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[InstCombine] Fold select (A &/| B), T, F if select B, T, F is foldable #76621

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Merged
merged 2 commits into from
Dec 31, 2023
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[InstCombine] Fold select (A &/| B), T, F if select B, T, F is foldable #76621

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f06526b
[InstCombine] Add pre-commit tests. NFC.
dtcxzyw Dec 31, 2023
ff104ef
[InstCombine] Fold `select (A &/| B), T, F` if `select B, T, F` is fo…
dtcxzyw Dec 31, 2023
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46 changes: 46 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3794,5 +3794,51 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
if (Instruction *I = foldBitCeil(SI, Builder))
return I;

// Fold:
// (select A && B, T, F) -> (select A, (select B, T, F), F)
// (select A || B, T, F) -> (select A, T, (select B, T, F))
// if (select B, T, F) is foldable.
// TODO: preserve FMF flags
auto FoldSelectWithAndOrCond = [&](bool IsAnd, Value *A,
Value *B) -> Instruction * {
if (Value *V = simplifySelectInst(B, TrueVal, FalseVal,
SQ.getWithInstruction(&SI)))
return SelectInst::Create(A, IsAnd ? V : TrueVal, IsAnd ? FalseVal : V);

// Is (select B, T, F) a SPF?
if (CondVal->hasOneUse() && SelType->isIntOrIntVectorTy()) {
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It would be profitable if we can fold more decomposed patterns here (e.g., foldAndOrOfICmps).
But I don't think it is a scalable approach.
See also #76623 (comment).

Value *LHS, *RHS;
if (ICmpInst *Cmp = dyn_cast<ICmpInst>(B))
if (Value *V = canonicalizeSPF(*Cmp, TrueVal, FalseVal, *this))
return SelectInst::Create(A, IsAnd ? V : TrueVal,
IsAnd ? FalseVal : V);
}

return nullptr;
};

Value *LHS, *RHS;
if (match(CondVal, m_And(m_Value(LHS), m_Value(RHS)))) {
if (Instruction *I = FoldSelectWithAndOrCond(/*IsAnd*/ true, LHS, RHS))
return I;
if (Instruction *I = FoldSelectWithAndOrCond(/*IsAnd*/ true, RHS, LHS))
return I;
} else if (match(CondVal, m_Or(m_Value(LHS), m_Value(RHS)))) {
if (Instruction *I = FoldSelectWithAndOrCond(/*IsAnd*/ false, LHS, RHS))
return I;
if (Instruction *I = FoldSelectWithAndOrCond(/*IsAnd*/ false, RHS, LHS))
return I;
} else {
// We cannot swap the operands of logical and/or.
// TODO: Can we swap the operands by inserting a freeze?
if (match(CondVal, m_LogicalAnd(m_Value(LHS), m_Value(RHS)))) {
if (Instruction *I = FoldSelectWithAndOrCond(/*IsAnd*/ true, LHS, RHS))
return I;
} else if (match(CondVal, m_LogicalOr(m_Value(LHS), m_Value(RHS)))) {
if (Instruction *I = FoldSelectWithAndOrCond(/*IsAnd*/ false, LHS, RHS))
return I;
}
}

return nullptr;
}
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