Hi everyone,I've noticed a slight problem in using the branch predictor that's causing it to update its state with the incorrect outcome of the branch. I'm looking at O3CPU here. The problem comes in bpred_unit_impl.hh in predict(). If the BTB doesn't supply a target address, then pred_taken is set to 'false'. This is done without updating the predict_record which could have the branch down as predicted taken. If the branch then isn't taken, the predictor is updated but with the possibly incorrect value from the predict_record.
I've attached a couple of patches to show what I mean. The first, 'test-bpred.patch' adds in a new trace flag 'TestBPred' simply to isolate the problem and show it up. The second, 'btb-invalid-fix-prediction.patch' contains the fix. For example, using build/ALPHA_SE/tests/opt/quick/00.hello/alpha/linux/o3-timing I get the following output first:
info: Entering event queue @ 0. Starting simulation...49000: system.cpu.BPredUnit: First predicted 0x120000170 taken, now not taken
target 0x120000170 52500: system.cpu.commit: Committed 0x120000170 53000: system.cpu.BPredUnit: Updating bpred for insn 0x120000170 as taken 53000: system.cpu.commit: Committed 0x120000174 53500: system.cpu.commit: Committed 0x120000178 ....As you can see, 0x120000170 obviously wasn't taken but the predictor is updated as though it was because the initial prediction was changed when the BTB couldn't supply a target address. After applying the fix we get this:
info: Entering event queue @ 0. Starting simulation...49000: system.cpu.BPredUnit: First predicted 0x120000170 taken, now not taken
target 0x120000170 52500: system.cpu.commit: Committed 0x12000017053000: system.cpu.BPredUnit: Updating bpred for insn 0x120000170 as not taken
53000: system.cpu.commit: Committed 0x120000174 53500: system.cpu.commit: Committed 0x120000178 ....Now the predictor is correctly updated with the knowledge that the branch wasn't taken.
Is my assessment of this right? Cheers Tim -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.
btb-invalid-fix-prediction.patch
Description: Binary data
test-bpred.patch
Description: Binary data
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