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[InstCombine] Preserve nuw on sub of geps (PR44419) 

Fix https://bugs.llvm.org/show_bug.cgi?id=44419 by preserving the
nuw on sub of geps. We only do this if the offset has a multiplication
as the final operation, as we can't be sure the operations is nuw
in the other cases without more thorough analysis.

Differential Revision: https://reviews.llvm.org/D72048
This commit is contained in:
Nikita Popov 2020-01-01 11:11:05 +01:00
parent 81a3d987ce
commit 0e322c8a1f
3 changed files with 17 additions and 5 deletions
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17
llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
View File

@ -1585,7 +1585,7 @@ Instruction *InstCombiner::visitFAdd(BinaryOperator &I) {
/// &A[10] - &A[0]: we should compile this to "10". LHS/RHS are the pointer
/// operands to the ptrtoint instructions for the LHS/RHS of the subtract.
Value *InstCombiner::OptimizePointerDifference(Value *LHS, Value *RHS,
Type *Ty) {
Type *Ty, bool IsNUW) {
// If LHS is a gep based on RHS or RHS is a gep based on LHS, we can optimize
// this.
bool Swapped = false;
@ -1653,6 +1653,15 @@ Value *InstCombiner::OptimizePointerDifference(Value *LHS, Value *RHS,
// Emit the offset of the GEP and an intptr_t.
Value *Result = EmitGEPOffset(GEP1);
// If this is a single inbounds GEP and the original sub was nuw,
// then the final multiplication is also nuw. We match an extra add zero
// here, because that's what EmitGEPOffset() generates.
Instruction *I;
if (IsNUW && !GEP2 && !Swapped && GEP1->isInBounds() &&
match(Result, m_Add(m_Instruction(I), m_Zero())) &&
I->getOpcode() == Instruction::Mul)
I->setHasNoUnsignedWrap();
// If we had a constant expression GEP on the other side offsetting the
// pointer, subtract it from the offset we have.
if (GEP2) {
@ -2051,13 +2060,15 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
Value *LHSOp, *RHSOp;
if (match(Op0, m_PtrToInt(m_Value(LHSOp))) &&
match(Op1, m_PtrToInt(m_Value(RHSOp))))
if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType()))
if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType(),
I.hasNoUnsignedWrap()))
return replaceInstUsesWith(I, Res);
// trunc(p)-trunc(q) -> trunc(p-q)
if (match(Op0, m_Trunc(m_PtrToInt(m_Value(LHSOp)))) &&
match(Op1, m_Trunc(m_PtrToInt(m_Value(RHSOp)))))
if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType()))
if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType(),
/* IsNUW */ false))
return replaceInstUsesWith(I, Res);
// Canonicalize a shifty way to code absolute value to the common pattern.

3
llvm/lib/Transforms/InstCombine/InstCombineInternal.h
View File

@ -369,7 +369,8 @@ public:
Instruction *visitFNeg(UnaryOperator &I);
Instruction *visitAdd(BinaryOperator &I);
Instruction *visitFAdd(BinaryOperator &I);
Value *OptimizePointerDifference(Value *LHS, Value *RHS, Type *Ty);
Value *OptimizePointerDifference(
Value *LHS, Value *RHS, Type *Ty, bool isNUW);
Instruction *visitSub(BinaryOperator &I);
Instruction *visitFSub(BinaryOperator &I);
Instruction *visitMul(BinaryOperator &I);

2
llvm/test/Transforms/InstCombine/sub-gep.ll
View File

@ -16,7 +16,7 @@ define i64 @test_inbounds([0 x i32]* %base, i64 %idx) {
define i64 @test_inbounds_nuw([0 x i32]* %base, i64 %idx) {
; CHECK-LABEL: @test_inbounds_nuw(
; CHECK-NEXT: [[P2_IDX:%.*]] = shl nsw i64 [[IDX:%.*]], 2
; CHECK-NEXT: [[P2_IDX:%.*]] = shl nuw nsw i64 [[IDX:%.*]], 2
; CHECK-NEXT: ret i64 [[P2_IDX]]
;
%p1 = getelementptr inbounds [0 x i32], [0 x i32]* %base, i64 0, i64 0