[Globals] Treat nobuiltin fns as maybe-derefined.

Callsites could be marked as `builtin` while calling `nobuiltin`
functions. This can lead to problems, if local optimizations apply
transformations based on the semantics of the builtin, but then IPO
treats the function as `nobuiltin` and applies a transform that breaks
builtin semantics (assumed earlier).

To avoid this, mark such functions as maybey-derefined, to avoid IPO
transforms on them that may break assumptions of earlier calls.

Fixes #57075
Fixes #48366

Reviewed By: ychen

Differential Revision: https://reviews.llvm.org/D97735

llvm/include/llvm/IR/GlobalValue.h

@@ -145,12 +145,20 @@ class GlobalValue : public Constant {
     case AppendingLinkage:
     case InternalLinkage:
     case PrivateLinkage:
-      return isInterposable();
+      // Optimizations may assume builtin semantics for functions defined as
+      // nobuiltin due to attributes at call-sites. To avoid applying IPO based
+      // on nobuiltin semantics, treat such function definitions as maybe
+      // derefined.
+      return isInterposable() || isNobuiltinFnDef();
     }
 
     llvm_unreachable("Fully covered switch above!");
   }
 
+  /// Returns true if the global is a function definition with the nobuiltin
+  /// attribute.
+  bool isNobuiltinFnDef() const;
+
 protected:
   /// The intrinsic ID for this subclass (which must be a Function).
   ///

llvm/lib/IR/Globals.cpp

@@ -261,6 +261,13 @@ void GlobalObject::setSection(StringRef S) {
   setGlobalObjectFlag(HasSectionHashEntryBit, !S.empty());
 }
 
+bool GlobalValue::isNobuiltinFnDef() const {
+  const Function *F = dyn_cast<Function>(this);
+  if (!F || F->empty())
+    return false;
+  return F->hasFnAttribute(Attribute::NoBuiltin);
+}
+
 bool GlobalValue::isDeclaration() const {
   // Globals are definitions if they have an initializer.
   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(this))

llvm/test/Transforms/DeadArgElim/pr49022-builtin-nobuiltin.ll

@@ -13,7 +13,7 @@ entry:
 define dso_local i32 @test_fn(ptr %ptr) {
 ; CHECK-LABEL: @test_fn(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    call void @_ZdlPv(ptr poison) #[[ATTR1:[0-9]+]]
+; CHECK-NEXT:    call void @_ZdlPv(ptr [[PTR:%.*]]) #[[ATTR1:[0-9]+]]
 ; CHECK-NEXT:    ret i32 1
 ;
 entry: