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Material from Section 4.3

# Material from Section 4.3 - 12-1 This Set 12-1 Material...

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This Set Material from Section 4.3 This set under construction. Outline Branch Prediction Overview One-Level Predictor Two-Level Correlating Predictor Other topics to be added. Sample Problems EE 4720 Lecture Transparency. Formatted 12:24, 14 November 2001 from lsli12.

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Branch and Target Prediction Motivation Branches occur frequently in code. At best, one cycle of branch delay; more with dependencies. Therefore, impact on CPI is large. Techniques Branch Prediction: Predict outcome of branch. (Taken or not taken.) Branch Target Prediction: Predict branch or other CTI’s target address. Branch Folding: Replace branch or other CTI with target instruction. EE 4720 Lecture Transparency. Formatted 12:24, 14 November 2001 from lsli12.
Branch Prediction Methods Covered One-level predictor ( m,n ) two-level correlating predictor or ( m,n ) predictor for short. EE 4720 Lecture Transparency. Formatted 12:24, 14 November 2001 from lsli12.

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Branch Prediction Idea Idea: Predict based on assumption that patterns hold. Example: LOOP: lw r1, 0(r2) ! Read random number, either 0 or 1. addi r2, r2, #4 slt r6, r2, r7 beqz r1, SKIP addi r3, r3, #1 SKIP: bneq r6, LOOP ! Loop executes 100 iterations. nop Second branch, bneq , taken 99 out of 100 executions. Pattern for bneq : T T T ... NT T T T First branch shows no pattern. EE 4720 Lecture Transparency. Formatted 12:24, 14 November 2001 from lsli12.
Prediction Accuracy SPEC89 benchmarks on IBM POWER (predecessor to PowerPC). 18% tomcatv spice SPEC89 benchmarks gcc li 2% 4% 6% 8% 10% 12% 14% 16% 0% 1% 5% 9% 9% 12% 5% 10% 18% nasa7 matrix300 doduc fpppp espresso eqntott 1% 0% Frequency of mispredictions FIGURE 4.14 Prediction accuracy of a 4096-entry two-bit prediction buffer for the SPEC89 benchmarks. EE 4720 Lecture Transparency. Formatted 12:24, 14 November 2001 from lsli12.

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nasa7 1% 0% matrix300 0% 0% tomcatv 1% 0% doduc spice SPEC89 benchmarks fpppp gcc espresso eqntott li 0% 2% 4% 6% 8% 10% 12% 14% 16% 18% 4096 entries: 2 bits per entry Unlimited entries: 2 bits per entry Frequency of mispredictions 5% 5% 9% 9% 9% 9% 12% 11% 5% 5% 18% 18% 10% 10% FIGURE 4.15 Prediction accuracy of a 4096-entry two-bit prediction buffer versus an infinite buffer for the SPEC89 benchmarks. EE 4720 Lecture Transparency. Formatted 12:24, 14 November 2001 from lsli12.
Branch Prediction Terminology Outcome: [of a branch instruction execution]. The outcome of the execution of a branch instruction. T: A taken branch. NT: or N A branch that is not taken. Prediction: [made by branch prediction hardware]. The predicted outcome of a branch. Misprediction: An incorrectly predicted outcome. Prediction Accuracy: [of a branch prediction scheme]. The number of correct predictions divided by the number of predictions. EE 4720 Lecture Transparency. Formatted 12:24, 14 November 2001 from lsli12.

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Branch Prediction Terminology (Continued) Speculative Execution: The execution of instructions following a predicted branch. Misprediction Recovery: Undoing the effect of speculatively executed instructions ...
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