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[InstCombine] Treat umax as select(icmp eq x, 0), 1, x) in binop select fold. #65978

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[InstCombine] Treat umax as select(icmp eq x, 0), 1, x) in binop select fold. #65978

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aee299e
[Instcombine] Add tests for treating umax as a select. NFC
davemgreen Sep 11, 2023
dcf0716
[InstCombine] Treat umax as select(icmp eq x, 0), 1, x) in binop sele…
davemgreen Nov 16, 2023
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36 changes: 36 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1097,6 +1097,42 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
if (!LHSIsSelect && !RHSIsSelect)
return nullptr;

// Treat umax(x, y) as select(icmp(ugt, x, y), y, x), if it matches the other
// predicate in A.
auto TryMatchSelectFromMinMax =
[&](bool LHSIsSelect, Value *RHS, bool &RHSIsSelect, Value *A, Value *B,
Value *C, Value *&D, Value *&E, Value *&F) {
if (!LHSIsSelect || RHSIsSelect)
return;

Value *X;

// This is a special case for limit extremes - the general pattern
// require more careful handling for undef. These use EQ predicates with
// a limit value, as opposed to <= or >= predicates, and are apparently
// valid so long as the select has known non-equal operands or are the
// same Value.
CmpInst::Predicate Pred;
const APInt *C1, *C2;
if (match(RHS, m_MaxOrMin(m_Value(X), m_APInt(C2))) &&
match(A, m_c_ICmp(Pred, m_Specific(X), m_APInt(C1))) &&
Pred == ICmpInst::ICMP_EQ &&
((C1->isZero() && *C2 == *C1 + 1 &&
match(RHS, m_UMax(m_Value(F), m_Value(E)))) ||
(C1->isMaxValue() && *C2 == *C1 - 1 &&
match(RHS, m_UMin(m_Value(F), m_Value(E)))) ||
(C1->isMinSignedValue() && *C2 == *C1 + 1 &&
match(RHS, m_SMax(m_Value(F), m_Value(E)))) ||
(C1->isMaxSignedValue() && *C2 == *C1 - 1 &&
match(RHS, m_SMin(m_Value(F), m_Value(E))))) &&
(isKnownNonEqual(B, E, DL) || B == E)) {
RHSIsSelect = true;
D = A;
}
};
TryMatchSelectFromMinMax(LHSIsSelect, RHS, RHSIsSelect, A, B, C, D, E, F);
TryMatchSelectFromMinMax(RHSIsSelect, LHS, LHSIsSelect, D, E, F, A, B, C);

FastMathFlags FMF;
BuilderTy::FastMathFlagGuard Guard(Builder);
if (isa<FPMathOperator>(&I)) {
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