[clang] [clang] Function type attribute to prevent CFI instrumentation (PR #135836)

[Paul Kirth via cfe-commits]

================
@@ -3006,6 +3006,10 @@ static LValue EmitFunctionDeclLValue(CodeGenFunction &CGF, const Expr *E,
                                      GlobalDecl GD) {
   const FunctionDecl *FD = cast<FunctionDecl>(GD.getDecl());
   llvm::Constant *V = CGF.CGM.getFunctionPointer(GD);
+  if (E->getType()->isCFIUncheckedCalleeFunctionType()) {
+    if (auto *GV = dyn_cast<llvm::GlobalValue>(V))
+      V = llvm::NoCFIValue::get(GV);
+  }
   CharUnits Alignment = CGF.getContext().getDeclAlign(FD);
   return CGF.MakeAddrLValue(V, E->getType(), Alignment,
----------------
ilovepi wrote:

since it get's used in the return, you could cache `E->getType()`.

```suggestion
  auto ETy = E->getType();
  if (ETy->isCFIUncheckedCalleeFunctionType()) {
    if (auto *GV = dyn_cast<llvm::GlobalValue>(V))
      V = llvm::NoCFIValue::get(GV);
  }
  CharUnits Alignment = CGF.getContext().getDeclAlign(FD);
  return CGF.MakeAddrLValue(V, ETy, Alignment,
```
Feel free to use a better name and explicit type.

https://github.com/llvm/llvm-project/pull/135836