中科大_高性能处理器体系结构_L6_动态分支预测.pdf

发布时间：2022-06-13 发布人：admin 分类：说明书资料大小：0.21M 资料格式：pdf 举报版权申诉

d096a1a5-79ae-4d1c-af2b-b6592f3aa887.pdf-第1页.png

第1页 / 共34页

d096a1a5-79ae-4d1c-af2b-b6592f3aa887.pdf-第2页.png

第2页 / 共34页

d096a1a5-79ae-4d1c-af2b-b6592f3aa887.pdf-第3页.png

第3页 / 共34页

d096a1a5-79ae-4d1c-af2b-b6592f3aa887.pdf-第4页.png

第4页 / 共34页

d096a1a5-79ae-4d1c-af2b-b6592f3aa887.pdf-第5页.png

第5页 / 共34页

d096a1a5-79ae-4d1c-af2b-b6592f3aa887.pdf-第6页.png

第6页 / 共34页

d096a1a5-79ae-4d1c-af2b-b6592f3aa887.pdf-第7页.png

第7页 / 共34页

d096a1a5-79ae-4d1c-af2b-b6592f3aa887.pdf-第8页.png

第8页 / 共34页

文本预览

High Performance Processor Architecture Lecture on (CS05162) Dynamic Branch Prediction Scheme An Hong han@ustc.edu.cn Fall 2007 University of Science and Technology of China Department of Computer Science and Technology

Outline Bimodal Branch Prediction Scheme Two-Level Branch Prediction Scheme 混合预测算法的例子：Alpha 21264的分支预测器 2013-01-29 CS of USTC AN Hong 2

Dynamic Prediction(1)：利用单个分支自身历史（基于模式的预测） Dynamic Prediction ：Use run-time information to make prediction − example: Branch Prediction Buffer(1-位预测器) 2013-01-29 CS of USTC AN Hong 3

Dynamic Prediction： 1-bit BHT Branch History Table − Lower bits of PC address index table of 1-bit values − Says whether or not branch taken last time − No address check Problem: in a loop, 1-bit BHT will cause two mispredictions : − End of loop case, when it exits instead of looping as before − First time through loop on next time through code, when it predicts exit instead of looping 2013-01-29 CS of USTC AN Hong 4

Dynamic Prediction： 1-bit BHT （Branch Prediction Buffer） Pros: − Small. 1 bit per entry can fit lots of entries − Always returns a prediction Cons: − aliasing between branches − one bit of state mispredicts many branches for (I =0; I<10; I++) { a = a + 1; } Two mispredictions per loop invocation 2013-01-29 CS of USTC AN Hong 5

Dynamic Prediction(2)：Bimodal Branch Prediction Scheme（2bits BHT, 2-位饱和预测器） Solution: 2-bit predictor where change prediction only if get misprediction twice: Use extra state to reduce mispredictions at loop ends T = Taken N = Not Taken Predict Taken Predict Not Taken T NT T NT NT T T Predict Taken Predict Not Taken BHT 11 10 00 01 Red: stop, not taken Green: go, taken Adds hysteresis to decision making process NT 2-bit Saturating Up-down Counter 2013-01-29 CS of USTC AN Hong 6

Bimodal Branch Prediction Scheme Strategy: Based on the direction the branch went the last few times it was executed. − Based on a little self-history pattern − Based on a counter Works well: − when each branch is strongly biased in a particular direction. − For scientific/engineering applications where program execution is dominated by inner-loops. 2013-01-29 CS of USTC AN Hong 7

Bimodal Branch Prediction Scheme 例1：…NNNTNNN… − 1-位预测器，出现2次预测错 − 2-位预测器，出现1次预测错例2：TNTNTN…., 初始状态为01的2-位预测器，出现100%预测错 BHT方法准确度 − Mispredict because either: Wrong guess for that branch Got branch history of wrong branch when index the table − 4096 entry table programs vary from 1% misprediction (nasa7, tomcatv) to 18% (eqntott), with spice at 9% and gcc at 12% − 4096 about as good as infinite table (in Alpha 21164) − 2-bit已经足够, n-bit (n>2)与2-bit效果差不多 2013-01-29 CS of USTC AN Hong 8

分享到：

赞收藏

资料库

中科大_高性能处理器体系结构_L6_动态分支预测.pdf

相关推荐

课程资源

热门标签

最新资料