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[InstCombine] Fold icmp samesign u{gt/lt} (X +nsw C2), C -> icmp s{gt/lt} X, (C - C2) #169960

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[InstCombine] Fold icmp samesign u{gt/lt} (X +nsw C2), C -> icmp s{gt/lt} X, (C - C2) #169960

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Pre-commit tests
wermos Nov 28, 2025
445fdd5
Fold `icmp samesign u{gt/lt} (add nsw X, C2), C` -> `icmp s{gt/lt} X,…
wermos Nov 28, 2025
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19 changes: 10 additions & 9 deletions llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3132,7 +3132,7 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,

Value *Op0, *Op1;
Instruction *Ext0, *Ext1;
const CmpInst::Predicate Pred = Cmp.getPredicate();
const CmpPredicate Pred(Cmp.getCmpPredicate());
if (match(Add,
m_Add(m_CombineAnd(m_Instruction(Ext0), m_ZExtOrSExt(m_Value(Op0))),
m_CombineAnd(m_Instruction(Ext1),
Expand Down Expand Up @@ -3167,20 +3167,21 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,

// If the add does not wrap, we can always adjust the compare by subtracting
// the constants. Equality comparisons are handled elsewhere. SGE/SLE/UGE/ULE
// are canonicalized to SGT/SLT/UGT/ULT.
if ((Add->hasNoSignedWrap() &&
(Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SLT)) ||
(Add->hasNoUnsignedWrap() &&
(Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_ULT))) {
// has been canonicalized to SGT/SLT/UGT/ULT.
CmpInst::Predicate ChosenPred = Pred.getPreferredSignedPredicate();
if ((Add->hasNoSignedWrap() && (ChosenPred == ICmpInst::ICMP_SGT ||
ChosenPred == ICmpInst::ICMP_SLT)) ||
(Add->hasNoUnsignedWrap() && (ChosenPred == ICmpInst::ICMP_UGT ||
ChosenPred == ICmpInst::ICMP_ULT))) {
bool Overflow;
APInt NewC =
Cmp.isSigned() ? C.ssub_ov(*C2, Overflow) : C.usub_ov(*C2, Overflow);
APInt NewC = ICmpInst::isSigned(ChosenPred) ? C.ssub_ov(*C2, Overflow)
: C.usub_ov(*C2, Overflow);
// If there is overflow, the result must be true or false.
// TODO: Can we assert there is no overflow because InstSimplify always
// handles those cases?
Comment on lines 3180 to 3181
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@wermos wermos Nov 30, 2025 •
edited
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Can this comment be removed? If I understood this comment correctly, then it is obvious that there are cases where an overflow can happen, so this defensive code is needed. Specifically, the @icmp_samesign_with_nsw_add_neg test causes a ssub overflow to happen.

if (!Overflow)
// icmp Pred (add nsw X, C2), C --> icmp Pred X, (C - C2)
return new ICmpInst(Pred, X, ConstantInt::get(Ty, NewC));
return new ICmpInst(ChosenPred, X, ConstantInt::get(Ty, NewC));
}

if (ICmpInst::isUnsigned(Pred) && Add->hasNoSignedWrap() &&
Expand Down
62 changes: 62 additions & 0 deletions llvm/test/Transforms/InstCombine/icmp-add.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -3440,3 +3440,65 @@ define i1 @val_is_aligend_pred_mismatch(i32 %num) {
%_0 = icmp sge i32 %num.masked, %num
ret i1 %_0
}

define i1 @icmp_samesign_with_nsw_add(i32 %arg0) {
; CHECK-LABEL: @icmp_samesign_with_nsw_add(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[V1:%.*]] = icmp sgt i32 [[ARG0:%.*]], 25
; CHECK-NEXT: ret i1 [[V1]]
;
entry:
%v0 = add nsw i32 %arg0, -18
%v1 = icmp samesign ugt i32 %v0, 7
ret i1 %v1
}

; Negative test; Fold shouldn't fire since -124 - 12 causes signed overflow
define i1 @icmp_samesign_with_nsw_add_neg(i8 %arg0) {
; CHECK-LABEL: @icmp_samesign_with_nsw_add_neg(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[TMP0:%.*]] = add i8 [[ARG0:%.*]], -121
; CHECK-NEXT: [[V1:%.*]] = icmp ult i8 [[TMP0]], 123
; CHECK-NEXT: ret i1 [[V1]]
;
entry:
%v0 = add nsw i8 %arg0, 12
%v1 = icmp samesign ugt i8 %v0, -124
ret i1 %v1
}

define i1 @icmp_with_nuw_add(i32 %arg0) {
; CHECK-LABEL: @icmp_with_nuw_add(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[V1:%.*]] = icmp ult i32 [[ARG0:%.*]], 11
; CHECK-NEXT: ret i1 [[V1]]
;
entry:
%v0 = add nuw i32 %arg0, 7
%v1 = icmp ult i32 %v0, 18
ret i1 %v1
}

define i1 @icmp_partial_negative_samesign_ult_to_slt(i8 range(i8 -1, 5) %x) {
; CHECK-LABEL: @icmp_partial_negative_samesign_ult_to_slt(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i8 [[X:%.*]], 2
; CHECK-NEXT: ret i1 [[CMP]]
;
entry:
%add = add nsw i8 %x, -5
%cmp = icmp samesign ult i8 %add, -3
ret i1 %cmp
}

define i1 @icmp_pos_samesign_slt_to_ult(i8 range(i8 1, 5) %x) {
; CHECK-LABEL: @icmp_pos_samesign_slt_to_ult(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[CMP:%.*]] = icmp samesign ult i8 [[X:%.*]], 2
; CHECK-NEXT: ret i1 [[CMP]]
;
entry:
%add = add nsw i8 %x, 1
%cmp = icmp samesign slt i8 %add, 3
ret i1 %cmp
}
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