ValueMapper: Rename RF_MoveDistinctMDs => RF_ReuseAndMutateDistinctMDs, NFC

Rename the `RF_MoveDistinctMDs` flag passed into `MapValue` and
`MapMetadata` to `RF_ReuseAndMutateDistinctMDs` in order to more
precisely describe its effect and clarify the header documentation.

Found this while helping to investigate PR48841, which pointed out an
unsound use of the flag in `CloneModule()`. For now I've just added a
FIXME there, but I'm hopeful that the new (more precise) name will
prevent other similar errors.

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/CloneModule.cpp b/llvm/lib/Transforms/Utils/CloneModule.cpp
index 6de679b..487cd4e 100644
--- a/llvm/lib/Transforms/Utils/CloneModule.cpp
+++ b/llvm/lib/Transforms/Utils/CloneModule.cpp
@@ -120,9 +120,13 @@ std::unique_ptr<Module> llvm::CloneModule(
 
     SmallVector<std::pair<unsigned, MDNode *>, 1> MDs;
     G.getAllMetadata(MDs);
+
+    // FIXME: Stop using RF_ReuseAndMutateDistinctMDs here, since it's unsound
+    // to mutate metadata that is still referenced by the source module unless
+    // the source is about to be discarded (see IRMover for a valid use).
     for (auto MD : MDs)
-      GV->addMetadata(MD.first,
-                      *MapMetadata(MD.second, VMap, RF_MoveDistinctMDs));
+      GV->addMetadata(MD.first, *MapMetadata(MD.second, VMap,
+                                             RF_ReuseAndMutateDistinctMDs));
 
     if (G.isDeclaration())
       continue;