[llvm] [FuncAttrs] Relax norecurse attribute inference (PR #139943)

[Usha Gupta via llvm-commits]

================
@@ -2322,8 +2343,39 @@ PreservedAnalyses PostOrderFunctionAttrsPass::run(LazyCallGraph::SCC &C,
     Functions.push_back(&N.getFunction());
   }
 
-  auto ChangedFunctions =
-      deriveAttrsInPostOrder(Functions, AARGetter, ArgAttrsOnly);
+  bool NoFunctionsAddressIsTaken = false;
+  // Check if any function in the whole program has its address taken or has
+  // potentially external linkage.
+  // We use this information when inferring norecurse attribute: If there is
+  // no function whose address is taken and all functions have internal
+  // linkage, there is no path for a callback to any user function.
+  if (IsLTOPostLink || ForceLTOFuncAttrs) {
+    bool AnyFunctionsAddressIsTaken = false;
+    // Get the parent Module of the Function
+    Module &M = *C.begin()->getFunction().getParent();
+    for (Function &F : M) {
+      // We only care about functions defined in user program whose addresses
+      // escape, making them potential callback targets.
+      if (F.isDeclaration())
+        continue;
+
+      // If the function is already marked as norecurse, this should not block
+      // norecurse inference even though it may have external linkage.
+      // For ex: main() in C++.
+      if (F.doesNotRecurse())
+        continue;
----------------
usha1830 wrote:

Hi @nikic, @david-arm

Apologies, it took me sometime to get back to this.
Do you have any suggestions on how we can move forward here?

Should we create an issue to update the LangRef with this stricter requirement?
Or fix the RPO and PO function attribute pass by walking the call graph ?(Possible fix approach in https://github.com/llvm/llvm-project/pull/139943#discussion_r2189750082)

https://github.com/llvm/llvm-project/pull/139943