[CodeGen] Don't re-sign null member function pointers (#4131)

rdar://86731786
(cherry picked from commit e3be7a5)

Author: ahatanaka

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -1070,11 +1070,16 @@ ItaniumCXXABI::EmitMemberPointerConversion(const CastExpr *E,
       QualType srcType = E->getSubExpr()->getType();
       const auto &curAuthInfo = CGM.getMemberFunctionPointerAuthInfo(srcType);
       llvm::Constant *memFnPtr = llvm::ConstantExpr::getExtractValue(src, 0);
-      llvm::Constant *constPtr =
-          pointerAuthResignConstant(cast<llvm::User>(memFnPtr)->getOperand(0),
-                                    curAuthInfo, newAuthInfo, CGM);
-      constPtr = llvm::ConstantExpr::getPtrToInt(constPtr, memFnPtr->getType());
-      src = llvm::ConstantExpr::getInsertValue(src, constPtr, 0);
+      if (memFnPtr->getNumOperands() == 0) {
+        // src must be a pair of null pointers.
+        assert(isa<llvm::ConstantInt>(memFnPtr) && "constant int expected");
+      } else {
+        llvm::Constant *constPtr = pointerAuthResignConstant(
+            memFnPtr->getOperand(0), curAuthInfo, newAuthInfo, CGM);
+        constPtr =
+            llvm::ConstantExpr::getPtrToInt(constPtr, memFnPtr->getType());
+        src = llvm::ConstantExpr::getInsertValue(src, constPtr, 0);
+      }
     }
 
   // Under Itanium, reinterprets don't require any additional processing.

clang/test/CodeGenCXX/ptrauth-member-function-pointer.cpp

@@ -386,3 +386,11 @@ void test_builtin_ptrauth_type_discriminator() {
 MethodTy1 gmethod0 = reinterpret_cast<MethodTy1>(&Base0::nonvirtual0);
 MethodTy0 gmethod1 = reinterpret_cast<MethodTy0>(&Derived0::nonvirtual5);
 MethodTy0 gmethod2 = reinterpret_cast<MethodTy0>(&Derived0::virtual1);
+
+// CHECK: define void @_Z15testConvertNullv(
+// CHECK: %[[T:.*]] = alloca { i64, i64 },
+// store { i64, i64 } zeroinitializer, { i64, i64 }* %[[T]],
+
+void testConvertNull() {
+  VariadicMethodTy0 t = (VariadicMethodTy0)(MethodTy0{});
+}