[llvm] [IR] Reduce memory usage in metadata use tracking (NFC) (PR #221866)

[Alexis Engelke via llvm-commits]

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@@ -1636,6 +1636,11 @@ class LLVMContextImpl {
   DenseMap<Value *, ValueAsMetadata *> ValuesAsMetadata;
   DenseMap<Metadata *, MetadataAsValue *> MetadataAsValues;
   DenseSet<DIArgList *, DIArgListInfo> DIArgLists;
+  // Maps a tracked metadata reference pointer (void *Ref) to its current index
+  // in ReplaceableMetadataImpl::UseMap. Only populated for replaceable metadata
+  // instances in Large Mode (IsLarge == true). Instantiated eagerly in the
+  // constructor and reset to std::nullopt at teardown.
+  std::optional<DenseMap<void *, unsigned>> MetadataUseMap;
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aengelke wrote:

I'm a bit worried that this map will never shrink.

Also, std::optional seems pointless here, just use an empty map for the start and clear the map in the destructor if that helps?

https://github.com/llvm/llvm-project/pull/221866