to execute, is that they tend to introduce bottlenecks where the code optimizer (during the compiling stage) is not able to do as effective a job as it might otherwise do. When the compiler optimizer encounters a branch, it cannot effectively combine instructions from beyond that branch with instructions from prior to that branch. This then does not allow the compiled code to be properly optimized for run-time performance. 
     Therefore, there is a need in the art for a system and method for optimizing run-time code in the face of branches and switch statements. 
     SUMMARY OF THE INVENTION 
     These and other objects, features and technical advantages are achieved in one embodiment of the invention in which the program that is being compiled is run through a compiler/optimizer to observe the selection of the various branches in the face of a set of data. The system and method of this invention addresses the performance impact of switch statements inside loops by collecting data on the behavior of those switch statements during a particular training run of the program. The collected data is then used by the optimizer to revise the object code produced for that program so that when a particular case within the switch statement is selected, another case may be the most likely. The system is then biased to run the most likely selection that will flow from the first selected case as though a switch statement did not exist, and the data is prefetched from memory using this assumption. During run-time when the conditions are present for a branch, then the branch path is followed, at a cost of some processing time. Thus, in concept the idea is to bias the switch statement cases so that they tend to flow easily into the case which is most likely to follow. 
     Therefore, it is a technical advantage of this invention that for likely cases (branches) that follow other cases, branches can be avoided in the codes, thereby saving clock cycles. 
     It is a still further technical advantage that the code can be optimized over several cases for the situation where a particular case is followed by another expected case. 
     It is a technical advantage of this invention that the code when it executes looks to past history to make a determination as to the most probable next history, thereby allowing the instruction memory cache to be preloaded with the data required for the expected next branch. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates the overall system and method of the invention; 
     FIG. 2 illustrates a typical CPU system structure in which the invention could operate; 
     FIGS. 3A and 3B illustrate the typical compiler invocation and the resulting code format; 
     FIGS. 4A and 4B illustrate the modified computer program instruction and the resulting code format; 
     FIG. 5A is a chart of the frequency of selection of each branch; 
     FIG. 5B is a register of the last case seen; 
     FIG. 6 illustrates the program step which controls the last phase of the system; 
     FIG. 7 illustrates the flow diagram of the modification of code based upon the frequency table of FIG. 5A; and 
     FIGS. 8A, 8B, 9A, 9B, 10A, 10B, 11A, 11B, 12A, 12B, 13A and 13B are charts showing the interim steps arriving at the chart of FIG. 5A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before beginning a discussion of the invention, it would perhaps be helpful to review the structure of a typical computing system. Such a typical system is shown in FIG. 2, having CPU 21, cache memory 22, bus 26 connecting main memory 24, to the cache and perhaps several devices 23, 25, connected to the bus. CPU 21, under control of a program, such as Program 30, would typically attempt the retrieve information (data) from the main memory 24. Such an information fetch would typically require 50 nCPU cycles to achieve. However, if the desired information is located in cache 22, then the time requirement could be reduced to about 2 CPU cycles, depending upon the relative speeds of the CPU and the memory 24. 
     There is shown in FIG. 1 system 10 for optimizing code, such as program source code 11, which code is passed through a compiler and optimizer 12A to produce executable code 13A. Compiler and optimizer 12A in the normal mode would operate on code using CC PROGRAM.C (shown in FIG. 3A) to produce executable code 31. The compiled code would contain switch statements, as shown in FIG. 3B. 
     As the code is run, table 50 (as will be detailed hereinafter) is created as shown in FIG. 5A. Table 50 is labeled for the various switch possibilities and has columns for the selection of those possibilities. Initially, before the program is run on the test data, the table of counters will be zero. Once the compiler and optimizer has produced executable code containing the information in FIGS. 5A and 5B, that executable code is run on sample data 14, FIG. 1. As the executable code is running, the system is collecting the information necessary for table 50 to be created and at the end of the execution run, the result of that information is output into an information file 15 (FIG. 1). Information in file 15 is then read by compiler and optimizer 12B, which is the same compiler and optimizer as 12A with a different command line as shown in FIG. 6. 
     FIG. 7 shows an overview of compiler and optimizer 12B, using the data in file 15, as it produces revised executable code. This revised code will run faster than the original executable code by virtue of anticipating the branches of the switch statements and thus having the necessary data prefetched into cache 22. 
     As shown in FIG. 7, each subroutine of the program is read as shown in block 701. Then, in block 702, information file 15 is checked to see if there is any information on that subroutine. In particular, the system is looking for the type of information that was discussed with respect to FIGS. 5A and 5B. Using the information from file 15 and the source code, there is produced, via block 702, object code which has switch statements that are improved over the switch statements that would have been produced without that information. 
     FIGS. 8A-13B illustrate one example of how switch information is gathered and recorded. When a program begins running (FIG. 8A), table 50 of switch values contains all zeros because no switches have occurred. The prior switch condition, shown in register 51 (FIG. 8B) is set to an illegal value indicating that there is no prior switch condition. 
     As the system runs, it detects the switch condition (selected branch) at a point in time and records that condition (branch A) in register 51, as shown in FIG. 9B. Note that file 50 in FIG. 8A is still all blanks because no actual transition has occurred. 
     In FIG. 10A, the system sees a switch to branch B (now recorded in register 51 of FIG. 10B) and, using the previous value A from register 51 of FIG. 9B, the system records the A to B switch by putting a &#34;1&#34; in box 1001. 
     At some later time the system detects a switch to branch C and records that switch in register 51, FIG. 11B. At the same time a &#34;1&#34; is put in box 1101 indicating a branch B to branch C transition. 
     Assume now a passage of time and during this time switch transitions have been detected and recorded. In such a situation table 50 might look like FIG. 12A with 150 A to B transitions now recorded in box 1001, 200 B to C transitions in box 1101, 50 C to B transitions in box 1201 and 8 branch C to branch C transitions in box 1202. 
     At the next transition (perhaps the last transition of the program) there is a branch C to branch B transition. Now, as shown in FIG. 13A, box 1201 contains the number 51 and register 51 (FIG. 13B) contains a B. 
     At this point it is apparent that the transition from branch A to branch B (box 1001) is more likely to occur than is a transition from branch A to itself, to branch C or to branch D. When the system is executing within branch B, a transition to branch C (box 1101) is most likely to occur. The system is not likely to go from B to A or from B to D, nor is it likely to remain in branch B, since these all show zeros. Similarly, if the system is in branch C, it is more likely, per box 1201, to transition to branch B then it is to transition to any other branch. The next most likely branch from C would be to itself per box 1202. 
     It should be noted that although it is hard to tell from the transition tables, there can be a transition from one state to the same state, as shown by the number 3 in box 1203 and the number 8 in box 1202 of FIG. 13A. 
     Using this data, we can, as discussed above with respect to FIG. 1, use information file 15 in conjunction with compiler and optimizer 12B to generate revised executable code 13B, taking advantage of the pattern of execution recorded for the switch statements. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.