[llvm] [ObjCARC] Don't sink objc_retain past releasing atomics (PR #184113)

[John McCall via llvm-commits]

================
@@ -67,7 +68,15 @@ bool llvm::objcarc::CanDecrementRefCount(const Instruction *Inst,
                                          const Value *Ptr,
                                          ProvenanceAnalysis &PA,
                                          ARCInstKind Class) {
-  // First perform a quick check if Class can not touch ref counts.
+  // Atomic RMW and CmpXchg instructions with release or stronger ordering
+  // publish memory to other threads, which may then read the stored pointer and
+  // release it. Treat these as potentially decrementing refcounts.
+  if (const auto *RMW = dyn_cast<AtomicRMWInst>(Inst))
+    return isReleaseOrStronger(RMW->getOrdering());
+  if (const auto *CmpXchg = dyn_cast<AtomicCmpXchgInst>(Inst))
+    return isReleaseOrStronger(CmpXchg->getSuccessOrdering());
----------------
rjmccall wrote:

You need to test both orderings for cmpxchg.

Is this function only used for sinking? If it's also used for hoisting, I think you should probably just use `isStrongerThanMonotonic`.

https://github.com/llvm/llvm-project/pull/184113