[llvm] [MergeFunc] Remove discardables function before writing alias or thunk. (PR #128865)

[Teresa Johnson via llvm-commits]

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@@ -875,9 +876,14 @@ void MergeFunctions::writeAlias(Function *F, Function *G) {
   ++NumAliasesWritten;
 }
 
-// Replace G with an alias to F if possible, or a thunk to F if
-// profitable. Returns false if neither is the case.
-bool MergeFunctions::writeThunkOrAlias(Function *F, Function *G) {
+// If needed, replace G with an alias to F if possible, or a thunk to F if
+// profitable. Returns false if neither is the case. If \p G is not needed (e.g.
+// it is discardable and linkonce_odr), \p G is removed directly.
+bool MergeFunctions::writeThunkOrAliasIfNeeded(Function *F, Function *G) {
+  if (G->isDiscardableIfUnused() && G->use_empty() && !MergeFunctionsPDI) {
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teresajohnson wrote:

Is G's lack of uses created by this pass? Why not let a subsequent DCE invocation remove it? Also, if MergeFunctionsPDI is enabled, but G is discardable and unused, wouldn't DCE still go ahead and remove?

https://github.com/llvm/llvm-project/pull/128865