Abstract:
A method and system for preparing branch instruction of a computer program, for compiling and execution in a computer system, in which each transfer instruction is split into two instructions: a control transfer preparation instruction and a control transfer instruction, wherein the control transfer preparation instruction contains the transfer address and is placed by the compiler several instructions ahead of the control transfer instruction, so that the number of clock cycles in the pipeline between transfer condition generation and transfer itself would be reduced.

Description:
This application claims benefit of provisional application Ser. No. 60/071,388 filed Jan. 15, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     Pipeline processor systems are well known in the art. See, e.g., Parallel and Distributed Computing Handbook, A. Y. H. Zomaya, Ed. (McGraw-Hill 1996), which is hereby incorporated by reference. One of the most essential factors affecting processor performance is the delay between the pipeline stage at which a transfer condition becomes known (E stage) and the stage whose operation depends on occurrence of the condition (F stage). In spite of hardware facilities that perform almost perfect dynamic branch direction prediction, mispredictions have a profound impact on CPU performance. Therefore, reduction of this delay, i.e., reduction of the number of clock cycles in the pipeline between transfer condition generation and transfer itself, has an important bearing on CPU performance. 
     SUMMARY OF THE INVENTION 
     The present invention achieves a decrease in the delay between transfer condition generation and branch execution by commencing execution of two or more branches that follow a transfer instruction before determination of the transfer condition. One of the branches may be moved forward along the system&#39;s main pipeline while other branches begin to execute in parallel along additional pipelines initialized by the system. 
     In accordance with a first preferred embodiment of the present invention, each transfer instruction is split into two instructions: a control transfer preparation instruction and a control transfer instruction. The control transfer preparation instruction contains the transfer address and is placed by the compiler several instructions ahead of the control transfer instruction. Execution of the control transfer preparation instruction initializes an additional parallel pipeline which duplicates a certain initial part of the main pipeline and executes instructions from the branch. Once the additional pipeline is filled, it is frozen pending determination of the transfer condition. The control transfer instruction is executed when the control transfer condition becomes known. If control is to be transferred to the branch, the number of the additional pipeline whose execution should be continued on the main pipeline is indicated in the control transfer instruction. 
     In a second preferred embodiment, a first portion of the additional pipeline, which does not use the contents of any register that may be modified by the main pipeline, is filled and frozen. A second portion of the additional pipeline, which may be affected by the contents of a register that may be modified by the main pipeline, is reexecuted every clock cycle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a typical five-stage pipeline system; 
     FIG. 2 a  represents a segment of program code before application of the present invention; and 
     FIG. 2 b  represents one illustrative embodiment of the segment of program code after application of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic representation of a typical five-stage pipeline system. As shown in FIG. 1, pipeline  10  comprises a program counter  12 , an instruction cache  14 , a register file  16 , an ALU  18 , a data cache  20 , and a multiplexer  22 . In addition, pipeline  10  comprises an instruction register  24 , a first source register RSA, a second source register RSB, a first destination register RD 3 , a result register  26 , a second destination register RD 4 , a final result register  28 , and a third destination register RD 5 . 
     During operation, in a first clock cycle, program counter  12  causes the next instruction to be read from instruction cache  14  into instruction register  24 . 
     In a second clock cycle, the instruction in instruction register  24  is decoded into an opcode and three addresses. The first address is that of the first source operand, the second address is that of the second source operand, and the third address is that of the destination operand. The first two addresses are supplied to register file  16  which provides the contents of the addressed registers to source registers RSA and RSB, respectively. The third address is supplied to RD 3 . 
     In a third clock cycle, the content of RSA and RSB are supplied to ALU  18 . The result is clocked into result register  26 . Also on the third clock cycle, the contents of RD 3  are clocked into RD 4 . 
     In a fourth clock cycle, if the operation is a load or store operation, the output of result register  26  is used as an address for a data cache access; if the instruction is a load, the data read out of data cache  20  is clocked out into final result register  28 . If the operation is not a load or store operation, the system does nothing this stage (i.e., top branch of multiplexer  22  is enabled). Also on the fourth clock cycle, the contents of RD 4  are clocked into RD 5 . 
     In a fifth clock cycle, register file  16  is updated by writing the contents of final register  28  to the register identified by the address in RD 5 . 
     FIG. 2 a  represents a segment of program code before application of the present invention. Instruction  100  of the segment is a transfer instruction that transfers the program to instruction  200  if the contents of register Rt&lt; 0 . 
     FIG. 2 b  represents one illustrative embodiment of the segment of program code after application of the present invention. As shown in FIG. 2 b , the compiler has divided transfer instruction  100  into two instructions: a control transfer preparation instruction  84  and a control transfer instruction  102 . 
     Execution of transfer preparation instruction  84  in the main pipeline causes the system to initialize an additional pipeline. The program counter of the additional pipeline is set to the first instruction in the branch (i.e., instruction  200 ). During ensuing clock cycles, the branch instructions execute in the additional pipeline as execution of the rest of the program continues in the main pipeline. 
     In a first preferred embodiment, once the additional pipeline is filled, it is frozen pending resolution of the transfer condition. In this preferred embodiment, actions performed in an additional pipeline should not use registers whose contents may be changed in the main pipeline. 
     In some cases, however, it may be desirable to include actions in the additional pipeline that use registers that may be modified by the main pipeline. For example, assume that prior to an RF read it is desired to calculate an effective RF address by adding the value of a modifiable base pointer to a register number in an operation code. In the first preferred embodiment, this action should not be executed as part of an additional pipeline since the value of the modifiable base pointer might be changed by the main pipeline. In such cases, the second preferred embodiment described below may instead be employed. 
     In the second preferred embodiment, the additional pipeline is divided into two portions. The first portion comprises those stages of the pipeline that are upstream from any operation that accesses a register that may be modified by the main pipeline. The second portion comprises those stages that access a register that may be modified by the main pipeline and all stages downstream from such stages. 
     In the second preferred embodiment, the first portion of the additional pipeline is filled and frozen, as described above in connection with the first preferred embodiment. The second portion of the additional pipeline reexecutes every clock cycle. In this way, the second portion of the additional pipeline is certain to operate on the latest contents of any register that may have been modified by the main pipeline. Advantageously, because the first portion of the additional pipeline is filled and frozen, the increased ICache throughput required to operate the additional pipeline is minimized. 
     Control transfer instruction  102  is executed when the transfer condition becomes known. If control is to be transferred to the branch, the number of the additional pipeline whose execution should be continued on the main pipeline is indicated in the control transfer instruction. 
     It should be noted that the present invention contemplates that more than one additional pipeline may operate simultaneously depending on the system&#39;s capabilities and the number and location of branches in the program. It should also be noted that the approach of the present invention requires an increase in ICACHE throughput. In addition, the direct (fall through) path may be moved forward along the main pipeline, which also leads to performance enhancement. 
     Thus, in accordance with the present invention, the branch turns out to be already moved forward along the pipeline when transfer control instruction  102  is executed. This leads to a reduction in the gap between the moment of the control transfer condition appearance and the target instruction. 
     The number of stages a branch may be moved forward, i.e., the additional pipeline length, depends on CPU architecture and additional hardware which can be used for this purpose. In particular, in some architectures, for example VLIW, a branch may be moved forward up to operand read from the register file. Further lengthening of a pipeline is possible, but requires additional read ports.