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EECS 470

EECS 470. Branch Prediction Lecture 6 Coverage: Chapter 3. Parts of the predictor. Direction Predictor For conditional branches Predicts whether the branch will be taken Examples: Always taken; backwards taken Address Predictor Predicts the target address (use if predicted taken)

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EECS 470

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  1. EECS 470 Branch Prediction Lecture 6 Coverage: Chapter 3

  2. Parts of the predictor • Direction Predictor • For conditional branches • Predicts whether the branch will be taken • Examples: • Always taken; backwards taken • Address Predictor • Predicts the target address (use if predicted taken) • Examples: • BTB; Return Address Stack; Precomputed Branch • Recovery logic Ref: The Precomputed BranchArchitecture

  3. Characteristics of branches • Individual branches differ • Loops tend not to exit • Unoptimized code: not-taken • Optimized code: taken • If-statements: • Tend to be less predictable • Unconditional branches • Still need address prediction

  4. Example gzip: • gzip: loop branch A@ 0x1200098d8 • Executed: 1359575 times • Taken: 1359565 times • Not-taken: 10 times • % time taken: 99% - 100% Easy to predict (direction and address)

  5. Example gzip: • gzip: if branch B@ 0x12000fa04 • Executed: 151409 times • Taken: 71480 times • Not-taken: 79929 times • % time taken: ~49% Easy to predict? (maybe not/ maybe dynamically)

  6. Easy to predict Easy to predict Example: gzip A B 0 100 Direction prediction: always taken Accuracy: ~73 %

  7. Branch Backwards Most backward branches are heavily NOT-TAKEN Forward branches slightly more likely to be TAKEN Ref: The Effects of Predicated Execution on Branch Prediction

  8. NT T Using history • 1-bit history (direction predictor) • Remember the last direction for a branch Branch History Table branchPC How big is the BHT?

  9. Example: gzip A B 0 100 Direction prediction: always taken Accuracy: ~73 % How many times will branch A mispredict? How many times will branch B mispredict?

  10. SN T NT ST Using history • 2-bit history (direction predictor) Branch History Table branchPC How big is the BHT?

  11. Example: gzip A B 0 100 Direction prediction: always taken Accuracy: ~73 % How many times will branch A mispredict? How many times will branch B mispredict?

  12. Using History Patterns ~80 percent of branches are either heavily TAKEN or heavily NOT-TAKEN For the other 20%, we need to look a patterns of reference to see if they are predictable using a more complex predictor Example: gcc has a branch that flips each time T(1) NT(0)10101010101010101010101010101010101010

  13. NT T Local history branchPC Branch History Table Pattern History Table 10101010 What is the prediction for this BHT 10101010? When do I update the tables?

  14. NT T Local history branchPC Branch History Table Pattern History Table 01010101 On the next execution of this branch instruction, the branch history table is 01010101, pointing to a different pattern What is the accuracy of a flip/flop branch 0101010101010…?

  15. Global history Pattern History Table Branch History Register 01110101 if (aa == 2) aa = 0; if (bb == 2) bb = 0; if (aa != bb) { … How can branches interfere with each other?

  16. Must read! Gshare predictor branchPC Pattern History Table Branch History Register xor 01110101 Ref: Combining Branch Predictors

  17. Bimod predictor Global history reg branchPC xor Choice predictor PHT skewed taken PHT skewed Not-taken mux

  18. Hybrid predictors Global/gshare predictor (much more state) Local predictor (e.g. 2-bit) Prediction 1 Prediction 2 Selection table (2-bit state machine) Prediction How do you select which predictor to use? How do you update the various predictor/selector?

  19. Overriding Predictors • Big predictors are slow, but more accurate • Use a single cycle predictor in fetch • Start the multi-cycle predictor • When it completes, compare it to the fast prediction. • If same, do nothing • If different, assume the slow predictor is right and flush pipline. • Advantage: reduced branch penalty for those branches mispredicted by the fast predictor and correctly predicted by the slow predictor

  20. Pipelined Gshare Predictor • How can we get a pipelined global prediction by stage 1? • Start in stage –2 • Don’t have the most recent branch history… • Access multiple entries • E.g. if we are missing last three branches, get 8 histories and pick between them during fetch stage. Ref: Reconsidering Complex Branch Predictors coming soon (to be published Feb 2003)

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