chapter3-m4-ziavras

# Ziavras comparing branch predictors of bits in mn

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Unformatted text preview: istory bits & # of required bits from the branch address S. Ziavras Comparing Branch Predictors • • • • • # of bits in (m,n) predictor: bit 2m x n x E – E: # of prediction entries selected by the branch of prediction entries selected by the branch address (0,2) predictor: 2-bit predictor without global history Example Example 1: # of bits for (0,2) with 4K entries of bits for (0 with 4K entries 20 x 2 x 4K = 8K Example 2: # of bits for predictor in last figure 22 x 2 x 16 = 128 Example 3: # of branch-selected entries (E) in the (2 predictor with 8K bits in the prediction buffer (2,2) predictor with 8K bits in the prediction buffer 22 x 2 x E = 8K E = 1K S. Ziavras Comparison of 2-bit Predictors Non-correlating predictors Often outperforms 2-bit predictor with ∞ entries S. Ziavras Tournament Predictors • • • • • Use multiple predictors, usually – 1 based on global info. & – 1 based on local info. & combine them with a selector Better accuracy at medium sizes (8K-32Kbits) acc at medi si (8K Uses very large #s of prediction bits effectively Most popular in class of multilevel branch predictors popular in class of multilevel branch predictors – Many levels of prediction tables together with an algorithm for choosing among multiple predictors Existing tournament predictors (as in next figure): – A 2-bit saturating counter per branch to choose between predictors between 2 predictors S. Ziavras State Transition Diagram 2-bit counter/branch Pr. 1 wrong Pr. 2 correct Counter incremented if “predicted” predictor is correct & other predictor is wrong; decremented if opposite occurs S. Ziavras Comparison Results for SPEC89 Local 2-bit predictors Correlating predictors predictors Tournament predictors Conditional branch misprediction rate vs. total size S. Ziavras...
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## This document was uploaded on 02/09/2014.

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