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JIT: Avoid introducing data races in RBO #78714

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JIT: Avoid introducing data races in RBO #78714

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JIT: Avoid introducing data races in RBO
jakobbotsch Nov 22, 2022
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jakobbotsch Nov 22, 2022
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16 changes: 13 additions & 3 deletions src/coreclr/jit/redundantbranchopts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1961,16 +1961,26 @@ bool Compiler::optRedundantRelop(BasicBlock* const block)
continue;
}

// If the lcl defined here is live out, forward sub is problematic.
// We'll either create a redundant tree (as the original won't be dead)
// or lose the def (if we actually move the RHS tree).
// If the lcl defined here is live out we'll create a redundant tree
// (as the original won't be dead).
//
if (VarSetOps::IsMember(this, block->bbLiveOut, prevTreeLclDsc->lvVarIndex))
{
JITDUMP(" -- prev tree lcl V%02u is live-out\n", prevTreeLcl);
continue;
}

// The local may have uses between the def and the JTRUE. Duplicating
// reads is not allowed and in any case duplicating it in this case is
// probably not be worth it from a profitability perspective.
//
LclSsaVarDsc* ssaDefDsc = prevTreeLclDsc->GetPerSsaData(prevTreeLHS->AsLclVarCommon()->GetSsaNum());
if (ssaDefDsc->GetNumUses() >= 2)
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@SingleAccretion SingleAccretion Nov 22, 2022

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Notably, this it is not a 100% fix, since we do not count composite uses.

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@jakobbotsch jakobbotsch Nov 22, 2022

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Ah, well that's unfortunate as I was trying to simplify the substitution that happens below.
Do we see the composite case here?

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@SingleAccretion SingleAccretion Nov 22, 2022 •
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It is theoretically possible, though I suspect quite hard to encounter in practice.

(A reproduction would go along the lines of complex field reinterpretations s.t. we end up with LCL_FLD<parent> as the "unaccounted" use)

{
JITDUMP(" -- prev tree lcl V%02u has up to %d uses\n", prevTreeLcl, ssaDefDsc->GetNumUses());
continue;
}

JITDUMP(" -- prev tree is viable candidate for relop fwd sub!\n");
candidateTree = prevTreeRHS;
candidateStmt = prevStmt;
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