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[InstCombine] Merge constant offset geps across variable geps #156326

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Merged
merged 4 commits into from
Sep 3, 2025
Merged

[InstCombine] Merge constant offset geps across variable geps #156326

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579e9eb
Add additional flag tests
nikic Sep 1, 2025
d478b01
[InstCombine] Merge constant offset geps across variable geps
nikic Jul 31, 2025
16f3d7b
Use intersectForOffsetAdd for final GEP
nikic Sep 2, 2025
684375b
adjust comment style
nikic Sep 2, 2025
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10 changes: 10 additions & 0 deletions llvm/include/llvm/IR/GEPNoWrapFlags.h
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Expand Up @@ -84,6 +84,16 @@ class GEPNoWrapFlags {
return Res;
}

/// Given (gep (gep p, x), y), determine the nowrap flags for
/// (gep (gep, p, y), x).
GEPNoWrapFlags intersectForReassociate(GEPNoWrapFlags Other) const {
GEPNoWrapFlags Res = *this & Other;
// We can only preserve inbounds and nusw if nuw is also set.
if (!Res.hasNoUnsignedWrap())
return none();
return Res;
}

bool operator==(GEPNoWrapFlags Other) const { return Flags == Other.Flags; }
bool operator!=(GEPNoWrapFlags Other) const { return !(*this == Other); }

Expand Down
59 changes: 59 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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Expand Up @@ -2674,6 +2674,62 @@ static Instruction *canonicalizeGEPOfConstGEPI8(GetElementPtrInst &GEP,
return nullptr;
}

/// Combine constant offsets separated by variable offsets.
/// ptradd (ptradd (ptradd p, C1), x), C2 -> ptradd (ptradd p, x), C1+C2
static Instruction *combineConstantOffsets(GetElementPtrInst &GEP,
InstCombinerImpl &IC) {
if (!GEP.hasAllConstantIndices())
return nullptr;

GEPNoWrapFlags NW = GEPNoWrapFlags::all();
SmallVector<GetElementPtrInst *> Skipped;
auto *InnerGEP = dyn_cast<GetElementPtrInst>(GEP.getPointerOperand());
while (true) {
if (!InnerGEP)
return nullptr;

NW = NW.intersectForReassociate(InnerGEP->getNoWrapFlags());
if (InnerGEP->hasAllConstantIndices())
break;

if (!InnerGEP->hasOneUse())
return nullptr;

Skipped.push_back(InnerGEP);
InnerGEP = dyn_cast<GetElementPtrInst>(InnerGEP->getPointerOperand());
}

// The two constant offset GEPs are directly adjacent: Let normal offset
// merging handle it.
if (Skipped.empty())
return nullptr;

// FIXME: This one-use check is not strictly necessary. Consider relaxing it
// if profitable.
if (!InnerGEP->hasOneUse())
return nullptr;

// Don't bother with vector splats.
Type *Ty = GEP.getType();
if (InnerGEP->getType() != Ty)
return nullptr;

const DataLayout &DL = IC.getDataLayout();
APInt Offset(DL.getIndexTypeSizeInBits(Ty), 0);
if (!GEP.accumulateConstantOffset(DL, Offset) ||
!InnerGEP->accumulateConstantOffset(DL, Offset))
return nullptr;

IC.replaceOperand(*Skipped.back(), 0, InnerGEP->getPointerOperand());
for (GetElementPtrInst *SkippedGEP : Skipped)
SkippedGEP->setNoWrapFlags(NW);

return IC.replaceInstUsesWith(
GEP,
IC.Builder.CreatePtrAdd(Skipped.front(), IC.Builder.getInt(Offset), "",
NW.intersectForOffsetAdd(GEP.getNoWrapFlags())));
}

Instruction *InstCombinerImpl::visitGEPOfGEP(GetElementPtrInst &GEP,
GEPOperator *Src) {
// Combine Indices - If the source pointer to this getelementptr instruction
Expand All @@ -2685,6 +2741,9 @@ Instruction *InstCombinerImpl::visitGEPOfGEP(GetElementPtrInst &GEP,
if (auto *I = canonicalizeGEPOfConstGEPI8(GEP, Src, *this))
return I;

if (auto *I = combineConstantOffsets(GEP, *this))
return I;

