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[InstCombine] Fold trunc nuw i1 to false when dominated by icmp. #151961

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[InstCombine] Fold trunc nuw i1 to false when dominated by icmp. #151961

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f13dc12
[InstCombine] Add test for trunc with dominating icmp.
andjo403 Aug 4, 2025
424fe1e
[InstCombine] Fold trunc nuw i1 to false when dominated by icmp.
andjo403 Aug 4, 2025
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32 changes: 32 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -951,6 +951,38 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
}
}

// Fold Trunc nuw i1 With Dominating ICmp
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@dtcxzyw dtcxzyw Aug 5, 2025

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Suggested change
// Fold Trunc nuw i1 With Dominating ICmp
// Fold trunc nuw i1 with dominating icmp.

if (DestWidth == 1 && Trunc.hasNoUnsignedWrap()) {
auto HandleDomCond = [&](ICmpInst::Predicate DomPred,
const APInt *DomC) -> Instruction * {
ConstantRange DominatingCR =
ConstantRange::makeExactICmpRegion(DomPred, *DomC);
if (!DominatingCR.contains(APInt(SrcWidth, 1)))
return replaceInstUsesWith(Trunc, Builder.getFalse());
return nullptr;
};

for (BranchInst *BI : DC.conditionsFor(Src)) {
CmpPredicate DomPred;
const APInt *DomC;
if (!match(BI->getCondition(),
m_ICmp(DomPred, m_Specific(Src), m_APInt(DomC))))
continue;

BasicBlockEdge Edge0(BI->getParent(), BI->getSuccessor(0));
if (DT.dominates(Edge0, Trunc.getParent())) {
if (auto *V = HandleDomCond(DomPred, DomC))
return V;
} else {
BasicBlockEdge Edge1(BI->getParent(), BI->getSuccessor(1));
if (DT.dominates(Edge1, Trunc.getParent()))
if (auto *V =
HandleDomCond(CmpInst::getInversePredicate(DomPred), DomC))
return V;
}
}
}

if (DestWidth == 1 &&
(Trunc.hasNoUnsignedWrap() || Trunc.hasNoSignedWrap()) &&
isKnownNonZero(Src, SQ.getWithInstruction(&Trunc)))
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@andjo403 andjo403 Aug 4, 2025

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only added fold to false as the fold to true is handled here

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140 changes: 140 additions & 0 deletions llvm/test/Transforms/InstCombine/trunc.ll
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Expand Up @@ -1218,3 +1218,143 @@ define i2 @neg_trunc_nsw_i2_non_zero(i8 %1) {
%ret = trunc nsw i8 %1 to i2
ret i2 %ret
}

define i1 @trunc_nuw_i1_dominating_icmp_ne_1(i8 %x) {
; CHECK-LABEL: @trunc_nuw_i1_dominating_icmp_ne_1(
; CHECK-NEXT: [[ICMP_NOT:%.*]] = icmp eq i8 [[X:%.*]], 1
; CHECK-NEXT: br i1 [[ICMP_NOT]], label [[BB2:%.*]], label [[BB1:%.*]]
; CHECK: bb1:
; CHECK-NEXT: ret i1 false
; CHECK: bb2:
; CHECK-NEXT: ret i1 true
;
%icmp = icmp ne i8 %x, 1
br i1 %icmp, label %bb1, label %bb2
bb1:
%ret1 = trunc nuw i8 %x to i1
ret i1 %ret1
bb2:
%ret2 = trunc nuw i8 %x to i1
ret i1 %ret2
}

define i1 @trunc_nuw_i1_dominating_icmp_eq_1(i8 %x) {
; CHECK-LABEL: @trunc_nuw_i1_dominating_icmp_eq_1(
; CHECK-NEXT: [[ICMP:%.*]] = icmp eq i8 [[X:%.*]], 1
; CHECK-NEXT: br i1 [[ICMP]], label [[BB1:%.*]], label [[BB2:%.*]]
; CHECK: bb1:
; CHECK-NEXT: ret i1 true
; CHECK: bb2:
; CHECK-NEXT: ret i1 false
;
%icmp = icmp eq i8 %x, 1
br i1 %icmp, label %bb1, label %bb2
bb1:
%ret1 = trunc nuw i8 %x to i1
ret i1 %ret1
bb2:
%ret2 = trunc nuw i8 %x to i1
ret i1 %ret2
}

