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

[David Sherwood via llvm-commits]

================
@@ -2071,24 +2072,36 @@ static void addNoRecurseAttrs(const SCCNodeSet &SCCNodes,
   if (!F || !F->hasExactDefinition() || F->doesNotRecurse())
     return;
 
-  // If all of the calls in F are identifiable and are to norecurse functions, F
-  // is norecurse. This check also detects self-recursion as F is not currently
-  // marked norecurse, so any called from F to F will not be marked norecurse.
-  for (auto &BB : *F)
-    for (auto &I : BB.instructionsWithoutDebug())
+  if (F->hasAddressTaken())
----------------
david-arm wrote:

Hmm, thinking about this now I'm not sure it solves problems like this:

```
foo() -> bar() -> moo() -> foo()
```

or

```
boo() -> foo() -> bar() -> moo() -> boo()
```

where effectively foo() is recursive despite foo() not directly calling itself? I think the previous version of your patch solved this problem, although @nikic mentioned FunctionAttrs probably isn't the right place for deeper inspection. For a full analysis I think you still need to inspect the call graph. I'm not sure how useful this is, but you could check that all functions called by `foo()` don't make any function calls themselves? I don't know if this is a quick and easy check, or whether it's just as computationally expensive as your previous version?

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