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JIT: Use likelihoods in block reordering decision #101132

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merged 2 commits into from
Apr 17, 2024
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JIT: Use likelihoods in block reordering decision #101132

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d4381b9
Use likelihoods in block reordering decision
amanasifkhalid Apr 16, 2024
084b6d9
Rename + comments
amanasifkhalid Apr 17, 2024
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51 changes: 20 additions & 31 deletions src/coreclr/jit/fgopt.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3587,54 +3587,43 @@ bool Compiler::fgReorderBlocks(bool useProfile)
{
noway_assert(bPrev->KindIs(BBJ_COND));
//
// We will reverse branch if the taken-jump to bDest ratio (i.e. 'takenRatio')
// is more than 51%
// We will reverse branch if the true edge's likelihood is more than 51%.
//
// We will setup profHotWeight to be maximum bbWeight that a block
// could have for us not to want to reverse the conditional branch
// We will set up profHotWeight to be maximum bbWeight that a block
// could have for us not to want to reverse the conditional branch.
//
// We will consider all blocks that have less weight than profHotWeight to be
// uncommonly run blocks as compared with the hot path of bPrev taken-jump to bDest
// uncommonly run blocks compared to the weight of bPrev's true edge.
//
// We will check that the weight of the bPrev to bDest edge
// is more than twice the weight of the bPrev to block edge.
// We will check if bPrev's true edge weight
// is more than twice bPrev's false edge weight.
//
// bPrev --> [BB04, weight 31]
// bPrev --> [BB04, weight 100]
// | \.
// edgeToBlock -------------> O \.
// [min=8,max=10] V \.
// block --> [BB05, weight 10] \.
// falseEdge ---------------> O \.
// [likelihood=0.33] V \.
// block --> [BB05, weight 33] \.
// \.
// edgeToDest ----------------------------> O
// [min=21,max=23] |
// trueEdge ------------------------------> O
// [likelihood=0.67] |
// V
// bDest ---------------> [BB08, weight 21]
// bDest ---------------> [BB08, weight 67]
//
assert(bPrev->FalseTargetIs(block));
FlowEdge* edgeToDest = bPrev->GetTrueEdge();
FlowEdge* edgeToBlock = bPrev->GetFalseEdge();
noway_assert(edgeToDest != nullptr);
noway_assert(edgeToBlock != nullptr);
//
// Calculate the taken ratio
// A takenRatio of 0.10 means taken 10% of the time, not taken 90% of the time
// A takenRatio of 0.50 means taken 50% of the time, not taken 50% of the time
// A takenRatio of 0.90 means taken 90% of the time, not taken 10% of the time
//
double takenCount = edgeToDest->getLikelyWeight();
double notTakenCount = edgeToBlock->getLikelyWeight();
double totalCount = takenCount + notTakenCount;
FlowEdge* trueEdge = bPrev->GetTrueEdge();
FlowEdge* falseEdge = bPrev->GetFalseEdge();
noway_assert(trueEdge != nullptr);
noway_assert(falseEdge != nullptr);

// If the takenRatio (takenCount / totalCount) is greater or equal to 51% then we will reverse
// the branch
if (takenCount < (0.51 * totalCount))
// If we take the true branch more than half the time, we will reverse the branch.
if (trueEdge->getLikelihood() < 0.51)
{
reorderBlock = false;
}
else
{
// set profHotWeight
profHotWeight = edgeToBlock->getLikelyWeight() - 1;
profHotWeight = falseEdge->getLikelyWeight() - 1;
}
}
}
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