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

[Nikita Popov 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;
----------------
nikic wrote:

Hm, I don't think this is quite correct: The fact that the function is norecurse does not mean that it cannot introduce recursion for another function. Consider something like this:

```
fn norecurse() {
  a(0);
}

fn a(c) {
  if (c) {
    norecurse();
  }
}
```
Here `a(1)` will recursively call itself, but `norecurse()` will not.

To make it fit into your inference framework, the actual example would probably look something like this:
```
external fn norecurse() {
  a(0);
}

extern fn calls_norecurse()

internal fn a(c) {
  if (c) {
    call_norecurse();
  }
}
```

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