// For constant GEPs, use a more general offset-based folding approach.
Type *PtrTy = Src->getType()->getScalarType();
if (GEP.hasAllConstantIndices() &&
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154 changes: 143 additions & 11 deletions llvm/test/Transforms/InstCombine/gep-canonicalize-constant-indices.ll
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Expand Up @@ -50,10 +50,9 @@ define ptr @partialConstant2(ptr %p, i64 %a, i64 %b) {
; result = ((ptr) p + a) + 3
define ptr @merge(ptr %p, i64 %a) {
; CHECK-LABEL: @merge(
; CHECK-NEXT: [[TMP1:%.*]] = getelementptr inbounds nuw i8, ptr [[P:%.*]], i64 4
; CHECK-NEXT: [[TMP2:%.*]] = getelementptr inbounds i32, ptr [[TMP1]], i64 [[A:%.*]]
; CHECK-NEXT: [[TMP3:%.*]] = getelementptr inbounds nuw i8, ptr [[TMP2]], i64 8
; CHECK-NEXT: ret ptr [[TMP3]]
; CHECK-NEXT: [[TMP1:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[TMP2:%.*]] = getelementptr i8, ptr [[TMP1]], i64 12
; CHECK-NEXT: ret ptr [[TMP2]]
;
%1 = getelementptr inbounds i32, ptr %p, i64 1
%2 = getelementptr inbounds i32, ptr %1, i64 %a
Expand All @@ -67,13 +66,11 @@ define ptr @merge(ptr %p, i64 %a) {
; result = (ptr) ((ptr) ((ptr) ptr + a) + (a * b)) + 9
define ptr @nested(ptr %p, i64 %a, i64 %b) {
; CHECK-LABEL: @nested(
; CHECK-NEXT: [[TMP1:%.*]] = getelementptr inbounds nuw i8, ptr [[P:%.*]], i64 16
; CHECK-NEXT: [[TMP2:%.*]] = getelementptr inbounds i8, ptr [[TMP1]], i64 [[A:%.*]]
; CHECK-NEXT: [[TMP3:%.*]] = mul i64 [[A]], [[B:%.*]]
; CHECK-NEXT: [[TMP4:%.*]] = getelementptr inbounds nuw i8, ptr [[TMP2]], i64 128
; CHECK-NEXT: [[TMP5:%.*]] = getelementptr inbounds i16, ptr [[TMP4]], i64 [[TMP3]]
; CHECK-NEXT: [[TMP6:%.*]] = getelementptr inbounds nuw i8, ptr [[TMP5]], i64 16
; CHECK-NEXT: ret ptr [[TMP6]]
; CHECK-NEXT: [[TMP1:%.*]] = getelementptr i8, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i64 [[A]], [[B:%.*]]
; CHECK-NEXT: [[TMP3:%.*]] = getelementptr i16, ptr [[TMP1]], i64 [[TMP2]]
; CHECK-NEXT: [[TMP4:%.*]] = getelementptr i8, ptr [[TMP3]], i64 160
; CHECK-NEXT: ret ptr [[TMP4]]
;
%1 = getelementptr inbounds <3 x i32>, ptr %p, i64 1
%2 = getelementptr inbounds i8, ptr %1, i64 %a
Expand Down Expand Up @@ -125,3 +122,138 @@ define ptr @multipleUses3(ptr %p) {
%3 = getelementptr inbounds i32, ptr %1, i64 %2
ret ptr %3
}

define ptr @merge_nuw(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_nuw(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr nuw i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr nuw i8, ptr %p, i64 1
%gep2 = getelementptr nuw i32, ptr %gep1, i64 %a
%gep3 = getelementptr nuw i32, ptr %gep2, i64 1
ret ptr %gep3
}

define ptr @merge_nuw_inbounds(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_nuw_inbounds(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr inbounds nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr inbounds nuw i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr inbounds nuw i8, ptr %p, i64 1
%gep2 = getelementptr inbounds nuw i32, ptr %gep1, i64 %a
%gep3 = getelementptr inbounds nuw i32, ptr %gep2, i64 1
ret ptr %gep3
}

; It would be okay to preserve nusw here, as the constant addition does not
; overflow.
define ptr @merge_nuw_nusw(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_nuw_nusw(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr nusw nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr nuw i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr nusw nuw i8, ptr %p, i64 1
%gep2 = getelementptr nusw nuw i32, ptr %gep1, i64 %a
%gep3 = getelementptr nusw nuw i32, ptr %gep2, i64 1
ret ptr %gep3
}

