[DSE] Fix non-determinism due to address reuse

The malloc->calloc fold creates a new MemoryAccess, which may end
of at the same address as a previously deleted access inside
SkipStores.

To the most part, this is not a problem, because SkipStores is
normally only used together with MemDefs. Neither the old malloc
access nor the new calloc access will be part of MemDefs, so
there is no problem here.

However, SkipStores is also used in one more place: In the main
DSE loop, ToCheck entries are checked against it. Fix this by
not using SkipStores here, and instead using a separate set to
track deletions inside this loop. This way it is not affected
by the calloc optimization that happens outside it.

This is all pretty ugly, but I haven't found another good way to
fix it. Suggestions welcome.

No test case as I don't have a reliable DSE-only test-case for
this.

Fixes #84458.

Author: nikic

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -1698,7 +1698,9 @@ struct DSEState {
 
   /// Delete dead memory defs and recursively add their operands to ToRemove if
   /// they became dead.
-  void deleteDeadInstruction(Instruction *SI) {
+  void
+  deleteDeadInstruction(Instruction *SI,
+                        SmallPtrSetImpl<MemoryAccess *> *Deleted = nullptr) {
     MemorySSAUpdater Updater(&MSSA);
     SmallVector<Instruction *, 32> NowDeadInsts;
     NowDeadInsts.push_back(SI);
@@ -1719,6 +1721,8 @@ struct DSEState {
         if (IsMemDef) {
           auto *MD = cast<MemoryDef>(MA);
           SkipStores.insert(MD);
+          if (Deleted)
+            Deleted->insert(MD);
           if (auto *SI = dyn_cast<StoreInst>(MD->getMemoryInst())) {
             if (SI->getValueOperand()->getType()->isPointerTy()) {
               const Value *UO = getUnderlyingObject(SI->getValueOperand());
@@ -2167,14 +2171,15 @@ static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
     unsigned PartialLimit = MemorySSAPartialStoreLimit;
     // Worklist of MemoryAccesses that may be killed by KillingDef.
     SmallSetVector<MemoryAccess *, 8> ToCheck;
+    SmallPtrSet<MemoryAccess *, 8> Deleted;
     ToCheck.insert(KillingDef->getDefiningAccess());
 
     bool Shortend = false;
     bool IsMemTerm = State.isMemTerminatorInst(KillingI);
     // Check if MemoryAccesses in the worklist are killed by KillingDef.
     for (unsigned I = 0; I < ToCheck.size(); I++) {
       MemoryAccess *Current = ToCheck[I];
-      if (State.SkipStores.count(Current))
+      if (Deleted.contains(Current))
         continue;
 
       std::optional<MemoryAccess *> MaybeDeadAccess = State.getDomMemoryDef(
@@ -2221,7 +2226,7 @@ static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
           continue;
         LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n  DEAD: " << *DeadI
                           << "\n  KILLER: " << *KillingI << '\n');
-        State.deleteDeadInstruction(DeadI);
+        State.deleteDeadInstruction(DeadI, &Deleted);
         ++NumFastStores;
         MadeChange = true;
       } else {
@@ -2258,7 +2263,7 @@ static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
               Shortend = true;
               // Remove killing store and remove any outstanding overlap
               // intervals for the updated store.
-              State.deleteDeadInstruction(KillingSI);
+              State.deleteDeadInstruction(KillingSI, &Deleted);
               auto I = State.IOLs.find(DeadSI->getParent());
               if (I != State.IOLs.end())
                 I->second.erase(DeadSI);
@@ -2270,7 +2275,7 @@ static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
         if (OR == OW_Complete) {
           LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n  DEAD: " << *DeadI
                             << "\n  KILLER: " << *KillingI << '\n');
-          State.deleteDeadInstruction(DeadI);
+          State.deleteDeadInstruction(DeadI, &Deleted);
           ++NumFastStores;
           MadeChange = true;
         }