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[AA] Improve precision for monotonic atomic load/store operations #158169

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[AA] Improve precision for monotonic atomic load/store operations #158169

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05c40eb
[AA] A conservative fix for atomic instruciton.
Sep 11, 2025
4b63c54
[AA] Improve precision for monotonic atomic load/store operations
Sep 12, 2025
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19 changes: 16 additions & 3 deletions llvm/lib/Analysis/AliasAnalysis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -421,7 +421,7 @@ ModRefInfo AAResults::getModRefInfo(const LoadInst *L,
const MemoryLocation &Loc,
AAQueryInfo &AAQI) {
// Be conservative in the face of atomic.
if (isStrongerThan(L->getOrdering(), AtomicOrdering::Unordered))
if (isStrongerThanMonotonic(L->getOrdering()))
return ModRefInfo::ModRef;

// If the load address doesn't alias the given address, it doesn't read
Expand All @@ -431,6 +431,13 @@ ModRefInfo AAResults::getModRefInfo(const LoadInst *L,
if (AR == AliasResult::NoAlias)
return ModRefInfo::NoModRef;
}

assert(!isStrongerThanMonotonic(L->getOrdering()) &&
"Stronger atomic orderings should have been handled above!");

if (isStrongerThanUnordered(L->getOrdering()))
return ModRefInfo::ModRef;

// Otherwise, a load just reads.
return ModRefInfo::Ref;
}
Expand All @@ -439,7 +446,7 @@ ModRefInfo AAResults::getModRefInfo(const StoreInst *S,
const MemoryLocation &Loc,
AAQueryInfo &AAQI) {
// Be conservative in the face of atomic.
if (isStrongerThan(S->getOrdering(), AtomicOrdering::Unordered))
if (isStrongerThanMonotonic(S->getOrdering()))
return ModRefInfo::ModRef;

if (Loc.Ptr) {
Expand All @@ -457,7 +464,13 @@ ModRefInfo AAResults::getModRefInfo(const StoreInst *S,
return ModRefInfo::NoModRef;
}

// Otherwise, a store just writes.
assert(!isStrongerThanMonotonic(S->getOrdering()) &&
"Stronger atomic orderings should have been handled above!");

if (isStrongerThanUnordered(S->getOrdering()))
return ModRefInfo::ModRef;

// A store just writes.
return ModRefInfo::Mod;
}

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23 changes: 0 additions & 23 deletions llvm/test/Transforms/DeadStoreElimination/atomic-todo.ll
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63 changes: 50 additions & 13 deletions llvm/test/Transforms/DeadStoreElimination/atomic.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -37,9 +37,21 @@ define void @test4() {
ret void
}

; DSE unordered store overwriting non-atomic store (allowed)
; DSE doesn't remove monotonic store.
define void @test5() {
; CHECK-LABEL: @test5(
; CHECK-NEXT: store atomic i32 2, ptr @x monotonic, align 4
; CHECK-NEXT: store i32 1, ptr @x, align 4
; CHECK-NEXT: ret void
;
store atomic i32 2, ptr @x monotonic, align 4
store i32 1, ptr @x
ret void
}

; DSE unordered store overwriting non-atomic store (allowed)
define void @test6() {
; CHECK-LABEL: @test6(
; CHECK-NEXT: store atomic i32 1, ptr @x unordered, align 4
; CHECK-NEXT: ret void
;
Expand All @@ -49,8 +61,8 @@ define void @test5() {
}

; DSE no-op unordered atomic store (allowed)
define void @test6() {
; CHECK-LABEL: @test6(
define void @test7() {
; CHECK-LABEL: @test7(
; CHECK-NEXT: ret void
;
%x = load atomic i32, ptr @x unordered, align 4
Expand All @@ -60,8 +72,8 @@ define void @test6() {

; DSE seq_cst store (be conservative; DSE doesn't have infrastructure
; to reason about atomic operations).
define void @test7() {
; CHECK-LABEL: @test7(
define void @test8() {
; CHECK-LABEL: @test8(
; CHECK-NEXT: [[A:%.*]] = alloca i32, align 4
; CHECK-NEXT: store atomic i32 0, ptr [[A]] seq_cst, align 4
; CHECK-NEXT: ret void
Expand All @@ -73,8 +85,8 @@ define void @test7() {

; DSE and seq_cst load (be conservative; DSE doesn't have infrastructure
; to reason about atomic operations).
define i32 @test8() {
; CHECK-LABEL: @test8(
define i32 @test9() {
; CHECK-LABEL: @test9(
; CHECK-NEXT: [[A:%.*]] = alloca i32, align 4
; CHECK-NEXT: call void @randomop(ptr [[A]])
; CHECK-NEXT: store i32 0, ptr [[A]], align 4
Expand All @@ -88,10 +100,35 @@ define i32 @test8() {
ret i32 %x
}

; DSE across monotonic store (allowed as long as the eliminated store isUnordered)
define void @test10() {
; DSE across monotonic load (allowed if the monotonic load's address is NoAlias)
define i32 @test10() {
; CHECK-LABEL: test10
; CHECK-NOT: store i32 0
; CHECK: store i32 1
store i32 0, ptr @x
%x = load atomic i32, ptr @y monotonic, align 4
store i32 1, ptr @x
ret i32 %x
}

; DSE across monotonic load (blocked if the atomic load's address isn't NoAlias)
define i32 @test11(ptr %ptr) {
; CHECK-LABEL: @test11(
; CHECK-NEXT: store i32 0, ptr @x, align 4
; CHECK-NEXT: [[X:%.*]] = load atomic i32, ptr [[PTR:%.*]] monotonic, align 4
; CHECK-NEXT: store i32 1, ptr @x, align 4
; CHECK-NEXT: ret i32 [[X]]
;
store i32 0, ptr @x
%x = load atomic i32, ptr %ptr monotonic, align 4
store i32 1, ptr @x
ret i32 %x
}

; DSE across monotonic store (allowed as long as the eliminated store isUnordered)
define void @test12() {
; CHECK-LABEL: test12
; CHECK-NOT: store i32 0
; CHECK: store i32 1
store i32 0, ptr @x
store atomic i32 42, ptr @y monotonic, align 4
Expand All @@ -100,8 +137,8 @@ define void @test10() {
}

; DSE across monotonic load (forbidden since the eliminated store is atomic)
define i32 @test11() {
; CHECK-LABEL: @test11(
define i32 @test13() {
; CHECK-LABEL: @test13(
; CHECK-NEXT: store atomic i32 0, ptr @x monotonic, align 4
; CHECK-NEXT: [[X:%.*]] = load atomic i32, ptr @y monotonic, align 4
; CHECK-NEXT: store atomic i32 1, ptr @x monotonic, align 4
Expand All @@ -114,8 +151,8 @@ define i32 @test11() {
}

; DSE across monotonic store (forbidden since the eliminated store is atomic)
define void @test12() {
; CHECK-LABEL: @test12(
define void @test14() {
; CHECK-LABEL: @test14(
; CHECK-NEXT: store atomic i32 0, ptr @x monotonic, align 4
; CHECK-NEXT: store atomic i32 42, ptr @y monotonic, align 4
; CHECK-NEXT: store atomic i32 1, ptr @x monotonic, align 4
Expand Down