[llvm] [MemProf][PGO] Prevent dropping of profile metadata during optimization (PR #121359)

[Teresa Johnson via llvm-commits]

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@@ -3379,6 +3379,10 @@ void llvm::combineMetadata(Instruction *K, const Instruction *J,
           K->setMetadata(Kind,
             MDNode::getMostGenericAlignmentOrDereferenceable(JMD, KMD));
         break;
+      case LLVMContext::MD_memprof:
+      case LLVMContext::MD_callsite:
+        // Preserve !memprof and !callsite metadata on K.
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teresajohnson wrote:

> It isn't incorrect to keep one. Since it is profile metadata, it doesn't affect correctness. We can improve how this is combined in the future, but simply keeping one set for now is a reasonable heuristic.

To that end, it might be better to add methods to merge the memprof related metadata, as is done for some of the other metadata types (e.g. getMergedProfMetadata), so we can keep that handling centralized with other memprof code and add TODOs for more sophisticated merging. I will do that.

https://github.com/llvm/llvm-project/pull/121359