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JIT: Run liveness + DCE in tier0 #124060

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JIT: Run liveness + DCE in tier0 #124060

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4 changes: 4 additions & 0 deletions src/coreclr/jit/compiler.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4857,6 +4857,10 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
opts.optRepeatActive = false;
}
}
else if (opts.Tier0OptimizationEnabled())
{
fgTier0Liveness();
}

optLoopsCanonical = false;

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1 change: 1 addition & 0 deletions src/coreclr/jit/compiler.h
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Expand Up @@ -6698,6 +6698,7 @@ class Compiler
}

void fgSsaLiveness();
void fgTier0Liveness();
void fgAsyncLiveness();
void fgPostLowerLiveness();
PhaseStatus fgEarlyLiveness();
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49 changes: 49 additions & 0 deletions src/coreclr/jit/liveness.cpp
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Expand Up @@ -2828,6 +2828,55 @@ void Compiler::fgSsaLiveness()
liveness.Run();
}

//------------------------------------------------------------------------
// fgSsaLiveness: Run SSA liveness.
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Copilot AI Feb 5, 2026

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The comment incorrectly states "Run SSA liveness" which is a copy-paste error from the fgSsaLiveness function above. This should say something like "Run tier0 liveness" or "Run liveness for tier0 optimization" to accurately describe what this function does.

Suggested change
// fgSsaLiveness: Run SSA liveness.
// fgTier0Liveness: Run tier0 liveness.

Copilot uses AI. Check for mistakes.
//
void Compiler::fgTier0Liveness()
{
struct Tier0Liveness : public Liveness<Tier0Liveness>
{
enum
{
SsaLiveness = false,
ComputeMemoryLiveness = false,
IsLIR = false,
IsEarly = false,
};
Comment on lines +2836 to +2844
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Copilot AI Feb 5, 2026

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The PR title states "Run liveness + DCE in tier0", but dead code elimination (DCE) will not actually run with this configuration. DCE only runs when IsEarly=true, which causes the compiler to use the TryRemoveDeadStoreEarly path during liveness analysis. The Tier0Liveness struct sets IsEarly=false, so it will follow the standard liveness path without DCE.

At the call site, fgNodeThreading is NodeThreading::AllTrees, and with IsEarly=false, the liveness analysis follows the code path at lines 1265-1313 in liveness.cpp which does not perform dead store removal. The DCE path at lines 1314-1387 only executes when IsEarly=true.

If the intent is to run DCE in tier0, IsEarly should be set to true. If DCE is not intended, the PR title and description should be updated to remove the mention of DCE.

Copilot uses AI. Check for mistakes.

Tier0Liveness(Compiler* comp)
: Liveness(comp)
{
}
};

if (m_dfsTree == nullptr)
{
m_dfsTree = fgComputeDfs();
}

Tier0Liveness liveness(this);
liveness.Run();

// Rest of the compiler does not expect that we started tracking locals, so reset that state.
for (unsigned i = 0; i < lvaTrackedCount; i++)
{
lvaGetDesc(lvaTrackedToVarNum[i])->lvTracked = false;
}

lvaCurEpoch++;
lvaTrackedCount = 0;
lvaTrackedCountInSizeTUnits = 0;

for (BasicBlock* block : Blocks())
{
block->bbLiveIn = VarSetOps::UninitVal();
block->bbLiveOut = VarSetOps::UninitVal();
block->bbVarUse = VarSetOps::UninitVal();
block->bbVarDef = VarSetOps::UninitVal();
}
fgBBVarSetsInited = false;
}

//------------------------------------------------------------------------
// fgAsyncLiveness: Run async liveness.
//
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