[llvm-branch-commits] [llvm] [DSE] Make DSE eliminate stores to objects with a sized dead_on_return (PR #173694)

[Aiden Grossman via llvm-branch-commits]

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@@ -1206,11 +1217,24 @@ struct DSEState {
     return OW_None;
   }
 
-  bool isInvisibleToCallerAfterRet(const Value *V) {
+  bool isInvisibleToCallerAfterRet(const Value *V, const Value *Ptr,
+                                   const LocationSize StoreSize) {
     if (isa<AllocaInst>(V))
       return true;
 
     auto I = InvisibleToCallerAfterRet.insert({V, false});
+    if (I.second && InvisibleToCallerAfterRetBounded.contains(V)) {
+      int64_t ValueOffset;
+      const Value *BaseValue =
+          GetPointerBaseWithConstantOffset(Ptr, ValueOffset, DL);
+      assert(BaseValue == V);
+      // This store is only invisible after return if we are in bounds of the
+      // range marked dead.
+      if (ValueOffset + StoreSize.getValue() <=
+              InvisibleToCallerAfterRetBounded[BaseValue] &&
+          ValueOffset >= 0)
+        return true;
+    }
     if (I.second && isInvisibleToCallerOnUnwind(V) && isNoAliasCall(V))
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boomanaiden154 wrote:

We only end up in this case if we inserted into the array (i.e., the pointer is not explicitly marked `dead_on_return`) but is marked `dead_on_unwind`. For objects marked dead_on_return the underlying object will always be explicitly marked, so we will never insert at this point anyways. If they're not marked dead_on_return, we get the same behavior.

We also do not want to potentially insert true into `InvisibleToCallerAfterRet` because then we would not be looking at bounds information when we should be.

Also realizing now after looking at this we do not want to be inserting into `InvisibleToCallerAfterRet` at all before we check for bounded dead_on_return annotations because otherwise we might erroneously report false if we have two stores at the end of a function to the same underlying object. Fixed + regression test added.

https://github.com/llvm/llvm-project/pull/173694