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[InstCombine] Fold gep inbounds of null to null 

Effectively, this is what we were previously already doing when
the GEP was used in conjunction with a load or store, but this
fold can also be applied more generally:

> The only in bounds address for a null pointer in the default
> address-space is the null pointer itself.
This commit is contained in:
Nikita Popov 2020-12-23 21:18:48 +01:00
parent 87087a02ae
commit eb79fd3c92
4 changed files with 13 additions and 24 deletions
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25
llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
View File

@ -903,29 +903,14 @@ static Instruction *replaceGEPIdxWithZero(InstCombinerImpl &IC, Value *Ptr,
}
static bool canSimplifyNullStoreOrGEP(StoreInst &SI) {
if (NullPointerIsDefined(SI.getFunction(), SI.getPointerAddressSpace()))
return false;
auto *Ptr = SI.getPointerOperand();
if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Ptr))
if (GEPI->isInBounds())
Ptr = GEPI->getOperand(0);
return (isa<ConstantPointerNull>(Ptr) &&
!NullPointerIsDefined(SI.getFunction(), SI.getPointerAddressSpace()));
return isa<ConstantPointerNull>(SI.getPointerOperand()) &&
!NullPointerIsDefined(SI.getFunction(), SI.getPointerAddressSpace());
}
static bool canSimplifyNullLoadOrGEP(LoadInst &LI, Value *Op) {
if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) {
const Value *GEPI0 = GEPI->getOperand(0);
if (isa<ConstantPointerNull>(GEPI0) && GEPI->isInBounds() &&
!NullPointerIsDefined(LI.getFunction(), GEPI->getPointerAddressSpace()))
return true;
}
if (isa<UndefValue>(Op) ||
(isa<ConstantPointerNull>(Op) &&
!NullPointerIsDefined(LI.getFunction(), LI.getPointerAddressSpace())))
return true;
return false;
return isa<UndefValue>(Op) ||
(isa<ConstantPointerNull>(Op) &&
!NullPointerIsDefined(LI.getFunction(), LI.getPointerAddressSpace()));
}
Instruction *InstCombinerImpl::visitLoadInst(LoadInst &LI) {

6
llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
View File

@ -1855,6 +1855,12 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
Value *PtrOp = GEP.getOperand(0);
// The only pointer that is inbounds of null is null.
if (isa<ConstantPointerNull>(PtrOp) && GEP.isInBounds() &&
!NullPointerIsDefined(GEP.getFunction(),
PtrOp->getType()->getPointerAddressSpace()))
return replaceInstUsesWith(GEP, PtrOp);
// Eliminate unneeded casts for indices, and replace indices which displace
// by multiples of a zero size type with zero.
bool MadeChange = false;

3
llvm/test/Transforms/InstCombine/getelementptr.ll
View File

@ -1239,8 +1239,7 @@ define i32* @PR45084_extra_use(i1 %cond, %struct.f** %p) {
define i8* @gep_null_inbounds(i64 %idx) {
; CHECK-LABEL: @gep_null_inbounds(
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i8, i8* null, i64 [[IDX:%.*]]
; CHECK-NEXT: ret i8* [[GEP]]
; CHECK-NEXT: ret i8* null
;
%gep = getelementptr inbounds i8, i8* null, i64 %idx
ret i8* %gep

3
llvm/test/Transforms/InstCombine/store.ll
View File

@ -25,8 +25,7 @@ define void @test2(i32* %P) {
define void @store_at_gep_off_null_inbounds(i64 %offset) {
; CHECK-LABEL: @store_at_gep_off_null_inbounds(
; CHECK-NEXT: [[PTR:%.*]] = getelementptr inbounds i32, i32* null, i64 [[OFFSET:%.*]]
; CHECK-NEXT: store i32 undef, i32* [[PTR]], align 4
; CHECK-NEXT: store i32 undef, i32* null, align 536870912
; CHECK-NEXT: ret void
;
%ptr = getelementptr inbounds i32, i32 *null, i64 %offset