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[InstCombine] Support offsets in memset to load forwarding #151924

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[InstCombine] Support offsets in memset to load forwarding #151924

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5f56ddb
[InstCombine] Add pre-commit tests
pedroclobo Aug 3, 2025
0eed6ec
[InstCombine] Support offsets in `memset` to load forwarding
pedroclobo Aug 3, 2025
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12 changes: 8 additions & 4 deletions llvm/lib/Analysis/Loads.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -631,9 +631,13 @@ static Value *getAvailableLoadStore(Instruction *Inst, const Value *Ptr,
if (!Val || !Len)
return nullptr;

// TODO: Handle offsets.
Value *Dst = MSI->getDest();
if (!AreEquivalentAddressValues(Dst, Ptr))
// Handle offsets.
int64_t StoreOffset = 0, LoadOffset = 0;
const Value *StoreBase =
GetPointerBaseWithConstantOffset(MSI->getDest(), StoreOffset, DL);
const Value *LoadBase =
GetPointerBaseWithConstantOffset(Ptr, LoadOffset, DL);
if (StoreBase != LoadBase || LoadOffset < StoreOffset)
return nullptr;

if (IsLoadCSE)
Expand All @@ -645,7 +649,7 @@ static Value *getAvailableLoadStore(Instruction *Inst, const Value *Ptr,

// Make sure the read bytes are contained in the memset.
uint64_t LoadSize = LoadTypeSize.getFixedValue();
if ((Len->getValue() * 8).ult(LoadSize))
if ((Len->getValue() * 8).ult(LoadSize + (LoadOffset - StoreOffset) * 8))
return nullptr;

APInt Splat = LoadSize >= 8 ? APInt::getSplat(LoadSize, Val->getValue())
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13 changes: 2 additions & 11 deletions llvm/test/Analysis/GlobalsModRef/memset-escape.ll
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Expand Up @@ -7,23 +7,14 @@ target triple = "x86_64-apple-macosx10.10.0"
@a = internal global [3 x i32] zeroinitializer, align 4
@b = common global i32 0, align 4

; The important thing we're checking for here is the reload of (some element of)
; @a after the memset.
; The important thing we're checking here is that the value from the memset
; rather than the preceding store is forwarded.

define i32 @main() {
; CHECK-LABEL: define noundef i32 @main(
; CHECK-SAME: ) local_unnamed_addr #[[ATTR0:[0-9]+]] {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: store i32 1, ptr getelementptr inbounds nuw (i8, ptr @a, i64 8), align 4
; CHECK-NEXT: tail call void @llvm.memset.p0.i64(ptr noundef nonnull align 4 dereferenceable(12) @a, i8 0, i64 12, i1 false)
; CHECK-NEXT: store i32 3, ptr @b, align 4
; CHECK-NEXT: [[TMP0:%.*]] = load i32, ptr getelementptr inbounds nuw (i8, ptr @a, i64 8), align 4
; CHECK-NEXT: [[CMP1_NOT:%.*]] = icmp eq i32 [[TMP0]], 0
; CHECK-NEXT: br i1 [[CMP1_NOT]], label %[[IF_END:.*]], label %[[IF_THEN:.*]]
; CHECK: [[IF_THEN]]:
; CHECK-NEXT: tail call void @abort()
; CHECK-NEXT: unreachable
; CHECK: [[IF_END]]:
; CHECK-NEXT: ret i32 0
;
entry:
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40 changes: 36 additions & 4 deletions llvm/test/Transforms/InstCombine/load-store-forward.ll
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Expand Up @@ -365,20 +365,52 @@ define i32 @load_after_memset_unknown(ptr %a, i8 %byte) {
ret i32 %v
}

; TODO: Handle load at offset.
define i32 @load_after_memset_0_offset(ptr %a) {
; CHECK-LABEL: @load_after_memset_0_offset(
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(16) [[A:%.*]], i8 0, i64 16, i1 false)
; CHECK-NEXT: [[GEP:%.*]] = getelementptr i8, ptr [[A]], i64 4
; CHECK-NEXT: [[V:%.*]] = load i32, ptr [[GEP]], align 4
; CHECK-NEXT: ret i32 [[V]]
; CHECK-NEXT: ret i32 0
;
call void @llvm.memset.p0.i64(ptr %a, i8 0, i64 16, i1 false)
%gep = getelementptr i8, ptr %a, i64 4
%v = load i32, ptr %gep
ret i32 %v
}

define i32 @load_after_memset_1_offset(ptr %a) {
; CHECK-LABEL: @load_after_memset_1_offset(
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(16) [[A:%.*]], i8 1, i64 16, i1 false)
; CHECK-NEXT: ret i32 16843009
;
call void @llvm.memset.p0.i64(ptr %a, i8 1, i64 16, i1 false)
%gep = getelementptr i8, ptr %a, i64 4
%v = load i32, ptr %gep
ret i32 %v
}

define i1 @load_after_memset_0_offset_i1(ptr %a) {
; CHECK-LABEL: @load_after_memset_0_offset_i1(
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(16) [[A:%.*]], i8 0, i64 16, i1 false)
; CHECK-NEXT: ret i1 false
;
call void @llvm.memset.p0.i64(ptr %a, i8 0, i64 16, i1 false)
%gep = getelementptr i1, ptr %a, i64 12
%v = load i1, ptr %gep
ret i1 %v
}

define i8 @neg_load_after_memset_0_neg_offset(ptr %a) {
; CHECK-LABEL: @neg_load_after_memset_0_neg_offset(
; CHECK-NEXT: [[GEP:%.*]] = getelementptr i8, ptr [[A:%.*]], i64 2
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(16) [[GEP]], i8 0, i64 16, i1 false)
; CHECK-NEXT: [[V:%.*]] = load i8, ptr [[A]], align 1
; CHECK-NEXT: ret i8 [[V]]
;
%gep = getelementptr i8, ptr %a, i64 2
call void @llvm.memset.p0.i64(ptr %gep, i8 0, i64 16, i1 false)
%v = load i8, ptr %a
ret i8 %v
}

define i32 @load_after_memset_0_offset_too_large(ptr %a) {
; CHECK-LABEL: @load_after_memset_0_offset_too_large(
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(16) [[A:%.*]], i8 0, i64 16, i1 false)
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