1 / 24

CPE 432 Computer Design 6 – ILP: Part II – Branch Prediction

CPE 432 Computer Design 6 – ILP: Part II – Branch Prediction. Dr. Gheith Abandah Adapted from the slides of Prof. David Patterson, University of California, Berkeley. Outline. Static Branch Prediction Dynamic Branch Prediction Branch History Table Correlated Branch Prediction

peggy
Download Presentation

CPE 432 Computer Design 6 – ILP: Part II – Branch Prediction

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. CPE 432 Computer Design 6 – ILP: Part II – Branch Prediction Dr. Gheith Abandah Adapted from the slides of Prof. David Patterson, University of California, Berkeley

  2. Outline • Static Branch Prediction • Dynamic Branch Prediction • Branch History Table • Correlated Branch Prediction • Tournament Predictors • Branch Target Buffer CPE 432, 6-ILP2

  3. Static Branch Prediction • We learned how to schedule code around delayed branch • To reorder code around branches, need to predict branch statically when compile • Simplest scheme is to predict a branch as taken • Average misprediction = untaken branch frequency = 34% SPEC • More accurate scheme predicts branches using profile information collected from earlier runs, and modify prediction based on last run: CPE 432, 6-ILP2 Integer Floating Point

  4. Dynamic Branch Prediction • Why does prediction work? • Underlying algorithm has regularities • Data that is being operated on has regularities • Instruction sequence has redundancies that are artifacts of way that humans/compilers think about problems • Is dynamic branch prediction better than static branch prediction? • Seems to be • There are a small number of important branches in programs which have dynamic behavior • Performance = ƒ(accuracy, cost of misprediction) CPE 432, 6-ILP2

  5. Branch History Table • Use the lower k bits of the PC to address index the table Branch Address k <n> Branch History Table (BHT) 2k entries Predict Taken or not Taken CPE 432, 6-ILP2

  6. Taken Taken Not taken 0 1 Not taken Predict not taken Predict taken BHT, n=1 • Problem: in a loop, 1-bit BHT will cause two mispredictions (avg is 9 iterations before exit): • 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 CPE 432, 6-ILP2

  7. T Predict Taken Predict Taken T NT NT NT Predict Not Taken Predict Not Taken T T NT BHT, n=2 • Solution: 2-bit scheme where change prediction only if get misprediction twice • Red: stop, not taken • Green: go, taken • Adds hysteresis to decision making process CPE 432, 6-ILP2

  8. BHT Accuracy • Mispredict because either: • Wrong guess for that branch • Got branch history of wrong branch when index the table • 4096 entry table: Integer CPE 432, 6-ILP2 Floating Point

  9. Outline • Static Branch Prediction • Dynamic Branch Prediction • Branch History Table • Correlated Branch Prediction • Tournament Predictors • Branch Target Buffer CPE 432, 6-ILP2

  10. Correlated Branch Prediction • Aka Two-Level Predictors • Motivation • if (a==2) a=0; • if (b==2) b=0; • if (a != b) {…} Can predict that Condition (3) is not true if Conditions (1) and (2) are true. CPE 432, 6-ILP2

  11. Correlated Branch Prediction • Idea: record m most recently executed branches as taken or not taken, and use that pattern to select the proper n-bit branch history table • In general, (m,n) predictor means record last m branches to select between 2m history tables, each with n-bit counters • Thus, old 2-bit BHT is a (0,2) predictor • Global Branch History: m-bit shift register keeping T/NT status of last m branches. • Each entry in table has mn-bit predictors. CPE 432, 6-ILP2

  12. Correlating Branch Prediction • (2,2) predictor • – Behavior of recent branches selects between four predictions of next branch, updating just that prediction Branch address 4 2-bits per branch predictor Prediction 2-bit global branch history CPE 432, 6-ILP2

  13. Correlated Branch Prediction Example: Find the size of (2, 2) predictor with k=10 Size = 2m * n * 2k = 4 * 2 * 1K = 8 Kbits CPE 432, 6-ILP2

  14. Accuracy of Different Schemes 20% 4096 Entries 2-bit BHT Unlimited Entries 2-bit BHT 1024 Entries (2,2) BHT 18% 16% 14% 12% 11% Frequency of Mispredictions 10% 8% 6% 6% 6% 6% 5% 5% 4% 4% 2% 1% 1% 0% 0% nasa7 matrix300 tomcatv doducd spice fpppp gcc expresso eqntott li 4,096 entries: 2-bits per entry Unlimited entries: 2-bits/entry 1,024 entries (2,2) CPE 432, 6-ILP2

  15. Outline • Static Branch Prediction • Dynamic Branch Prediction • Branch History Table • Correlated Branch Prediction • Tournament Predictors • Branch Target Buffer CPE 432, 6-ILP2

  16. Tournament Predictors • Multilevel branch predictor • Use n-bit saturating counter to choose between predictors • Usual choice between global and local predictors CPE 432, 6-ILP2

  17. Tournament Predictors Tournament predictor using, say, 4K 2-bit counters indexed by local branch address. Chooses between: • Global predictor • 4K entries index by history of last 12 branches (212 = 4K) • Each entry is a standard 2-bit predictor • Local predictor • Local history table: 1024 10-bit entries recording last 10 branches, index by branch address • The pattern of the last 10 occurrences of that particular branch used to index table of 1K entries with 3-bit saturating counters CPE 432, 6-ILP2

  18. Alpha 21264 Branch Predictor <2> 4K Global <12> BHR <2> 4K Selector k=12 addr Select Prediction <3> <10> 1K 1K 10 addr Local Size = 4K*2 + 4K*2 + 1K*10 + 1K*3 = 29 Kbits CPE 432, 6-ILP2

  19. Comparing Predictors (Fig. 2.8) • Advantage of tournament predictor is ability to select the right predictor for a particular branch • Particularly crucial for integer benchmarks. • A typical tournament predictor will select the global predictor almost 40% of the time for the SPEC integer benchmarks and less than 15% of the time for the SPEC FP benchmarks CPE 432, 6-ILP2

  20. Outline • Static Branch Prediction • Dynamic Branch Prediction • Branch History Table • Correlated Branch Prediction • Tournament Predictors • Branch Target Buffer CPE 432, 6-ILP2

  21. Branch Target Buffers (BTB) Branch target calculation is costly and stalls the instruction fetch. BTB stores PCs the same way as caches The PC of a branch is sent to the BTB When a match is found the corresponding Predicted PC is returned If the branch was predicted taken, instruction fetch continues at the returned predicted PC

  22. Branch Target Buffers

  23. Pentium 4 Misprediction Rate (per 1000 instructions, not per branch) 6% misprediction rate per branch SPECint (19% of INT instructions are branch) 2% misprediction rate per branch SPECfp(5% of FP instructions are branch) SPECint2000 SPECfp2000 CPE 432, 6-ILP2

  24. Dynamic Branch Prediction Summary • Prediction becoming important part of execution • Branch History Table: 2 bits for loop accuracy • Correlation: Recently executed branches correlated with next branch • Either different branches (GA) • Or different executions of same branches (PA) • Tournament predictors take insight to next level, by using multiple predictors • usually one based on global information and one based on local information, and combining them with a selector • In 2006, tournament predictors using  30K bits are in processors like the Power5 and Pentium 4 • Branch Target Buffer: include branch address & prediction CPE 432, 6-ILP2

More Related