[llvm] [llvm][GlobalOpt] Optimize statically resolvable IFuncs (PR #80606)

[Jon Roelofs via llvm-commits]

================
@@ -2404,6 +2405,42 @@ static bool OptimizeEmptyGlobalCXXDtors(Function *CXAAtExitFn) {
   return Changed;
 }
 
+static Function *hasSideeffectFreeStaticResolution(GlobalIFunc &IF) {
+  Function *Resolver = IF.getResolverFunction();
+  if (!Resolver)
+    return nullptr;
+
+  Function *Callee = nullptr;
+  for (BasicBlock &BB : *Resolver) {
+    if (any_of(BB, [](Instruction &I) { return I.mayHaveSideEffects(); }))
+      return nullptr;
+
+    if (auto *Ret = dyn_cast<ReturnInst>(BB.getTerminator()))
+      if (auto *F = dyn_cast<Function>(Ret->getReturnValue())) {
+        if (!Callee)
+          Callee = F;
+        if (F != Callee)
+          return nullptr;
+      }
+  }
+
+  return Callee;
+}
+
+/// Find IFuncs that have resolvers that always point at the same statically
+/// known callee, and replace their callers with a direct call.
+static bool OptimizeStaticIFuncs(Module &M) {
+  bool Changed = false;
+  for (GlobalIFunc &IF : M.ifuncs())
+    if (Function *Callee = hasSideeffectFreeStaticResolution(IF))
+      if (!IF.use_empty()) {
+        IF.replaceAllUsesWith(Callee);
+        NumIFuncsResolved++;
----------------
jroelofs wrote:

Is it always legal to erase the `IFunc` afterward?

https://github.com/llvm/llvm-project/pull/80606