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JIT: Repair profile after tail-merging #110656

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JIT: Repair profile after tail-merging #110656

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JIT: Repair profile after tail-merging
jakobbotsch Dec 12, 2024
e43cce3
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jakobbotsch Jan 6, 2025
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17 changes: 12 additions & 5 deletions src/coreclr/jit/compiler.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4657,11 +4657,6 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
//
DoPhase(this, PHASE_CLONE_FINALLY, &Compiler::fgCloneFinally);

// Drop back to just checking profile likelihoods.
//
activePhaseChecks &= ~PhaseChecks::CHECK_PROFILE;
activePhaseChecks |= PhaseChecks::CHECK_LIKELIHOODS;

// Do some flow-related optimizations
//
if (opts.OptimizationEnabled())
Expand All @@ -4672,6 +4667,11 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
return fgHeadTailMerge(true);
});

// Drop back to just checking profile likelihoods.
//
activePhaseChecks &= ~PhaseChecks::CHECK_PROFILE;
activePhaseChecks |= PhaseChecks::CHECK_LIKELIHOODS;

// Merge common throw blocks
//
DoPhase(this, PHASE_MERGE_THROWS, &Compiler::fgTailMergeThrows);
Expand All @@ -4680,6 +4680,13 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
//
DoPhase(this, PHASE_EARLY_UPDATE_FLOW_GRAPH, &Compiler::fgUpdateFlowGraphPhase);
}
else
{
// Drop back to just checking profile likelihoods.
//
activePhaseChecks &= ~PhaseChecks::CHECK_PROFILE;
activePhaseChecks |= PhaseChecks::CHECK_LIKELIHOODS;
}

// Promote struct locals
//
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12 changes: 12 additions & 0 deletions src/coreclr/jit/fgehopt.cpp
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Expand Up @@ -2437,7 +2437,19 @@ PhaseStatus Compiler::fgTailMergeThrows()
case BBJ_SWITCH:
{
JITDUMP("*** " FMT_BB " now branching to " FMT_BB "\n", predBlock->bbNum, canonicalBlock->bbNum);
FlowEdge* prevEdge = fgGetPredForBlock(nonCanonicalBlock, predBlock);
weight_t removedWeight = predBlock->bbWeight * prevEdge->getLikelihood();
fgReplaceJumpTarget(predBlock, nonCanonicalBlock, canonicalBlock);

// In practice, when we have true profile data, we can repair it locally here, since the no-return
// call means that there is no contribution from nonCanonicalBlock to any of its successors.
// Note that this might not be the case if we have profile data from e.g. synthesis, so this
// repair is best-effort only.
canonicalBlock->bbWeight += removedWeight;
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I should probably add a helper for this, but the pattern we've been following for propagating profile data is like so:

if (canonicalBlock->hasProfileWeight())
{
    canonicalBlock->setBBProfileWeight(canonicalBlock->bbWeight + removedWeight);
}

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Updated to follow this pattern. Hopefully at some point hasProfileWeight will be always true and we can get rid of those checks.

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The block weight solver @AndyAyersMS implemented propagates the BBF_PROF_WEIGHT flag to all blocks reachable via normal flow here, so if we start using it relatively early on -- such as a replacement for the current fgComputeBlockWeights and optSetBlockWeights phases -- I imagine we can begin requiring blocks to have profile-derived weights at most flow modification sites.

if (canonicalBlock->bbWeight > BB_ZERO_WEIGHT)
{
canonicalBlock->RemoveFlags(BBF_RUN_RARELY);
}
updated = true;
}
break;
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8 changes: 8 additions & 0 deletions src/coreclr/jit/fgopt.cpp
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Expand Up @@ -6576,6 +6576,14 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
FlowEdge* const newEdge = fgAddRefPred(crossJumpTarget, predBlock);
predBlock->SetKindAndTargetEdge(BBJ_ALWAYS, newEdge);
}

// For tail merge we have a common successor of predBlock and
// crossJumpTarget, so the profile update can be done locally.
crossJumpTarget->bbWeight += predBlock->bbWeight;
if (crossJumpTarget->bbWeight > BB_ZERO_WEIGHT)
{
crossJumpTarget->RemoveFlags(BBF_RUN_RARELY);
}
}

// We changed things
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