[llvm] [MergeFuncs] Don't introduce calls to {linkonce,weak}_odr functions. (PR #125050)

[Florian Hahn via llvm-commits]

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@@ -891,8 +891,10 @@ bool MergeFunctions::writeThunkOrAlias(Function *F, Function *G) {
 
 // Merge two equivalent functions. Upon completion, Function G is deleted.
 void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
-  if (F->isInterposable()) {
-    assert(G->isInterposable());
+  if (F->isInterposable() || G->hasWeakODRLinkage() ||
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fhahn wrote:

Unfortunately replacing all reference of `isInterposable` gives worse results, in particular, if we have a function `F` that is not interposable and not linkonce/weak ODR and another function `G` that is link once/weak ODR, then `G` should be able to call `F`.

I also updated the comparator to order ODR functions before non-ODR functions to consistently support the case above.

https://github.com/llvm/llvm-project/pull/125050