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

[Nikita Popov via llvm-commits]

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@@ -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;
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nikic wrote:

@david-arm norecurse *is* being inferred for your example already, as long as the language mode is C++ and `foo` is internal, see https://clang.godbolt.org/z/afEPr1Yhe (internalization could be as a result of LTO).

Please note that there are two different places that infer norecurse. It is inferred both during the normal post-order FunctionAttrs pass, *and* the RPO FunctionAttrs pass, which propagates the norecurse from main downwards.

(Generally, what you want to do probably fits better into the RPO FunctionAttrs pass in any case, because it is a module rather than CGSCC pass.)

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