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[InstCombine]: Canonicalize to a mask when trunc nuw #163628

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Merged
merged 4 commits into from
Oct 17, 2025
Merged

[InstCombine]: Canonicalize to a mask when trunc nuw #163628

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07bdb2c
[InstCombine]: Canonicalize to a mask when trunc nuw
kper Oct 15, 2025
6a7e2bd
[InstCombine]: Use other `use2` function
kper Oct 15, 2025
0e2d832
[InstCombine]: Do not mask when trunc nuw
kper Oct 16, 2025
df5680c
[InstCombine]: Run formatter
kper Oct 17, 2025
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4 changes: 2 additions & 2 deletions llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1481,13 +1481,13 @@ Instruction *InstCombinerImpl::foldICmpTruncConstant(ICmpInst &Cmp,
return new ICmpInst(Pred, Y, ConstantInt::get(SrcTy, C.logBase2()));
}

if (Cmp.isEquality() && Trunc->hasOneUse()) {
if (Cmp.isEquality() && (Trunc->hasOneUse() || Trunc->hasNoUnsignedWrap())) {
// Canonicalize to a mask and wider compare if the wide type is suitable:
// (trunc X to i8) == C --> (X & 0xff) == (zext C)
if (!SrcTy->isVectorTy() && shouldChangeType(DstBits, SrcBits)) {
Constant *Mask =
ConstantInt::get(SrcTy, APInt::getLowBitsSet(SrcBits, DstBits));
Value *And = Builder.CreateAnd(X, Mask);
Value *And = Trunc->hasNoUnsignedWrap() ? X : Builder.CreateAnd(X, Mask);
Constant *WideC = ConstantInt::get(SrcTy, C.zext(SrcBits));
return new ICmpInst(Pred, And, WideC);
}
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30 changes: 30 additions & 0 deletions llvm/test/Transforms/InstCombine/icmp-trunc.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
; RUN: opt < %s -passes=instcombine -S -data-layout="n8" | FileCheck %s --check-prefixes=CHECK,DL8

declare void @use(i8)
declare void @use2(i4)

define i1 @ult_2(i32 %x) {
; CHECK-LABEL: @ult_2(
Expand Down Expand Up @@ -785,3 +786,32 @@ define <2 x i1> @uge_nsw_non_splat(<2 x i32> %x) {
ret <2 x i1> %r
}

define i1 @trunc_icmp(i8 %a0) {
; CHECK-LABEL: @trunc_icmp(
; CHECK-NEXT: [[TZ:%.*]] = tail call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[A0:%.*]], i1 false)
; CHECK-NEXT: [[TR:%.*]] = trunc nuw i8 [[TZ]] to i4
; CHECK-NEXT: [[C:%.*]] = icmp eq i8 [[A0]], 0
; CHECK-NEXT: call void @use2(i4 [[TR]])
; CHECK-NEXT: ret i1 [[C]]
;
%tz = tail call range(i8 0, 9) i8 @llvm.cttz.i8(i8 %a0, i1 false)
%tr = trunc i8 %tz to i4
%c = icmp eq i4 %tr, 8
call void @use2(i4 %tr)
ret i1 %c
}

define i1 @do_not_mask_trunc_eq_i32_i8(i32 %x) {
; DL64-LABEL: @do_not_mask_trunc_eq_i32_i8(
; DL64-NEXT: [[R:%.*]] = icmp eq i32 [[X:%.*]], 42
; DL64-NEXT: ret i1 [[R]]
;
; DL8-LABEL: @do_not_mask_trunc_eq_i32_i8(
; DL8-NEXT: [[T:%.*]] = trunc nuw i32 [[X:%.*]] to i8
; DL8-NEXT: [[R:%.*]] = icmp eq i8 [[T]], 42
; DL8-NEXT: ret i1 [[R]]
;
%t = trunc nuw i32 %x to i8
%r = icmp eq i8 %t, 42
ret i1 %r
}