JP2010152843A - Circuit for estimating reliability of branch prediction and method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To estimate the reliability of branch prediction with high accuracy. <P>SOLUTION: The branch prediction reliability estimation circuit in a processor includes: an instruction processing pipeline 30 for performing pipeline processing of an input program; a branch predictor 40 for predicting a branch direction of a conditional branch instruction included in the program; and a pipeline gating circuit 20 for determining whether to permit speculation processing execution of an instruction in accordance with a prediction reliability to the conditional branch instruction. The branch prediction reliability estimation circuit is provided with: a reliability determining means for determining the reliability of the prediction from the prediction reliability to the conditional branch instruction; a reliability updating means for updating the reliability of the prediction to the conditional branch instruction; and an allowable threshold storage circuit 12 for storing an allowable threshold to be used to determine the reliability. The reliability determining means compares the allowable threshold input from the allowable threshold storage circuit 12 with the reliability to determine the reliability. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a branch prediction reliability estimation circuit and a method thereof in a processor, particularly a processor having a pipeline structure and performing speculative processing.

  Traditionally, processors have improved processing power by increasing instruction level parallelism. The pipeline structure of the processor is one way to obtain instruction level parallelism. However, the conditional branch instruction does not know the subsequent instruction until the instruction is executed, which hinders performance improvement by the pipeline structure. A branch predictor is required to solve this problem. The branch predictor predicts the branch direction of the conditional branch instruction, and the processor speculatively processes subsequent instruction sequences based on the prediction of the branch predictor. When the prediction by the branch predictor is successful, the processor is not affected by the performance degradation caused by the conditional branch instruction.

  However, if the branch predictor fails in prediction, the instruction sequence erroneously speculatively executed is stopped and deleted. The power consumed for speculative execution of the wrong instruction is wasted, and the power efficiency of the processor is reduced.

  On the other hand, a method called pipeline gating stops speculative processing of a conditional branch instruction with low reliability of prediction. Improve processor power efficiency by reducing wasteful speculative execution. However, if the conditional branch instruction that succeeds in prediction is determined to be unreliable and the speculative processing of the subsequent instruction is not performed, the processing time of the processor is extended compared to the case where the speculative processing is performed, and the effect of improving the power efficiency It will fade. Thus, the effect of pipeline gating depends on the determination of the reliability of prediction, and a method for estimating the reliability with higher accuracy is required.

  The technique for estimating the reliability of prediction is a method called Jacobsen, Rotenberg, and Smith Confidence Estimator (JRS Estimator), or a method of estimating the reliability of branch prediction by using the internal state of a branch predictor having a saturation counter. Has been proposed.

  The JRS Estimator stores n-bit reliability or reliability in the Correct / Incorrect Register History Table. When branch prediction is performed, one reliability is selected, and the presence or absence of reliability is determined from the reliability selected by a device called a reduction function. In Non-Patent Document 1, a plurality of reliability estimation methods have been tried. Among them, a method using a resetting counter shows the highest estimation accuracy.

  In the reliability update by the method using the reset counter, 1 is added to the reliability value when the branch predictor succeeds in prediction, and the reliability value is reset to 0 when the prediction fails.

  The reliability is determined to be reliable if the reliability is equal to or higher than a threshold value, and not reliable if the reliability is less than the threshold value. However, if the prediction fails, the reliability is reset to the minimum value of 0. Therefore, updating the reliability to a value larger than the threshold is an implementation problem and does not affect the accuracy of estimating the reliability. .

  FIG. 2 shows an example of reliability update means and reliability determination means by a method using a resetting counter. A binary number that is a state name represents a reliability value. In FIG. 2, it is determined that the prediction is reliable only when 7 (111), that is, when the reliability is the maximum value. If the prediction is successful, 1 is added to the reliability, and if the prediction fails, the reliability is reset to 0.

