[llvm] 6d54f1e - [AA] Consider read-only provenance capture for synchronization effects (#197157)

[via llvm-commits]

Author: Nikita Popov
Date: 2026-05-13T09:02:18+02:00
New Revision: 6d54f1e565894fd3e0f3fb18e4cee7e4b90bb9b7

URL: https://github.com/llvm/llvm-project/commit/6d54f1e565894fd3e0f3fb18e4cee7e4b90bb9b7
DIFF: https://github.com/llvm/llvm-project/commit/6d54f1e565894fd3e0f3fb18e4cee7e4b90bb9b7.diff

LOG: [AA] Consider read-only provenance capture for synchronization effects (#197157)

If only read-only provenance is captured, this means that another thread
may only read the object, not write to it. As such, we can also model
synchronizing operations as only reading the location (and thus allow
reordering of reads, but not writes, across the synchronization).

Added: 
    

Modified: 
    llvm/lib/Analysis/AliasAnalysis.cpp
    llvm/test/Analysis/BasicAA/atomics.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/AliasAnalysis.cpp b/llvm/lib/Analysis/AliasAnalysis.cpp
index 82f1b7c3a4fd3..f3aa16ff2f790 100644
--- a/llvm/lib/Analysis/AliasAnalysis.cpp
+++ b/llvm/lib/Analysis/AliasAnalysis.cpp
@@ -469,13 +469,19 @@ ModRefInfo llvm::getSyncEffects(AAResults *AA, const MemoryLocation &Loc,
   // an effect if the object is only captured *later*. As such, set I to null
   // and ReturnCaptures to true here.
   const Value *Obj = getUnderlyingObject(Loc.Ptr);
-  if (capturesNothing(AAQI.CA->getCapturesBefore(
-          Obj, /*I=*/nullptr, /*OrAt=*/true, /*ReturnCaptures=*/true)))
+  CaptureComponents CC = AAQI.CA->getCapturesBefore(
+      Obj, /*I=*/nullptr, /*OrAt=*/true, /*ReturnCaptures=*/true);
+  if (capturesNothing(CC))
     return ModRefInfo::NoModRef;
 
+  // If only read provenance was captured, other threads may only read the
+  // object.
+  ModRefInfo MR =
+      capturesReadProvenanceOnly(CC) ? ModRefInfo::Ref : ModRefInfo::ModRef;
+
   // If Loc is a constant memory location, the synchronization operation
   // definitely could not modify it.
-  return AA->getModRefInfoMask(Loc);
+  return MR & AA->getModRefInfoMask(Loc);
 }
 
 ModRefInfo AAResults::getModRefInfo(const LoadInst *L,

diff  --git a/llvm/test/Analysis/BasicAA/atomics.ll b/llvm/test/Analysis/BasicAA/atomics.ll
index 0aedd7f3816ba..ab0556ddf62c2 100644
--- a/llvm/test/Analysis/BasicAA/atomics.ll
+++ b/llvm/test/Analysis/BasicAA/atomics.ll
@@ -83,6 +83,32 @@ define void @alloca_escape_after(ptr %x) {
   ret void
 }
 
+; CHECK-LABEL: Function: alloca_escape_after_readonly:
+; CHECK:  Just Ref:  Ptr: i32* %a	<->  fence acq_rel
+; CHECK:  Both ModRef:  Ptr: i32* %x	<->  fence acq_rel
+; CHECK:  Just Ref:  Ptr: i32* %a	<->  %1 = atomicrmw add ptr %x, i32 1 acq_rel, align 4
+; CHECK:  Both ModRef:  Ptr: i32* %x	<->  %1 = atomicrmw add ptr %x, i32 1 acq_rel, align 4
+; CHECK:  Just Ref:  Ptr: i32* %a	<->  %2 = cmpxchg ptr %x, i32 0, i32 1 acq_rel monotonic, align 4
+; CHECK:  Both ModRef:  Ptr: i32* %x	<->  %2 = cmpxchg ptr %x, i32 0, i32 1 acq_rel monotonic, align 4
+; CHECK:  Just Ref:  Ptr: i32* %a	<->  %3 = load atomic i32, ptr %x acquire, align 4
+; CHECK:  Both ModRef:  Ptr: i32* %x	<->  %3 = load atomic i32, ptr %x acquire, align 4
+; CHECK:  Just Ref:  Ptr: i32* %a	<->  store atomic i32 0, ptr %x release, align 4
+; CHECK:  Both ModRef:  Ptr: i32* %x	<->  store atomic i32 0, ptr %x release, align 4
+define void @alloca_escape_after_readonly(ptr %x) {
+  %a = alloca i32
+  store i32 0, ptr %a
+
+  fence acq_rel
+  atomicrmw add ptr %x, i32 1 acq_rel
+  cmpxchg ptr %x, i32 0, i32 1 acq_rel monotonic
+  load atomic i32, ptr %x acquire, align 4
+  store atomic i32 0, ptr %x release, align 4
+
+  call void @escape(ptr captures(address, read_provenance) %a)
+
+  ret void
+}
+
 ; CHECK-LABEL: Function: alloca_no_escape_aliasing:
 ; CHECK:  Both ModRef:  Ptr: i32* %a	<->  %1 = atomicrmw add ptr %a, i32 1 monotonic, align 4
 ; CHECK:  Both ModRef:  Ptr: i32* %a	<->  %2 = cmpxchg ptr %a, i32 0, i32 1 monotonic monotonic, align 4