; Can't preserve nusw on the final GEP
define ptr @merge_nuw_nusw_overflow(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_nuw_nusw_overflow(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr nusw nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr nuw i8, ptr [[GEP2]], i64 -2305843009213693952
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr nusw nuw i8, ptr %p, i64 u0x7000000000000000
%gep2 = getelementptr nusw nuw i32, ptr %gep1, i64 %a
%gep3 = getelementptr nusw nuw i8, ptr %gep2, i64 u0x7000000000000000
ret ptr %gep3
}

define ptr @merge_missing_nuw1(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_missing_nuw1(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr i8, ptr %p, i64 1
%gep2 = getelementptr nuw i32, ptr %gep1, i64 %a
%gep3 = getelementptr nuw i32, ptr %gep2, i64 1
ret ptr %gep3
}

define ptr @merge_missing_nuw2(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_missing_nuw2(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr nuw i8, ptr %p, i64 1
%gep2 = getelementptr i32, ptr %gep1, i64 %a
%gep3 = getelementptr nuw i32, ptr %gep2, i64 1
ret ptr %gep3
}

define ptr @merge_missing_nuw3(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_missing_nuw3(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr nuw i8, ptr %p, i64 1
%gep2 = getelementptr nuw i32, ptr %gep1, i64 %a
%gep3 = getelementptr i32, ptr %gep2, i64 1
ret ptr %gep3
}

define ptr @merge_nuw_missing_inbounds(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_nuw_missing_inbounds(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr nuw i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr nuw i8, ptr %p, i64 1
%gep2 = getelementptr inbounds nuw i32, ptr %gep1, i64 %a
%gep3 = getelementptr inbounds nuw i32, ptr %gep2, i64 1
ret ptr %gep3
}

define ptr @merge_nuw_missing_nusw(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_nuw_missing_nusw(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr nuw i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr nusw nuw i8, ptr %p, i64 1
%gep2 = getelementptr nuw i32, ptr %gep1, i64 %a
%gep3 = getelementptr nusw nuw i32, ptr %gep2, i64 1
ret ptr %gep3
}

define ptr @merge_inbounds_missing_nuw(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_inbounds_missing_nuw(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr inbounds nuw i8, ptr %p, i64 1
%gep2 = getelementptr inbounds i32, ptr %gep1, i64 %a
%gep3 = getelementptr inbounds nuw i32, ptr %gep2, i64 1
ret ptr %gep3
}

define ptr @merge_nusw_missing_nuw(ptr %p, i64 %a) {
; CHECK-LABEL: @merge_nusw_missing_nuw(
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[A:%.*]]
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2]], i64 5
; CHECK-NEXT: ret ptr [[GEP3]]
;
%gep1 = getelementptr nusw nuw i8, ptr %p, i64 1
%gep2 = getelementptr nusw i32, ptr %gep1, i64 %a
%gep3 = getelementptr nusw nuw i32, ptr %gep2, i64 1
ret ptr %gep3
}
4 changes: 1 addition & 3 deletions llvm/test/Transforms/InstCombine/gepofconstgepi8.ll
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Expand Up @@ -85,10 +85,8 @@ define ptr @test_zero_sext_add_nsw(ptr %base, i32 %a) {
; CHECK-LABEL: define ptr @test_zero_sext_add_nsw(
; CHECK-SAME: ptr [[BASE:%.*]], i32 [[A:%.*]]) {
; CHECK-NEXT: entry:
; CHECK-NEXT: [[P1:%.*]] = getelementptr i8, ptr [[BASE]], i64 -4
; CHECK-NEXT: [[TMP0:%.*]] = sext i32 [[A]] to i64
; CHECK-NEXT: [[TMP1:%.*]] = getelementptr i32, ptr [[P1]], i64 [[TMP0]]
; CHECK-NEXT: [[P2:%.*]] = getelementptr i8, ptr [[TMP1]], i64 4
; CHECK-NEXT: [[P2:%.*]] = getelementptr i32, ptr [[BASE]], i64 [[TMP0]]
; CHECK-NEXT: ret ptr [[P2]]
;
entry:
Expand Down
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