Patent Document 1 discloses a method using a branch predictor having a saturation counter, which uses a saturation counter compared to the method using a reset counter in Non-Patent Document 1. The method and principle are the same, and it is inferior to the method using a reset counter in terms of the accuracy of estimation of reliability.
Special table 2008-529191 gazette E. Jacobsen, E. Rotenberg, and JESmith; Assigning Confidence to Conditional Branch Predictions. 1996. Proc. Of 29th Annual International Symposium on Microarchitecture (MICRO-29) p.142-152

  However, the invention according to the related technique has a problem that the estimation accuracy of the reliability is low. Due to this problem, speculative processing is erroneously stopped and the processing speed of the processor is reduced.

  The reason is that the method using the resetting counter cannot tolerate the specific behavior of the conditional branch instruction. In the method using the reset counter, the reliability is reset to 0 when the branch prediction for a certain conditional branch instruction fails. For this reason, when the next branch prediction for the conditional branch instruction is performed, it is determined that the prediction is not reliable.

  However, there are many conditional branch instructions that temporarily behave in a program. For example, many loop processes existing in a program always require a process to exit the loop. The process of exiting the loop is realized by a conditional branch instruction for constructing the loop branching in a direction opposite to the branching direction in which the loop is performed.

  For this reason, in the loop processing, when a unique branching behavior different from the normal branching direction occurs, the method using the reset counter resets the reliability. However, this anomalous behavior is temporary and the prediction becomes successful again. Despite the success of the prediction, many prediction opportunities for reliability are required for the prediction of the conditional branch instructions constituting the loop processing to regain the trust. Until the reliability is restored, the speculative processing of the processor stops erroneously, so that the processing capacity of the processor decreases.

  The present invention has been made in view of the above, and an object of the present invention is to provide a branch prediction reliability estimation circuit and a method for performing branch prediction reliability estimation with higher accuracy than the related art.

  In order to solve the above-described problem, a branch prediction reliability estimation circuit according to the present invention predicts an instruction processing pipeline that pipelines an input program and a branch direction of a conditional branch instruction included in the program. A branch prediction reliability estimation circuit in a processor having a branch predictor and a pipeline gating circuit that determines whether or not to execute speculative processing according to the presence or absence of prediction reliability for the conditional branch instruction, Reliability determination means for determining the reliability of the prediction from the reliability of prediction for the conditional branch instruction; reliability update means for updating the reliability of the prediction for the conditional branch instruction; and And an allowable threshold value storage circuit for storing an allowable threshold value used for the determination, wherein the reliability determination means is input from the allowable threshold value storage circuit. And judging the presence or absence of said reliability by comparing the reliability and the tolerance threshold.

  In order to solve the above-described problem, a branch prediction reliability estimation method according to the present invention predicts an instruction processing pipeline that pipelines an input program and a branch direction of a conditional branch instruction included in the program. A branch prediction reliability estimation method in a processor having a branch predictor and a pipeline gating circuit that determines whether or not to execute speculative processing of an instruction according to the presence or absence of prediction reliability for the conditional branch instruction, An allowable threshold value storing procedure for storing an allowable threshold value used when determining the presence or absence of the reliability of the prediction from the reliability of the prediction for the conditional branch instruction; a reliability determination procedure for determining the presence or absence of the reliability; and the condition A reliability update procedure for updating the reliability of the prediction for a branch instruction, and the reliability determination procedure includes the allowable threshold value storage procedure. Compared to et the inputted allowable threshold and the reliability and judging the presence or absence of the reliability.

  According to the present invention, the reliability determination means allows a specific branching behavior of the conditional branch instruction by allowing a certain prediction failure, and corrects the prediction reliability for the normal branching behavior correctly. Can be determined. At the same time, by using the reliability update means, if the number of prediction failures is unacceptable, the reliability is reset to the lowest state, so that the conditional branch instruction that once lost the trust avoids regaining the trust in a short period of time. Accordingly, it is possible to provide a branch prediction reliability estimation circuit and a method thereof that perform highly accurate branch prediction reliability estimation.

[Configuration of First Embodiment]
A first embodiment of the present invention will be described with reference to FIG. Here, FIG. 3 is a block diagram showing the configuration of the first exemplary embodiment of the present invention.

  According to FIG. 3, the first embodiment of the present invention is a reliability estimation having means for determining the reliability of prediction for a conditional branch instruction of the present invention and means for updating the reliability of prediction for a conditional branch instruction. The circuit 10 includes a reliability storage circuit 11, an allowable threshold storage circuit 12, a pipeline gating circuit 20, an instruction processing pipeline 30, and a branch predictor 40.

