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release/20.x: [InstCombine] Do not keep samesign when speculatively executing icmps (#127007) #127391

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Merged
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Feb 18, 2025
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release/20.x: [InstCombine] Do not keep samesign when speculatively executing icmps (#127007) #127391

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5 changes: 5 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5609,6 +5609,11 @@ Instruction *InstCombinerImpl::foldICmpWithMinMax(Instruction &I,
return false;
return std::nullopt;
};
// Remove samesign here since it is illegal to keep it when we speculatively
// execute comparisons. For example, `icmp samesign ult umax(X, -46), -32`
// cannot be decomposed into `(icmp samesign ult X, -46) or (icmp samesign ult
// -46, -32)`. `X` is allowed to be non-negative here.
Pred = static_cast<CmpInst::Predicate>(Pred);
auto CmpXZ = IsCondKnownTrue(simplifyICmpInst(Pred, X, Z, Q));
auto CmpYZ = IsCondKnownTrue(simplifyICmpInst(Pred, Y, Z, Q));
if (!CmpXZ.has_value() && !CmpYZ.has_value())
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24 changes: 24 additions & 0 deletions llvm/test/Transforms/InstCombine/umax-icmp.ll
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Expand Up @@ -804,4 +804,28 @@ end:
ret void
}

define i1 @pr126974(i8 %x) {
; CHECK-LABEL: @pr126974(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[COND:%.*]] = icmp sgt i8 [[X:%.*]], -2
; CHECK-NEXT: br i1 [[COND]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
; CHECK: if.then:
; CHECK-NEXT: [[CMP:%.*]] = icmp ne i8 [[X]], -1
; CHECK-NEXT: ret i1 [[CMP]]
; CHECK: if.else:
; CHECK-NEXT: ret i1 false
;
entry:
%cond = icmp sgt i8 %x, -2
br i1 %cond, label %if.then, label %if.else

if.then:
%umax = call i8 @llvm.umax.i8(i8 %x, i8 -46)
%cmp = icmp samesign ult i8 %umax, -32
ret i1 %cmp

if.else:
ret i1 false
}

declare i32 @llvm.umax.i32(i32, i32)
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