A STUDY OF BRANCH PREDICTION STRATEGIES JAMES E.SMITH Presented By: Prasanth Kanakadandi

INTRODUCTION Break the smooth flow of instruction fetch and instruction execution Cause Delay Delay reduced by branch prediction More delay if prediction is wrong

PROGRAMS USED IN THE STUDY 1. ADVAN: Calculates the solution of three simultaneous partial differential equations 2. SCI2:Performs matrix inversion 3. SINCOS: Converts a series of points from polar to Cartesian coordinates 4. SORTST: Sorts a list of 10,000 integers 5. GIBSON: An artificial program that compiles to instructions that roughly satisfy the so called GIBSON mix 6. TBLLNK: Processes a linked list and contains a variety of conditional branches

BRANCH PREDICTION STRATEGIES Two types: Static Strategies Dynamic Strategies Two approaches: Predict that branches are always taken Predict that branches are never taken

STRATEGY 1 Predict that all branches will be taken Results: Program Prediction Accuracy ADVAN 99.4 GIBSON 65.4 SCI SINCOS 80.2 SORTST 57.4 TBLLNK 61.5

Majority of branches taken Success rates vary Program Sensitivity: The algorithm, the programmer and the compiler have an effect on the number of branches taken

Strategy 2 Predict that a branch will be decided the same way as its last execution If not previously executed, predict that it will be taken Not realizable

RESULTS FOR STRATEGY 2 Program Prediction Accuracy ADVAN 98.9 GIBSON 97.9 SCI SINCOS 76.2 SORTST 81.7 TBLLNK 91.7

STRATEGY 1a Predict that all branches with certain opcodes will be taken; predict that others will not be taken Examine which branches are usually taken Results: Program Prediction Accuracy Strategy 1 ADVAN GIBSON SCI SINCOS SORTST TBLLNK

STRATEGY 3 Predict that all backward branches will be taken; predict that all forward branches will not be taken Loops terminated by backward branches Program Sensitivity is high Disadvantage: Prediction is slower

RESULTS FOR STRATEGY 3 Program Prediction Accuracy ADVAN 81.9 GIBSON 98.0 SCI SINCOS 82.5 TBLLNK 84.9

DYNAMIC PREDICTION STRATEGIES

STRATEGY 4 Maintain a table of the most recently used branch instructions that are not taken If a branch instruction is in the table, predict that it will not be taken. Remove table entries if they are taken, and use LRU replacement to add new entries

RESULTS FOR STRATEGY 4 Program Prediction Accuracy Table Size ADVAN GIBSON SCI SINCOS SORTST TBLLNK

A VARIATION TO STRATEGY 4 Faster predictions Instruction words compared with associative memory Observe whether next word is in sequence or out-of-sequence If out-of-sequence,address of the out- of-sequence word is given

Instruction fetch can begin at that location Prediction made before decomposing instruction word into separate instructions

STRATEGY 5 Maintain a bit for each instruction in the cache. If the instruction is a branch instruction, the bit is used to record if it was taken on its last execution. Branches are predicted to be decided as on their last execution; if a branch has not been executed, it is predicted to be taken

RESULTS FOR STRATEGY 5 Program Prediction Accuracy ADVAN 98.9 GIBSON 97.0 SCI SINCOS 76.1 SORTST 81.7 TBLLNK 91.7

IMPROVED DYNAMIC STRATEGIES Use random access memory instead of associative memory Deal with anomalous decisions more effectively

STRATEGY 6 Hash the branch instruction address to m bits and use this index to address a RAM containing the outcome of the most recent branch instruction indexing the same location. Predict that the branch outcome will be the same.

Default prediction controlled by initializing memory to all 0’s or 1’s Results: Program Prediction Accuracy ADVAN 98.9 GIBSON 97.9 SCI SINCOS 76.2 SORTST 81.7 TBLLNK 91.8

STRATEGY 7 Use strategy 6 with twos compliment counts instead of a single bit. Predict that the branch will be taken if the sign bit is 1. Increment the count when a branch is taken; decrement it when a branch is not taken

RESULTS FOR STRATEGY 7 Program Prediction Accuracy 2 Bit Counter 3 Bit Counter ADVAN GIBSON SCI SINCOS SORTST TBLLNK

HIERARCHICAL PREDICTION Time penalty Need for a level of confidence to be attached to branch prediction

AN EXAMPLE For instruction prefetch, if the prediction is incorrect, delay is 6 cp and if the prediction is correct, delay is 3 cp For instruction prefetch and instruction preissue, no delay if the prediction is correct. If the prediction is incorrect, delay is 12 cp

70 % of the branches can be predicted correctly Half of the branches with 50% accuracy (Set A) Other half of with 90% accuracy (Set B)

Possible Strategies and average delays Prefetch for all branches: (0.3 * 6 cp) + (0.7 * 3 cp) = 3.9 cp Prefetch and preissue for all branches: (0.3 * 12 cp) + (0.7 * 0) = 3.6 cp Prefetch for branches in A and prefetch and preissue for branches in B: 0.5 * [(0.5 * 3 cp) + (0.5 * 6 cp)] * [(0.1 * 12 cp) + (0.9 * 0 )] = 2.85 cp

HIERARCHICAL PREDICTION FOR STRATEGY 7 Strategy 7 provides natural way for implementing hierarchical prediction. If a counter is at its maximum value when a prediction is made, the last prediction must have been that the branch be taken, and it must have been correct. A following similar prediction is likely to be correct High confidence prediction at extreme counter values

RESULTS Program Prediction Accuracy % Prediction % Correct %Correct % Correct at extremes at extremes not at extremes overall ADVAN GIBSON SCI SINCOS SORTST TBLLNK (Results with 3 bit counter and 16-word RAM)

CONCLUSION