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

[Nikita Popov 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) {
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nikic wrote:

I'd go for a symbolic evaluation approach. Basically go through the instructions, try to constant fold (or instsimplify) them, plus handle the specific feature pattern you are looking for. GlobalOpt does something similar for evaluating global_ctors (https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Utils/Evaluator.cpp), though I expect that what you need here would be much simpler. (The hard part of evaluating global_ctors is mutating global state, which presumably isn't relevant here.)

Doing this kind of speculative optimization + rollback is something we try to avoid, as it is expensive. I don't think we use function cloning for this purpose anywhere. (IPSCCP does clone functions, but not as an analysis mechanism.)

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