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

[Florian Hahn 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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fhahn wrote:

It is done by the pass, when merging a function we may update all callers of one functions to call another, leaving around some unused functions.

GlobalDCE would pick those up, but MergeFunctions is run after GlobalDCE, presumably to avoid spending time on trying to merge dead functions.

MergeFunc already removes some functions if they become dead.

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