  The instruction processing pipeline 30 is a device that processes an instruction sequence input to the processor, and is connected to the reliability estimation circuit 10, the reliability storage circuit 11, the pipeline gating circuit 20, and the branch predictor 40.

  The instruction processing pipeline 30 outputs the address of the conditional branch instruction to the branch predictor 40 and the reliability storage circuit 11 when the conditional branch instruction is input. The instruction processing pipeline 30 outputs the address of the conditional branch instruction and the branch result to the branch predictor 40 and the reliability estimation circuit 10 when the execution of the conditional branch instruction is completed.

  The branch predictor 40 is connected to the instruction processing pipeline 30, performs branch prediction, and outputs the result to the instruction processing pipeline 30. The branch predictor 40 updates the prediction when the branch result of the conditional branch instruction is input.

  The reliability storage circuit 11 is connected to the instruction processing pipeline 30 and the reliability estimation circuit 10. The reliability storage circuit 11 reads the reliability from the address of the input conditional branch instruction and outputs the reliability to the reliability estimation circuit 10. The reliability storage circuit 11 receives and stores the conditional branch instruction address and the updated reliability from the reliability estimation circuit 10.

  The reliability estimation circuit 10 is connected to the reliability storage circuit 11, the allowable threshold storage circuit 12, the pipeline gating circuit 20, and the instruction processing pipeline 30, and when the reliability is input from the reliability storage circuit 11. Then, the allowable threshold value input from the allowable threshold value storage circuit 12 is compared with the reliability to determine the presence or absence of reliability, and the determination result is output to the pipeline gating device 20. The reliability estimation circuit 10 calculates the reliability using the address of the conditional branch instruction executed from the instruction processing pipeline 30 and the allowable threshold value input from the allowable threshold value storage circuit 12 when the branch result is input. Update and output to the reliability storage circuit 11.

  The allowable threshold value storage circuit 12 stores an allowable threshold value used for determination of reliability in the reliability estimation circuit 10.

  The pipeline gating circuit 20 is connected to the reliability estimation circuit 10 and the instruction processing pipeline 30, and the reliability information of the conditional branch instruction input from the reliability estimation circuit 10 and the instruction processing pipeline 30. Control of stop and restart of instruction fetch by the instruction processing pipeline 30 is performed based on the information about the executed conditional branch instruction.

[Operation of First Embodiment]
FIG. 1 shows an example of reliability update means and reliability determination means performed by the reliability estimation circuit 10. A binary number that is a state name represents a reliability value. The allowable threshold value in FIG. 1 is 4 (100), and the reliability estimation circuit 10 determines that the reliability is input when the reliability input from the allowable threshold value storage circuit 12 is 4 (100), for example. When the reliability input from 12 is 3 (011), it is determined that it is not reliable.

  When the state is updated, the reliability estimation circuit 10 updates the reliability to 6 (110), for example, when the reliability is 5 (101) and the prediction is successful, and allows the conditional branch instruction address. If the reliability is 5 (101) and the prediction fails, the reliability is updated to 4 (100) and output to the allowable threshold storage circuit 12 together with the address of the conditional branch instruction. If the prediction fails at 4 (100), the reliability is reset to 0 (000) and output to the allowable threshold value storage circuit 12 together with the address of the conditional branch instruction.

[Other embodiments]
Next, a second embodiment of the present invention will be described with reference to the drawings. Constituent elements similar to those in the first embodiment are denoted by the same reference numerals as those in the drawing, and detailed description thereof is omitted. Here, FIG. 4 is a block diagram showing a configuration of the second embodiment.

  Referring to FIG. 4, the second embodiment includes a branch prediction storage circuit 41 in place of the branch predictor of the first embodiment.

  The branch prediction storage circuit 41 is a table having one or a plurality of entries, and one entry is composed of 1 bit. The entry stores the direction of branch prediction, for example, if the branch is 1, the branch is established, and if 0, the branch is not established.

