[WIP] C++: Adjust implicit '__cxa_bad_typeid' prototype to reality

[Jason Merrill]

On 3/24/25 7:02 AM, Thomas Schwinge wrote:
> Hi!
> 
> On 2025-03-21T15:46:01+0100, I wrote:
>> On 2025-03-19T14:25:49+0000, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>>> On Wed, 19 Mar 2025 at 14:21, Marek Polacek <polacek@redhat.com> wrote:
>>>> On Wed, Mar 19, 2025 at 12:38:31PM +0100, Thomas Schwinge wrote:
>>>>> In 2001 Subversion r40924 (Git commit 52a11cbfcf0cfb32628b6953588b6af4037ac0b6)
>>>>> "IA-64 ABI Exception Handling", '__cxa_bad_cast' changed from 'void *' to
>>>>> 'void' return type:
>>>>>
>>>>>      --- libstdc++-v3/libsupc++/exception_support.cc
>>>>>      +++ /dev/null
>>>>>      @@ -1,388 +0,0 @@
>>>>>      -[...]
>>>>>      -// Helpers for rtti. Although these don't return, we give them return types so
>>>>>      -// that the type system is not broken.
>>>>>      -extern "C" void *
>>>>>      -__cxa_bad_cast ()
>>>>>      -{
>>>>>      -  [...]
>>>>>      -}
>>>>>      -[...]
>>>>>
>>>>>      --- /dev/null
>>>>>      +++ libstdc++-v3/libsupc++/unwind-cxx.h
>>>>>      @@ -0,0 +1,163 @@
>>>>>      +[...]
>>>>>      +extern "C" void __cxa_bad_cast ();
>>>>>      +[...]
>>>>>
>>>>>      --- /dev/null
>>>>>      +++ libstdc++-v3/libsupc++/eh_aux_runtime.cc
>>>>>      @@ -0,0 +1,56 @@
>>>>>      +[...]
>>>>>      +extern "C" void
>>>>>      +__cxa_bad_cast ()
>>>>>      +{
>>>>>      +  [...]
>>>>>      +}
>>>>>      +[...]
>>>>>
>>>>> The implicit prototype in the C++ front end however wasn't likewise adjusted,
>>>>> and so for nvptx we generate code for 'void *' return type:
>>>>>
>>>>>      // BEGIN GLOBAL FUNCTION DECL: __cxa_bad_cast
>>>>>      .extern .func (.param .u64 %value_out) __cxa_bad_cast;
>>>>>
>>>>>      {
>>>>>      .param .u64 %value_in;
>>>>>      call (%value_in),__cxa_bad_cast;
>>>>>      trap;
>>>>>      // (noreturn)
>>>>>      exit;
>>>>>      // (noreturn)
>>>>>      ld.param.u64 %r30,[%value_in];
>>>>>      }
>>>>>
>>>>> ..., which is in conflict with the library code with 'void' return type:
>>>>>
>>>>>      // BEGIN GLOBAL FUNCTION DECL: __cxa_bad_cast
>>>>>      .visible .func __cxa_bad_cast;
>>>>>
>>>>>      // BEGIN GLOBAL FUNCTION DEF: __cxa_bad_cast
>>>>>      .visible .func __cxa_bad_cast
>>>>>      {
>>>>>      [...]
>>>>>      }
>>>>>
>>>>> ..., and we thus get execution test FAIL for 'g++.dg/rtti/dyncast2.C':
>>>>>
>>>>>      error   : Prototype doesn't match for '__cxa_bad_cast' in 'input file 7 at offset 51437', first defined in 'input file 7 at offset 51437'
>>>>>      nvptx-run: cuLinkAddData failed: device kernel image is invalid (CUDA_ERROR_INVALID_SOURCE, 300)
>>>>>
>>>>> With this patched, we get the expected: [...]
> 
>>>>> --- a/gcc/cp/rtti.cc
>>>>> +++ b/gcc/cp/rtti.cc
>>>>> @@ -198,7 +198,7 @@ throw_bad_cast (void)
>>>>>         fn = get_global_binding (name);
>>>>>         if (!fn)
>>>>>        fn = push_throw_library_fn
>>>>> -       (name, build_function_type_list (ptr_type_node, NULL_TREE));
>>>>> +       (name, build_function_type_list (void_type_node, NULL_TREE));
>>>>>       }
>>>>>
>>>>>     return build_cxx_call (fn, 0, NULL, tf_warning_or_error);
>>>>
>>>> LGTM, matches what I see in abi-eh.html from the itanium-cxx-abi.
> 
>> [...] I've now pushed to trunk branch:
>> commit 618c42d23726be6e2086d452d6718abe5e0daca8
>> "C++: Adjust implicit '__cxa_bad_cast' prototype to reality", [...]
> 
> So, a similar problem exists for '__cxa_bad_typeid'.  See the attached
> "[WIP] C++: Adjust implicit '__cxa_bad_typeid' prototype to reality" for
> what seemed to be the corresponding patch for that one.  However, that
> isn't sufficient; we run into internal errors like:
> 
>      [...]/g++.dg/rtti/typeid1.C: In function 'int main()':
>      [...]/g++.dg/rtti/typeid1.C:9:12: error: lvalue required as unary '&' operand
> 
> I got lost in the C++ front end code trying to understand how to resolve
> this mismatch.  Anyone able to advise, please?

This addition seems to resolve it:

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