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JIT: Fix edge likelihoods in fgOptimizeBranch #113235

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merged 1 commit into from
Mar 7, 2025
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JIT: Fix edge likelihoods in fgOptimizeBranch #113235

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17 changes: 9 additions & 8 deletions src/coreclr/jit/fgopt.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2768,15 +2768,18 @@ bool Compiler::fgOptimizeBranch(BasicBlock* bJump)
FlowEdge* const destFalseEdge = bDest->GetFalseEdge();
FlowEdge* const destTrueEdge = bDest->GetTrueEdge();

// bJump now falls through into the next block
// bJump now falls through into the next block.
// Note that we're deriving the false edge's likelihood from 'destTrueEdge',
// because the comparison in 'bJump' is flipped.
// Similarly, we will derive the true edge's likelihood from 'destFalseEdge'.
//
BasicBlock* const bDestFalseTarget = bJump->Next();
FlowEdge* const falseEdge = fgAddRefPred(bDestFalseTarget, bJump, destFalseEdge);
FlowEdge* const falseEdge = fgAddRefPred(bDestFalseTarget, bJump, destTrueEdge);

// bJump now jumps to bDest's normal jump target
//
fgRedirectTargetEdge(bJump, bDestNormalTarget);
bJump->GetTargetEdge()->setLikelihood(destTrueEdge->getLikelihood());
bJump->GetTargetEdge()->setLikelihood(destFalseEdge->getLikelihood());

bJump->SetCond(bJump->GetTargetEdge(), falseEdge);

Expand All @@ -2787,18 +2790,16 @@ bool Compiler::fgOptimizeBranch(BasicBlock* bJump)
// bJump no longer flows into bDest
//
bDest->decreaseBBProfileWeight(bJump->bbWeight);
bDestNormalTarget->decreaseBBProfileWeight(bJump->bbWeight * destFalseEdge->getLikelihood());
bDestFalseTarget->decreaseBBProfileWeight(bJump->bbWeight * destTrueEdge->getLikelihood());

// Propagate bJump's weight into its new successors
//
bDestNormalTarget->increaseBBProfileWeight(bJump->GetTrueEdge()->getLikelyWeight());
bDestFalseTarget->increaseBBProfileWeight(falseEdge->getLikelyWeight());
bDestNormalTarget->setBBProfileWeight(bDestNormalTarget->computeIncomingWeight());
bDestFalseTarget->setBBProfileWeight(bDestFalseTarget->computeIncomingWeight());

if ((bDestNormalTarget->NumSucc() > 0) || (bDestFalseTarget->NumSucc() > 0))
{
JITDUMP("fgOptimizeBranch: New flow out of " FMT_BB " needs to be propagated. Data %s inconsistent.\n",
fgPgoConsistent ? "is now" : "was already");
bJump->bbNum, fgPgoConsistent ? "is now" : "was already");
fgPgoConsistent = false;
}
}
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