  The branch prediction storage circuit 41 is connected to the reliability estimation circuit 10 and the instruction processing pipeline 30 and predicts the read branch according to the address input from the instruction processing pipeline 30. Output to.

  The instruction processing pipeline 30 outputs the address of the conditional branch instruction that has been executed to the reliability estimation circuit 10.

  The reliability estimation circuit 10 is connected to the branch prediction storage circuit 41 in addition to the connection in the first embodiment. The reliability estimation circuit 10 determines the presence / absence of prediction reliability from the prediction reliability for the conditional branch instruction, and updates the reliability and the direction of branch prediction from the input address and branch result. When the reliability is 0 and the prediction fails, the reliability estimation circuit 10 reverses the direction of branch prediction and stores it in the branch prediction storage circuit 41.

  That is, the reliability update means updates the value by increasing or decreasing the reliability value, increases the reliability when the prediction is successful, and increases the reliability when the prediction fails and the reliability is higher than the allowable threshold. Decrease and set the reliability to the lowest value if prediction fails and is below the acceptable threshold.

  When prediction fails after setting the reliability to the lowest value, the branch prediction update means reverses the direction of branch prediction when the branch is taken to when the branch is not taken, or when the branch is not taken and the branch is taken. To do. Thereafter, the reliability update means performs update by increasing or decreasing the value of reliability.

  FIG. 6 is a diagram illustrating an example of the reliability update unit and the reliability determination unit in the second embodiment. The state name is a combination of a 1-bit branch prediction direction and a 3-bit reliability value. In the branch prediction, a branch establishment is represented by 1 and a branch failure is represented by 0. The branch prediction is inverted only when the reliability is 0 and the prediction fails. It is also possible to use a plurality of the allowable thresholds depending on the branch prediction value. In FIG. 6, the allowable threshold for predicting that a branch is taken is set to 5 (101), and the allowable threshold for predicting that a branch is not taken is set to 4 (100).

  The branch prediction storage circuit 41 can be integrated with the reliability storage circuit 11.

  Next, a third embodiment of the present invention will be described with reference to the drawings. Constituent elements similar to those in the first embodiment are denoted by the same reference numerals as those in the drawing, and detailed description thereof is omitted. Here, FIG. 5 is a block diagram showing a configuration of the third embodiment.

  Referring to FIG. 5, in addition to the structure of the first embodiment, an allowable threshold value update circuit 13 and a temperature measurement circuit 50 are provided.

  Information of the temperature measurement circuit 50 is sequentially transmitted to the connected allowable threshold value update circuit 13.

  When the temperature is higher than a preset temperature threshold, the allowable threshold update circuit 13 decreases the above-described allowable threshold value input from the allowable threshold storage circuit 12 and outputs it to the allowable threshold storage circuit 12. The temperature threshold value may be plural. The updated allowable threshold value is used in the reliability estimation circuit 10.

  If the allowable threshold value is decreased, the number of conditional branch instructions that are determined to be reliable increases, and the number of times fetching stops increases. Power consumption is reduced, and the effect of lowering the processor temperature can be expected.

  Those skilled in the art can easily imagine that the signal input from the thermometer circuit 50 to the allowable threshold value update circuit 13 can be substituted by a deceleration command from another processor or a deceleration command from an operating system or software operating on the system.

  In addition, by improving the accuracy of reliability estimation, it is possible to reduce erroneous stoppage of speculative processing due to pipeline gating. As a result, it is possible to suppress the performance degradation of the processor and improve the power efficiency. The content rate of conditional branch instructions that are successfully predicted in a set of conditional branch instructions that are determined to be low in reliability is reduced, and a reduction in processing speed is suppressed.

  The present invention can improve the power efficiency of a processor by applying it to a processor, particularly a processor having a pipeline structure and performing speculative processing.

