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JIT: extend initial profile repair to more cases #84741

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Apr 13, 2023
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JIT: extend initial profile repair to more cases #84741

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69 changes: 66 additions & 3 deletions src/coreclr/jit/fgprofilesynthesis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -528,14 +528,28 @@ void ProfileSynthesis::RepairLikelihoods()
case BBJ_COND:
case BBJ_SWITCH:
{
weight_t sum = SumOutgoingLikelihoods(block);
// Repair if either likelihoods are inconsistent or block weight is zero.
//
weight_t const sum = SumOutgoingLikelihoods(block);
bool const consistent = Compiler::fgProfileWeightsEqual(sum, 1.0, epsilon);
bool const zero = Compiler::fgProfileWeightsEqual(block->bbWeight, 0.0, epsilon);

if (Compiler::fgProfileWeightsEqual(sum, 1.0, epsilon))
if (consistent && !zero)
{
// Leave as is.
break;
}

JITDUMP("Repairing likelihoods in " FMT_BB ", existing likelihood sum " FMT_WT "\n", block->bbNum, sum);
JITDUMP("Repairing likelihoods in " FMT_BB, block->bbNum);
if (!consistent)
{
JITDUMP("; existing likelihood sum: " FMT_WT, sum);
}
if (zero)
{
JITDUMP("; zero weight block");
}
JITDUMP("\n");

if (block->bbJumpKind == BBJ_COND)
{
Expand Down Expand Up @@ -1137,6 +1151,55 @@ void ProfileSynthesis::ComputeCyclicProbabilities(SimpleLoop* loop)
loop->m_head->bbNum, cyclicWeight, cyclicProbability, capped ? " [capped]" : "");

loop->m_cyclicProbability = cyclicProbability;

// Try and adjust loop exit likelihood to reflect capping.
// If there are multiple exits we just adjust the first one we can. This is somewhat arbitrary.
//
if (capped)
{
bool adjustedExit = false;

for (FlowEdge* const exitEdge : loop->m_exitEdges)
{
BasicBlock* const exitBlock = exitEdge->getSourceBlock();

if ((exitBlock->bbJumpKind == BBJ_COND) &&
!Compiler::fgProfileWeightsEqual(exitBlock->bbWeight, 0.0, epsilon))
{
JITDUMP("Will adjust likelihood of the exit edge from loop exit block " FMT_BB
" to reflect capping; current likelihood is " FMT_WT "\n",
exitBlock->bbNum, exitEdge->getLikelihood());

BasicBlock* const jump = exitBlock->bbJumpDest;
BasicBlock* const next = exitBlock->bbNext;
FlowEdge* const jumpEdge = m_comp->fgGetPredForBlock(jump, exitBlock);
FlowEdge* const nextEdge = m_comp->fgGetPredForBlock(next, exitBlock);
weight_t const continueLikelihood = cappedLikelihood / exitBlock->bbWeight;
weight_t const breakLikelihood = 1 - continueLikelihood;

if (jumpEdge == exitEdge)
{
jumpEdge->setLikelihood(breakLikelihood);
nextEdge->setLikelihood(continueLikelihood);
}
else
{
assert(nextEdge == exitEdge);
jumpEdge->setLikelihood(continueLikelihood);
nextEdge->setLikelihood(breakLikelihood);
}
adjustedExit = true;

JITDUMP("New likelihood is " FMT_WT "\n", exitEdge->getLikelihood());
break;
}
}

if (!adjustedExit)
{
JITDUMP("Unable to find suitable exit to carry off capped flow\n");
}
}
}

//------------------------------------------------------------------------
Expand Down