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[InstCombine] Refine nuw propagation in OptimizePointerDifference #147059

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Merged
merged 3 commits into from
Jul 4, 2025
Merged

[InstCombine] Refine nuw propagation in OptimizePointerDifference #147059

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c93e3b7
[InstCombine] Add pre-commit tests. NFC.
dtcxzyw Jul 4, 2025
44f1ddc
[InstCombine] Refine nuw propagation in `OptimizePointerDifference`
dtcxzyw Jul 4, 2025
fae37f7
[InstCombine] Add one-use check.
dtcxzyw Jul 4, 2025
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10 changes: 7 additions & 3 deletions llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2164,10 +2164,14 @@ Value *InstCombinerImpl::OptimizePointerDifference(Value *LHS, Value *RHS,

// If this is a single inbounds GEP and the original sub was nuw,
// then the final multiplication is also nuw.
if (auto *I = dyn_cast<Instruction>(Result))
if (auto *I = dyn_cast<OverflowingBinaryOperator>(Result))
if (IsNUW && match(Offset2, m_Zero()) && Base.LHSNW.isInBounds() &&
I->getOpcode() == Instruction::Mul)
I->setHasNoUnsignedWrap();
(I->use_empty() || I->hasOneUse()) && I->hasNoSignedWrap() &&
!I->hasNoUnsignedWrap() &&
((I->getOpcode() == Instruction::Mul &&
match(I->getOperand(1), m_NonNegative())) ||
I->getOpcode() == Instruction::Shl))
cast<Instruction>(I)->setHasNoUnsignedWrap();

// If we have a 2nd GEP of the same base pointer, subtract the offsets.
// If both GEPs are inbounds, then the subtract does not have signed overflow.
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69 changes: 69 additions & 0 deletions llvm/test/Transforms/InstCombine/sub-gep.ll
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Expand Up @@ -1089,3 +1089,72 @@ define <2 x i64> @splat_geps_multiple(ptr %base, i64 %idx0, <2 x i64> %idx1, <2
%d = sub <2 x i64> %gep2.int, %gep1.int
ret <2 x i64> %d
}

define i64 @nuw_ptrdiff_shl_nsw(ptr %base, i64 %idx) {
; CHECK-LABEL: @nuw_ptrdiff_shl_nsw(
; CHECK-NEXT: [[OFFSET:%.*]] = shl nuw nsw i64 [[IDX:%.*]], 3
; CHECK-NEXT: ret i64 [[OFFSET]]
;
%offset = shl nsw i64 %idx, 3
%gep = getelementptr inbounds i8, ptr %base, i64 %offset
%lhs = ptrtoint ptr %gep to i64
%rhs = ptrtoint ptr %base to i64
%diff = sub nuw i64 %lhs, %rhs
ret i64 %diff
}

define i64 @nuw_ptrdiff_shl_nonsw(ptr %base, i64 %idx) {
; CHECK-LABEL: @nuw_ptrdiff_shl_nonsw(
; CHECK-NEXT: [[OFFSET:%.*]] = shl i64 [[IDX:%.*]], 3
; CHECK-NEXT: ret i64 [[OFFSET]]
;
%offset = shl i64 %idx, 3
%gep = getelementptr inbounds i8, ptr %base, i64 %offset
%lhs = ptrtoint ptr %gep to i64
%rhs = ptrtoint ptr %base to i64
%diff = sub nuw i64 %lhs, %rhs
ret i64 %diff
}

define i64 @nuw_ptrdiff_mul_nsw_nneg_scale(ptr %base, i64 %idx) {
; CHECK-LABEL: @nuw_ptrdiff_mul_nsw_nneg_scale(
; CHECK-NEXT: [[OFFSET:%.*]] = mul nuw nsw i64 [[IDX:%.*]], 3
; CHECK-NEXT: ret i64 [[OFFSET]]
;
%offset = mul nsw i64 %idx, 3
%gep = getelementptr inbounds i8, ptr %base, i64 %offset
%lhs = ptrtoint ptr %gep to i64
%rhs = ptrtoint ptr %base to i64
%diff = sub nuw i64 %lhs, %rhs
ret i64 %diff
}

define i64 @nuw_ptrdiff_mul_nsw_unknown_scale(ptr %base, i64 %idx, i64 %scale) {
; CHECK-LABEL: @nuw_ptrdiff_mul_nsw_unknown_scale(
; CHECK-NEXT: [[OFFSET:%.*]] = mul nsw i64 [[IDX:%.*]], [[SCALE:%.*]]
; CHECK-NEXT: ret i64 [[OFFSET]]
;
%offset = mul nsw i64 %idx, %scale
%gep = getelementptr inbounds i8, ptr %base, i64 %offset
%lhs = ptrtoint ptr %gep to i64
%rhs = ptrtoint ptr %base to i64
%diff = sub nuw i64 %lhs, %rhs
ret i64 %diff
}

declare void @usei64(i64)

define i64 @nuw_ptrdiff_mul_nsw_nneg_scale_multiuse(ptr %base, i64 %idx) {
; CHECK-LABEL: @nuw_ptrdiff_mul_nsw_nneg_scale_multiuse(
; CHECK-NEXT: [[OFFSET:%.*]] = mul nsw i64 [[IDX:%.*]], 3
; CHECK-NEXT: call void @usei64(i64 [[OFFSET]])
; CHECK-NEXT: ret i64 [[OFFSET]]
;
%offset = mul nsw i64 %idx, 3
call void @usei64(i64 %offset)
%gep = getelementptr inbounds i8, ptr %base, i64 %offset
%lhs = ptrtoint ptr %gep to i64
%rhs = ptrtoint ptr %base to i64
%diff = sub nuw i64 %lhs, %rhs
ret i64 %diff
}
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