[llvm] [IR] Do not track metadata uses of ConstantData (PR #223303)

[Fangrui Song via llvm-commits]

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@@ -551,56 +558,44 @@ void ValueAsMetadata::handleRAUW(Value *From, Value *To) {
   assert(From != To && "Expected changed value");
   assert(&From->getContext() == &To->getContext() && "Expected same context");
 
-  LLVMContext &Context = From->getType()->getContext();
-  auto &Store = Context.pImpl->ValuesAsMetadata;
+  auto &Store = From->getContext().pImpl->ValuesAsMetadata;
   auto I = Store.find(From);
   if (I == Store.end()) {
     assert(!From->IsUsedByMD && "Expected From not to be used by metadata");
     return;
   }
 
-  // Remove old entry from the map.
   assert(From->IsUsedByMD && "Expected From to be used by metadata");
   From->IsUsedByMD = false;
   ValueAsMetadata *MD = I->second;
   assert(MD && "Expected valid metadata");
   assert(MD->getValue() == From && "Expected valid mapping");
   Store.erase(I);
 
-  if (isa<LocalAsMetadata>(MD)) {
-    if (auto *C = dyn_cast<Constant>(To)) {
-      // Local became a constant.
-      MD->replaceAllUsesWith(ConstantAsMetadata::get(C));
-      delete MD;
-      return;
-    }
-    if (getLocalFunctionMetadata(From) && getLocalFunctionMetadata(To) &&
-        getLocalFunctionMetadata(From) != getLocalFunctionMetadata(To)) {
-      // DISubprogram changed.
-      MD->replaceAllUsesWith(nullptr);
-      delete MD;
-      return;
-    }
-  } else if (!isa<Constant>(To)) {
-    // Changed to function-local value.
-    MD->replaceAllUsesWith(nullptr);
-    delete MD;
+  // The uses of a ConstantData node are not tracked, so the node can only be
+  // retyped in place.
+  if (isa<ConstantData>(From)) {
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MaskRay wrote:

The most common scenario is in `~Instruction() { ... ValueAsMetadata::handleRAUW(this, PoisonValue::get(getType())); }`

When an instruction referenced by `#dbg_value` is replaced with a PoisonValue (a ConstantData).

https://github.com/llvm/llvm-project/pull/223303