define i1 @trunc_nuw_i1_dominating_icmp_ne_0(i8 %x) {
; CHECK-LABEL: @trunc_nuw_i1_dominating_icmp_ne_0(
; CHECK-NEXT: [[ICMP_NOT:%.*]] = icmp eq i8 [[X:%.*]], 0
; CHECK-NEXT: br i1 [[ICMP_NOT]], label [[BB2:%.*]], label [[BB1:%.*]]
; CHECK: bb1:
; CHECK-NEXT: ret i1 true
; CHECK: bb2:
; CHECK-NEXT: ret i1 false
;
%icmp = icmp ne i8 %x, 0
br i1 %icmp, label %bb1, label %bb2
bb1:
%ret1 = trunc nuw i8 %x to i1
ret i1 %ret1
bb2:
%ret2 = trunc nuw i8 %x to i1
ret i1 %ret2
}

define i1 @trunc_nuw_i1_dominating_icmp_eq_0(i8 %x) {
; CHECK-LABEL: @trunc_nuw_i1_dominating_icmp_eq_0(
; CHECK-NEXT: [[ICMP:%.*]] = icmp eq i8 [[X:%.*]], 0
; CHECK-NEXT: br i1 [[ICMP]], label [[BB1:%.*]], label [[BB2:%.*]]
; CHECK: bb1:
; CHECK-NEXT: ret i1 false
; CHECK: bb2:
; CHECK-NEXT: ret i1 true
;
%icmp = icmp eq i8 %x, 0
br i1 %icmp, label %bb1, label %bb2
bb1:
%ret1 = trunc nuw i8 %x to i1
ret i1 %ret1
bb2:
%ret2 = trunc nuw i8 %x to i1
ret i1 %ret2
}

define i1 @neg_trunc_i1_dominating_icmp_ne_1(i8 %x) {
; CHECK-LABEL: @neg_trunc_i1_dominating_icmp_ne_1(
; CHECK-NEXT: [[ICMP_NOT:%.*]] = icmp eq i8 [[X:%.*]], 1
; CHECK-NEXT: br i1 [[ICMP_NOT]], label [[BB2:%.*]], label [[BB1:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[RET1:%.*]] = trunc i8 [[X]] to i1
; CHECK-NEXT: ret i1 [[RET1]]
; CHECK: bb2:
; CHECK-NEXT: ret i1 true
;
%icmp = icmp ne i8 %x, 1
br i1 %icmp, label %bb1, label %bb2
bb1:
%ret1 = trunc i8 %x to i1
ret i1 %ret1
bb2:
%ret2 = trunc i8 %x to i1
ret i1 %ret2
}

define i2 @neg_trunc_nuw_i2_dominating_icmp_ne_1(i8 %x) {
; CHECK-LABEL: @neg_trunc_nuw_i2_dominating_icmp_ne_1(
; CHECK-NEXT: [[ICMP_NOT:%.*]] = icmp eq i8 [[X:%.*]], 1
; CHECK-NEXT: br i1 [[ICMP_NOT]], label [[BB2:%.*]], label [[BB1:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[RET1:%.*]] = trunc nuw i8 [[X]] to i2
; CHECK-NEXT: ret i2 [[RET1]]
; CHECK: bb2:
; CHECK-NEXT: ret i2 1
;
%icmp = icmp ne i8 %x, 1
br i1 %icmp, label %bb1, label %bb2
bb1:
%ret1 = trunc nuw i8 %x to i2
ret i2 %ret1
bb2:
%ret2 = trunc nuw i8 %x to i2
ret i2 %ret2
}

define i1 @neg_trunc_nuw_i1_not_dominating_icmp_ne_1(i8 %x, i1 %y) {
; CHECK-LABEL: @neg_trunc_nuw_i1_not_dominating_icmp_ne_1(
; CHECK-NEXT: br i1 [[Y:%.*]], label [[BB1:%.*]], label [[BB0:%.*]]
; CHECK: bb0:
; CHECK-NEXT: [[ICMP_NOT:%.*]] = icmp eq i8 [[X:%.*]], 1
; CHECK-NEXT: br i1 [[ICMP_NOT]], label [[BB2:%.*]], label [[BB1]]
; CHECK: bb1:
; CHECK-NEXT: [[RET1:%.*]] = trunc nuw i8 [[X]] to i1
; CHECK-NEXT: ret i1 [[RET1]]
; CHECK: bb2:
; CHECK-NEXT: ret i1 true
;
br i1 %y, label %bb1, label %bb0
bb0:
%icmp = icmp ne i8 %x, 1
br i1 %icmp, label %bb1, label %bb2
bb1:
%ret1 = trunc nuw i8 %x to i1
ret i1 %ret1
bb2:
%ret2 = trunc nuw i8 %x to i1
ret i1 %ret2
}