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[InstCombine] Drop poison-generating flags when reusing existing or instruction #161504

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Merged
merged 2 commits into from
Oct 1, 2025
Merged

[InstCombine] Drop poison-generating flags when reusing existing or instruction #161504

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ebc8049
[InstCombine] Add pre-commit tests. NFC.
dtcxzyw Oct 1, 2025
498e81f
[InstCombine] Drop poison-generating flags when reusing existing or i…
dtcxzyw Oct 1, 2025
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7 changes: 7 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3080,6 +3080,13 @@ InstCombinerImpl::convertOrOfShiftsToFunnelShift(Instruction &Or) {
assert(ZextLowShlAmt->uge(HighSize) &&
ZextLowShlAmt->ule(Width - LowSize) && "Invalid concat");

// We cannot reuse the result if it may produce poison.
// Drop poison generating flags in the expression tree.
// Or
cast<Instruction>(U)->dropPoisonGeneratingFlags();
// Shl
cast<Instruction>(X)->dropPoisonGeneratingFlags();

FShiftArgs = {U, U, ConstantInt::get(Or0->getType(), *ZextHighShlAmt)};
break;
}
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26 changes: 26 additions & 0 deletions llvm/test/Transforms/InstCombine/funnel.ll
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Expand Up @@ -635,3 +635,29 @@ define i32 @test_rotl_and_neg_wrong_mask(i32 %x, i32 %shamt) {
%or = or i32 %shl, %shr
ret i32 %or
}

declare void @use(i16)

; Make sure the reused result does not produce poison.

define i16 @fshl_concat_vector_may_produce_poison(i4 %x, i12 %y) {
; CHECK-LABEL: @fshl_concat_vector_may_produce_poison(
; CHECK-NEXT: [[X_FR:%.*]] = freeze i4 [[X:%.*]]
; CHECK-NEXT: [[ZEXT_X:%.*]] = zext i4 [[X_FR]] to i16
; CHECK-NEXT: [[SLX:%.*]] = shl nuw i16 [[ZEXT_X]], 12
; CHECK-NEXT: [[ZEXT_Y:%.*]] = zext i12 [[Y:%.*]] to i16
; CHECK-NEXT: [[XY:%.*]] = or disjoint i16 [[SLX]], [[ZEXT_Y]]
; CHECK-NEXT: call void @use(i16 [[XY]])
; CHECK-NEXT: [[YX:%.*]] = call i16 @llvm.fshl.i16(i16 [[XY]], i16 [[XY]], i16 4)
; CHECK-NEXT: ret i16 [[YX]]
;
%x.fr = freeze i4 %x
%zext.x = zext i4 %x.fr to i16
%slx = shl nuw nsw i16 %zext.x, 12
%zext.y = zext i12 %y to i16
%xy = or disjoint i16 %slx, %zext.y
call void @use(i16 %xy)
%sly = shl nuw i16 %zext.y, 4
%yx = or disjoint i16 %sly, %zext.x
ret i16 %yx
}