[llvm] [InstCombine] Fold trunc nuw i1 to false when dominated by icmp. (PR #151961)
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Mon Aug 4 06:31:04 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Andreas Jonson (andjo403)
<details>
<summary>Changes</summary>
Proof: https://alive2.llvm.org/ce/z/4BOiFF
---
Full diff: https://github.com/llvm/llvm-project/pull/151961.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp (+32)
- (modified) llvm/test/Transforms/InstCombine/trunc.ll (+140)
``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index a43a6ee1f58b0..8529d7b30b56d 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -951,6 +951,38 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
}
}
+ // 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)))
diff --git a/llvm/test/Transforms/InstCombine/trunc.ll b/llvm/test/Transforms/InstCombine/trunc.ll
index dfe9d941f840c..a03aa88063247 100644
--- a/llvm/test/Transforms/InstCombine/trunc.ll
+++ b/llvm/test/Transforms/InstCombine/trunc.ll
@@ -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
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/151961
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