[llvm] [EarlyCSE] Support memset loads (PR #194268)
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Thu Apr 30 14:03:52 PDT 2026
https://github.com/nataliakokoromyti updated https://github.com/llvm/llvm-project/pull/194268
>From 13ef31156bdf7c8c638c7d99c31b48cff8759edd Mon Sep 17 00:00:00 2001
From: nataliakokoromyti <nataliakokoromyti at gmail.com>
Date: Mon, 27 Apr 2026 12:25:22 -0700
Subject: [PATCH 1/3] support memset loads
---
llvm/lib/Transforms/Scalar/EarlyCSE.cpp | 34 +++++++++-
llvm/test/Transforms/EarlyCSE/memset-load.ll | 65 ++++++++++++++++++++
2 files changed, 96 insertions(+), 3 deletions(-)
create mode 100644 llvm/test/Transforms/EarlyCSE/memset-load.ll
diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index ff3d55e5747e5..1950ff6978fab 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -821,6 +821,13 @@ class EarlyCSE {
Info.IsVolatile = false;
break;
}
+ } else if (isa<MemSetInst>(Inst)) {
+ auto *MI = cast<MemIntrinsic>(Inst);
+ Info.PtrVal = MI->getDest();
+ Info.MatchingId = 0;
+ Info.ReadMem = false;
+ Info.WriteMem = true;
+ Info.IsVolatile = MI->isVolatile();
}
}
}
@@ -885,9 +892,9 @@ class EarlyCSE {
// field in the MemIntrinsicInfo structure. That field contains
// non-negative values only.
int getMatchingId() const {
- if (IntrID != 0)
- return Info.MatchingId;
- return -1;
+ if (IntrID == 0)
+ return -1;
+ return Info.MatchingId;
}
Value *getPointerOperand() const {
@@ -1226,6 +1233,27 @@ 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;
+ 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;
+ // Byte-wise memset does not preserve out-of-band external pointer state.
+ if (SQ.DL.hasExternalState(MemInst.getValueType()))
+ 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.
diff --git a/llvm/test/Transforms/EarlyCSE/memset-load.ll b/llvm/test/Transforms/EarlyCSE/memset-load.ll
new file mode 100644
index 0000000000000..adb28b482c7f0
--- /dev/null
+++ b/llvm/test/Transforms/EarlyCSE/memset-load.ll
@@ -0,0 +1,65 @@
+; 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 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: [[V:%.*]] = load ptr addrspace(1), ptr addrspace(1) [[P]], align 8
+; CHECK-NEXT: ret ptr addrspace(1) [[V]]
+;
+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)
>From ce54872a4049e1cb467d540f0d9d2f5c67a5e487 Mon Sep 17 00:00:00 2001
From: nataliakokoromyti <nataliakokoromyti at gmail.com>
Date: Thu, 30 Apr 2026 13:56:18 -0700
Subject: [PATCH 2/3] restore getMatchingId
---
llvm/lib/Transforms/Scalar/EarlyCSE.cpp | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index 1950ff6978fab..728ca2864abdb 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -892,9 +892,9 @@ class EarlyCSE {
// field in the MemIntrinsicInfo structure. That field contains
// non-negative values only.
int getMatchingId() const {
- if (IntrID == 0)
- return -1;
- return Info.MatchingId;
+ if (IntrID != 0)
+ return Info.MatchingId;
+ return -1;
}
Value *getPointerOperand() const {
>From 406cae26fb8e4d00948bf59447520ab19e102096 Mon Sep 17 00:00:00 2001
From: nataliakokoromyti <nataliakokoromyti at gmail.com>
Date: Thu, 30 Apr 2026 14:03:42 -0700
Subject: [PATCH 3/3] Update llvm/lib/Transforms/Scalar/EarlyCSE.cpp
Co-authored-by: Nikita Popov <github at npopov.com>
---
llvm/lib/Transforms/Scalar/EarlyCSE.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index 728ca2864abdb..d72a5c4bd70e0 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -822,7 +822,7 @@ class EarlyCSE {
break;
}
} else if (isa<MemSetInst>(Inst)) {
- auto *MI = cast<MemIntrinsic>(Inst);
+ } else if (auto *MI = dyn_cast<MemSetInst>(Inst)) {
Info.PtrVal = MI->getDest();
Info.MatchingId = 0;
Info.ReadMem = false;
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