It is a figure which shows an example of the reliability update means and reliability determination means which are performed by the reliability estimation circuit of 1st Embodiment. It is a figure which shows an example of the reliability update means by the method using a reset counter, and a reliability determination means. It is a block diagram which shows the structure of 1st Embodiment. It is a block diagram which shows the structure of 2nd Embodiment. It is a block diagram which shows the structure of 3rd Embodiment. It is a figure which shows an example of the reliability update means and reliability determination means in 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Reliability estimation circuit 11 Reliability memory circuit 12 Allowable threshold memory circuit 13 Allowable threshold update circuit 20 Pipeline gating circuit 30 Instruction processing pipeline 40 Branch predictor 41 Branch prediction memory circuit 50 Temperature measurement circuit

Claims (16)

An instruction processing pipeline that pipelines an input program, a branch predictor that predicts the branch direction of a conditional branch instruction included in the program, and an instruction depending on whether or not the prediction reliability of the conditional branch instruction is reliable A pipeline gating circuit that determines whether or not to execute speculative processing, and a reliability estimation circuit for branch prediction in a processor having:
Reliability determination means for determining the reliability of the prediction from the reliability of the prediction for the conditional branch instruction;
Reliability update means for updating the reliability of the prediction for the conditional branch instruction;
An allowable threshold value storage circuit for storing an allowable threshold value used for the determination of the reliability;
With
A reliability estimation circuit for branch prediction, wherein the reliability determination unit determines the presence or absence of the reliability by comparing the reliability with the allowable threshold value input from the allowable threshold value storage circuit.   The reliability update means updates the value by increasing or decreasing the value of the reliability, increases the reliability when the prediction is successful, fails in the prediction, and the reliability is higher than the allowable threshold. 2. The branch prediction reliability estimation circuit according to claim 1, wherein the reliability is decreased, and the reliability is reset to a lowest value when the prediction fails and is equal to or less than the allowable threshold. .   The reliability update unit increases the reliability by 1 except when the reliability is the maximum value when the prediction is successful, and the prediction fails and the reliability is higher than the allowable threshold. 3. The reliability estimation of branch prediction according to claim 2, wherein the reliability is decreased by 1, and the reliability is reset to 0 when the prediction fails and the reliability is equal to or less than the allowable threshold. circuit. Branch prediction in a processor having an instruction processing pipeline that pipelines an input program, and a pipeline gating circuit that determines whether or not to execute speculative processing of an instruction according to the presence or absence of prediction reliability for a conditional branch instruction The reliability estimation circuit of
Branch prediction update means for updating the branch prediction direction of the conditional branch instruction from branch establishment to branch failure or from branch failure to branch establishment from the address input from the instruction processing pipeline and the branch result of the conditional branch instruction; ,
Reliability determination means for determining the reliability of the prediction from the reliability of the prediction for the conditional branch instruction;
Reliability update means for updating the reliability of the prediction for the conditional branch instruction;
An allowable threshold value storage circuit for storing an allowable threshold value used for the determination of the reliability;
With
A reliability estimation circuit for branch prediction, wherein the reliability determination unit determines the presence or absence of the reliability by comparing the reliability with the allowable threshold value input from the allowable threshold value storage circuit. The reliability update means performs update by increasing or decreasing the reliability value in each of branch establishment or branch failure, increases the reliability when the prediction is successful, fails the prediction, and Decreasing the reliability when the reliability is higher than the allowable threshold, resetting the reliability to the lowest value when the prediction fails and below the allowable threshold,
The branch prediction update means, when the prediction fails after resetting the reliability to the lowest value, when the branch is established, when the branch is not established, or when the branch is established, and when the branch is established Invert each
5. The reliability estimation circuit for branch prediction according to claim 4, wherein the reliability update means updates the reliability by increasing or decreasing the reliability value after the inversion.   When the prediction is successful, the reliability update unit increases the reliability by 1 except when the reliability is the maximum value, and the prediction fails and the reliability is higher than the allowable threshold. 6. The reliability estimation for branch prediction according to claim 5, wherein the reliability is decreased by 1, and the reliability is reset to 0 when the prediction fails and the reliability is equal to or less than the allowable threshold. circuit. Measuring means for measuring changes in the external environment;
Threshold update means for dynamically changing a threshold value for determining the reliability of the branch instruction;
With
