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

[David Sherwood via llvm-commits]

================
@@ -2071,24 +2072,42 @@ 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())
+  Module *M = F->getParent();
+  llvm::TargetLibraryInfoImpl TLII(llvm::Triple(M->getTargetTriple()));
+  llvm::TargetLibraryInfo TLI(TLII);
+  // If all of the calls in F are identifiable and can be proven to not
+  // callback F, F is norecurse. This check also detects self-recursion
+  // as F is not currently marked norecurse, so any call from F to F
+  // will not be marked norecurse.
+  for (auto &BB : *F) {
+    for (auto &I : BB.instructionsWithoutDebug()) {
       if (auto *CB = dyn_cast<CallBase>(&I)) {
+        if (F->hasAddressTaken())
----------------
david-arm wrote:

I thought about this some more after @nikic's comments and I don't think this is strong enough because the bail-out when SCCNodes.size() != 1 is of limited value. If `F` isn't internal (or private?) then it could be accessible outside the module even if the address isn't taken, and at the same time SCCNodes.size() will return 1 because SCC analysis gives up when encountering an unknown call. An example would be something like this:

a.c:
```
extern void moo();

void foo() {
  bar();
}

void bar() {
  moo();
}
```

b.c:
```
extern void foo();

void moo() {
  foo();
}
```

I think you can rewrite the code to be something like this:

```
      if (auto *CB = dyn_cast<CallBase>(&I)) {
        if (!Callee || Callee == F)
          return;

        if (Callee->doesNotRecurse())
          continue;

        LibFunc LF;
        if (Callee->isDeclaration()) {
          if (Callee->hasFnAttribute(Attribute::NoCallback) ||
              (NoFunctionsAddressIsTaken &&
               TLI.getLibFunc(Callee->getName(), LF)))
            continue;
          return;
        } else if (F->hasAddressTaken() || !F.hasLocalLinkage())
          return;
```


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