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[EarlyCSE] Support memset loads (#194268)

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This PR addresses the zero-`memset` case in EarlyCSE as discussed in
#194080. If we do a `memset` of zero and then load back from the same
base pointer, we can fold that load to `null`.
This commit is contained in:
nataliakokoromyti 2026-05-04 03:20:59 -07:00 committed by GitHub
parent a699b25b92
commit 15383dcdb4
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2 changed files with 118 additions and 0 deletions
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26
llvm/lib/Transforms/Scalar/EarlyCSE.cpp
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@ -821,6 +821,12 @@ private:
Info.IsVolatile = false;
break;
}
} else if (auto *MI = dyn_cast<MemSetInst>(Inst)) {
Info.PtrVal = MI->getDest();
Info.MatchingId = 0;
Info.ReadMem = false;
Info.WriteMem = true;
Info.IsVolatile = MI->isVolatile();
}
}
}
@ -1226,6 +1232,26 @@ Value *EarlyCSE::getMatchingValue(LoadValue &InVal, ParseMemoryInst &MemInst,
unsigned CurrentGeneration) {
if (InVal.DefInst == nullptr)
return nullptr;
if (auto *MSI = dyn_cast<MemSetInst>(InVal.DefInst)) {
if (!MemInst.isLoad() || MemInst.isVolatile() || !MemInst.isUnordered())
return nullptr;
if (MSI->isVolatile())
return nullptr;
auto *Val = dyn_cast<ConstantInt>(MSI->getValue());
if (!Val || !Val->isZero())
return nullptr;
auto Len = MSI->getLengthInBytes();
if (!Len)
return nullptr;
TypeSize LoadSize = SQ.DL.getTypeStoreSize(MemInst.getValueType());
if (LoadSize.isScalable() || Len->ult(LoadSize.getFixedValue()))
return nullptr;
if (!isOperatingOnInvariantMemAt(MemInst.get(), InVal.Generation) &&
!isSameMemGeneration(InVal.Generation, CurrentGeneration, InVal.DefInst,
MemInst.get()))
return nullptr;
return Constant::getNullValue(MemInst.getValueType());
}
if (InVal.MatchingId != MemInst.getMatchingId())
return nullptr;
// We don't yet handle removing loads with ordering of any kind.

92
llvm/test/Transforms/EarlyCSE/memset-load.ll Normal file
View file

@ -0,0 +1,92 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
; RUN: opt -S -passes='early-cse<memssa>' < %s | FileCheck %s
target datalayout = "pe1:64:64:64:32"
define ptr @load_from_zero_memset(ptr %p) {
; CHECK-LABEL: define ptr @load_from_zero_memset(
; CHECK-SAME: ptr [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 8, i1 false)
; CHECK-NEXT: ret ptr null
;
entry:
call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 8, i1 false)
%v = load ptr, ptr %p, align 8
ret ptr %v
}
define ptr @load_from_nonzero_memset(ptr %p) {
; CHECK-LABEL: define ptr @load_from_nonzero_memset(
; CHECK-SAME: ptr [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr [[P]], i8 1, i64 8, i1 false)
; CHECK-NEXT: [[V:%.*]] = load ptr, ptr [[P]], align 8
; CHECK-NEXT: ret ptr [[V]]
;
entry:
call void @llvm.memset.p0.i64(ptr %p, i8 1, i64 8, i1 false)
%v = load ptr, ptr %p, align 8
ret ptr %v
}
define ptr @load_from_zero_memset_with_clobber(ptr %p) {
; CHECK-LABEL: define ptr @load_from_zero_memset_with_clobber(
; CHECK-SAME: ptr [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 8, i1 false)
; CHECK-NEXT: call void @clobber(ptr [[P]])
; CHECK-NEXT: [[V:%.*]] = load ptr, ptr [[P]], align 8
; CHECK-NEXT: ret ptr [[V]]
;
entry:
call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 8, i1 false)
call void @clobber(ptr %p)
%v = load ptr, ptr %p, align 8
ret ptr %v
}
define ptr @load_from_volatile_zero_memset(ptr %p) {
; CHECK-LABEL: define ptr @load_from_volatile_zero_memset(
; CHECK-SAME: ptr [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 8, i1 true)
; CHECK-NEXT: [[V:%.*]] = load ptr, ptr [[P]], align 8
; CHECK-NEXT: ret ptr [[V]]
;
entry:
call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 8, i1 true)
%v = load ptr, ptr %p, align 8
ret ptr %v
}
define ptr @load_from_zero_memset_unknown_length(ptr %p, i64 %n) {
; CHECK-LABEL: define ptr @load_from_zero_memset_unknown_length(
; CHECK-SAME: ptr [[P:%.*]], i64 [[N:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 [[N]], i1 false)
; CHECK-NEXT: [[V:%.*]] = load ptr, ptr [[P]], align 8
; CHECK-NEXT: ret ptr [[V]]
;
entry:
call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 %n, i1 false)
%v = load ptr, ptr %p, align 8
ret ptr %v
}
define ptr addrspace(1) @load_from_zero_memset_external(ptr addrspace(1) %p) {
; CHECK-LABEL: define ptr addrspace(1) @load_from_zero_memset_external(
; CHECK-SAME: ptr addrspace(1) [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: call void @llvm.memset.p1.i64(ptr addrspace(1) [[P]], i8 0, i64 8, i1 false)
; CHECK-NEXT: ret ptr addrspace(1) null
;
entry:
call void @llvm.memset.p1.i64(ptr addrspace(1) %p, i8 0, i64 8, i1 false)
%v = load ptr addrspace(1), ptr addrspace(1) %p, align 8
ret ptr addrspace(1) %v
}
declare void @llvm.memset.p0.i64(ptr nocapture writeonly, i8, i64, i1 immarg)
declare void @llvm.memset.p1.i64(ptr addrspace(1) nocapture writeonly, i8, i64, i1 immarg)
declare void @clobber(ptr)