The branch prediction reliability estimation circuit according to claim 1, wherein the threshold update unit dynamically controls the speed and power consumption of the processor.   The measuring means measures the temperature of the system, and the threshold updating means dynamically changes a threshold value for determining the reliability of the branch instruction from the temperature input from the measuring means. The branch prediction reliability estimation circuit according to claim 7. An instruction processing pipeline that pipelines an input program, a branch predictor that predicts the branch direction of a conditional branch instruction included in the program, and an instruction depending on whether or not the prediction reliability of the conditional branch instruction is reliable A pipeline gating circuit that determines whether or not to execute speculative processing, and a branch prediction reliability estimation method in a processor having:
An allowable threshold value storing procedure for storing an allowable threshold value used when determining the reliability of the prediction from the reliability of the prediction for the conditional branch instruction;
A reliability determination procedure for determining the presence or absence of the reliability;
A reliability update procedure for updating the reliability of the prediction for the conditional branch instruction;
With
The reliability estimation method of branch prediction, wherein the reliability determination procedure determines the presence or absence of the reliability by comparing the reliability with the allowable threshold value input from the allowable threshold value storage procedure.   The reliability update procedure updates by increasing or decreasing the value of the reliability, increases the reliability when the prediction is successful, fails in the prediction, and the reliability is higher than the allowable threshold 10. The reliability estimation method for branch prediction according to claim 9, wherein the reliability is decreased in a case, and the reliability is reset to a lowest value when the prediction fails and is equal to or less than the allowable threshold. .   The reliability update procedure increases the reliability by 1 except when the reliability is the maximum value when the prediction is successful, and the prediction fails and the reliability is higher than the allowable threshold. 11. The reliability estimation of branch prediction according to claim 10, wherein the reliability is decreased by 1, and the reliability is reset to 0 when the prediction fails and the reliability is equal to or less than the allowable threshold. Method. Branch prediction in a processor having an instruction processing pipeline that pipelines an input program, and a pipeline gating circuit that determines whether or not to execute speculative processing of an instruction according to the presence or absence of prediction reliability for a conditional branch instruction The reliability estimation method of
A branch prediction update procedure for updating the branch prediction direction of the conditional branch instruction from branch establishment to branch failure or branch failure to branch establishment from the address input from the instruction processing pipeline and the branch result of the conditional branch instruction; ,
An allowable threshold value storing procedure for storing an allowable threshold value used when determining the reliability of the prediction from the reliability of the prediction for the conditional branch instruction;
A reliability determination procedure for determining the presence or absence of the reliability;
A reliability update procedure for updating the reliability of the prediction for the conditional branch instruction;
With
The reliability estimation method of branch prediction, wherein the reliability determination procedure determines the presence or absence of the reliability by comparing the reliability with the allowable threshold value input from the allowable threshold value storage procedure. The reliability update procedure is performed by increasing or decreasing the reliability value in each of branch establishment or branch failure, increasing the reliability when the prediction is successful, failing the prediction, and Decreasing the reliability when the reliability is higher than the allowable threshold, resetting the reliability to the lowest value when the prediction fails and below the allowable threshold,
In the branch prediction update procedure, when the prediction fails after resetting the reliability to the lowest value, the case where the branch is taken from the case where the branch is taken or the case where the branch is taken from the case where the branch is not taken Invert each
13. The reliability estimation method for branch prediction according to claim 12, wherein, in the reliability update procedure, after the inversion, the reliability is updated by increasing or decreasing the reliability value.   The reliability update procedure increases the reliability by 1 except when the reliability is the maximum value when the prediction is successful, and the prediction fails and the reliability is higher than the allowable threshold. 14. The reliability estimation of branch prediction according to claim 13, wherein the reliability is decreased by 1, and the reliability is reset to 0 when the previous prediction fails and the previous reliability is less than or equal to the allowable threshold. Method. A measurement procedure for measuring changes in the external environment;
A threshold update procedure for dynamically changing a threshold value for determining the reliability of the branch instruction;
With
The branch prediction reliability estimation method according to claim 9, wherein the threshold update procedure dynamically controls the speed and power consumption of the processor.   The measurement procedure measures a system temperature, and the threshold update procedure dynamically changes a threshold value for determining the reliability of the branch instruction from the temperature input from the measurement procedure. The branch prediction reliability estimation method according to claim 15.

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