[PATCH] C++: Adjust implicit '__cxa_bad_cast' prototype to reality

[Marek Polacek]

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:
> 
>      // BEGIN GLOBAL FUNCTION DECL: __cxa_bad_cast
>     -.extern .func (.param .u64 %value_out) __cxa_bad_cast;
>     +.extern .func __cxa_bad_cast;
> 
>      {
>     -.param .u64 %value_in;
>     -call (%value_in),__cxa_bad_cast;
>     +call __cxa_bad_cast;
>      trap;
>      // (noreturn)
>      exit;
>      // (noreturn)
>     -ld.param.u64 %r30,[%value_in];
>      }
> 
> ..., and execution test PASS.
> 
> 	gcc/cp/
> 	* rtti.cc (throw_bad_cast): Adjust implicit '__cxa_bad_cast'
> 	prototype to reality.
> ---
>  gcc/cp/rtti.cc | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/gcc/cp/rtti.cc b/gcc/cp/rtti.cc
> index dcf84f17163..a10fa383f79 100644
> --- 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.

The comment above mentions libstdc++/exception.cc but I don't see that
file.

Marek