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JIT: ensure magic divisors are always marked DONT_CSE for all targets #129856

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JIT: ensure magic divisors are always marked DONT_CSE for all targets #129856

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JIT: ensure magic divisors are always marked DONT_CSE for all targets
AndyAyersMS Jun 25, 2026
f11b2fa
JIT: extend UsesDivideByConstOptimized to negative magic and pow2 div…
AndyAyersMS Jun 25, 2026
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11 changes: 9 additions & 2 deletions src/coreclr/jit/gentree.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -10464,7 +10464,12 @@ bool GenTreeOp::UsesDivideByConstOptimized(Compiler* comp)
// x / -1 can't be optimized because INT_MIN / -1 is required to throw an exception.
return false;
}
else if (isPow2(divisorValue))
// Match the lowering acceptance for absolute value: lowering's TryLowerConstIntDivOrMod
// turns negative pow2 divisors into shift+neg and negative non-pow2 divisors into
// signed magic-divide sequences, both of which require the divisor be a literal.
size_t absDivisorValue = (divisorValue == SSIZE_T_MIN) ? static_cast<size_t>(divisorValue)
: static_cast<size_t>(std::abs(divisorValue));
if (isPow2(absDivisorValue))
{
return true;
}
Expand Down Expand Up @@ -10514,7 +10519,9 @@ bool GenTreeOp::UsesDivideByConstOptimized(Compiler* comp)

// TODO-ARM-CQ: Currently there's no GT_MULHI for ARM32
#if defined(TARGET_XARCH) || defined(TARGET_ARM64) || defined(TARGET_LOONGARCH64) || defined(TARGET_RISCV64)
if (!comp->opts.MinOpts() && ((divisorValue >= 3) || !isSignedDivide))
// For signed divides also accept negative magic divisors (e.g. -3, -5); lowering generates
// a reciprocal-multiply sequence for those, see TryLowerConstIntDivOrMod.
if (!comp->opts.MinOpts() && (!isSignedDivide || (divisorValue >= 3) || (divisorValue <= -3)))
{
// All checks pass we can perform the division operation using a reciprocal multiply.
return true;
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1 change: 1 addition & 0 deletions src/coreclr/jit/morph.cpp
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Expand Up @@ -11396,6 +11396,7 @@ GenTree* Compiler::fgMorphSmpOpOptional(GenTreeOp* tree, bool* optAssertionPropD
DEBUG_DESTROY_NODE(tree);
return op1;
}
tree->AsOp()->CheckDivideByConstOptimized(this);
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break;

case GT_UDIV:
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