Re: Ping: cacheflush.2

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

Hi Martin,

Thanks!  It's good to learn some GCC internal details :)

Cheers,

Alex

On 12/18/20 5:51 PM, Martin Sebor wrote:
> On 12/18/20 3:42 AM, Alejandro Colomar (man-pages) wrote:
>> Hi Martin,
>>
>> I sent you an email, but I received a "delivery failure".
>> If you're reading this from a list, could you answer, please?
>>
>> Thanks,
>>
>> Alex
>>
>> On 12/14/20 11:34 PM, Alejandro Colomar (man-pages) wrote:
>>> Hello Martin,
>>>
>>> Thanks for the correction!
>>> Then the prototypes that changes from 'char *' to 'void *' in r269082
>>> were not exposed to the user, right?
>>> I guess then those are just internal implementation where GCC did use
>>> 'char *'.
> 
> __builtin___clear_cache was added to GCC in r126535 (the __builtin_
> prefix is added by the macro):
> 
> +DEF_EXT_LIB_BUILTIN    (BUILT_IN_CLEAR_CACHE, "__clear_cache",
> BT_FN_VOID_PTR_PTR, ATTR_NOTHROW_LIST)
> 
> The BT_FN_VOID_PTR_PTR macro describes its signature as returning
> void and taking two void pointer arguments.  AFAIK, this has never
> changed.  Contrary to that, the manual entry for the built-in added
> in the same revision documented it as taking two char*.  That was
> corrected to void* in r269082 to match.
> 
> There's a GCC internal declaration of __clear_cache (apparently
> provided in libgcc for VxWorks).  It was added in r264479 and
> it also used char*.  This was also changed to void* in r269082
> to match the built-in.  Looks like this __clear_cache has just
> been removed from libgcc in GCC 11:
> https://gcc.gnu.org/pipermail/gcc-cvs/2020-December/338478.html
> 
>>>
>>> Where is the actual prototype exposed to the user declared?
> 
> Built-in functions are declared implicitly by GCC.  They have no
> explicit declarations like user-defined functions.  The implicit
> internal "declarations" are specified in the GCC internal file
> gcc/builtins.def, where they are hidden behind layers of macros.
> For example, on the GCC 10 branch, the declaration for
> __builtin___clear_cache is here:
> 
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/builtins.def;h=fa8b0641ab13b36f983c591a7020f6b432e5fb3d;hb=refs/heads/releases/gcc-10#l837
> 
> 
> Martin
> 
>>>
>>> Thanks,
>>>
>>> Alex
>>>
>>> P.S.: Michael, wait for a patch revision (v6).
>>>
>>> On 12/14/20 10:13 PM, Martin Sebor wrote:
>>>> On 12/11/20 11:14 AM, Alejandro Colomar (man-pages) via Gcc wrote:
>>>>> It looks like GCC recently moved from 'char *' to 'void *'.
>>>>> This SO question[1] (4 years ago) quotes the GCC docs
>>>>> and they had 'char *'.
>>>>
>>>> __builtin___clear_cache in GCC has always been declared to take
>>>> void*.  The signature in the manual was recently corrected to match
>>>> the implementation, i.e., from char* to void*, in r269082.
>>>>
>>>> Martin
>>>>
>>>>> Maybe Clang hasn't noticed the change.
>>>>> I'll report a bug.
>>>>>
>>>>> [1]: https://stackoverflow.com/q/35741814/6872717
>>>>>
>>>>> On 12/9/20 8:15 PM, Alejandro Colomar (man-pages) wrote:
>>>>>> Hi Heinrich,
>>>>>>
>>>>>> It looks like a bug (or at least an undocumented divergence from
>>>>>> GCC) in
>>>>>> Clang/LLVM.  Or I couldn't find the documentation for it.
>>>>>>
>>>>>> Clang uses 'char *':
>>>>>> https://github.com/llvm/llvm-project/blob/7faf62a80bfc3a9dfe34133681fcc31f8e8d658b/clang/include/clang/Basic/Builtins.def#L583
>>>>>>
>>>>>>
>>>>>>
>>>>>> GCC uses 'void *':
>>>>>> https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
>>>>>>
>>>>>> I CCd Clang and GCC lists; maybe they know about that divergence.
>>>>>>
>>>>>> Cheers,
>>>>>>
>>>>>> Alex
>>>>>>
>>>>>> On 12/9/20 7:48 PM, Heinrich Schuchardt wrote:
>>>>>>> On 12/9/20 7:34 PM, Alejandro Colomar (man-pages) wrote:
>>>>>>>> Hi Heinrich & Michael,
>>>>>>>>
>>>>>>>> What about the following?:
>>>>>>>>
>>>>>>>> [
>>>>>>>> NOTES
>>>>>>>>           GCC provides a similar function, which may be useful on
>>>>>>>> archi‐
>>>>>>>>           tectures that lack this system call:
>>>>>>>>
>>>>>>>>               void __builtin___clear_cache(void *begin, void *end);
>>>>>>>> ]
>>>>>>>
>>>>>>> I just checked building with Clang/LLVM. There the arguments are of
>>>>>>> type
>>>>>>> (char *). See the following error output:
>>>>>>>
>>>>>>> +arch/sandbox/cpu/cache.c:19:26: error: passing 'uint8_t *' (aka
>>>>>>> 'unsigned char *') to parameter of type 'char *' converts between
>>>>>>> pointers to integer types with different sign
>>>>>>> [-Werror,-Wpointer-sign]
>>>>>>> +        __builtin___clear_cache(state->ram_buf,
>>>>>>> +                                ^~~~~~~~~~~~~~
>>>>>>> +arch/sandbox/cpu/cache.c:20:12: error: passing 'uint8_t *' (aka
>>>>>>> 'unsigned char *') to parameter of type 'char *' converts between
>>>>>>> pointers to integer types with different sign
>>>>>>> [-Werror,-Wpointer-sign]
>>>>>>> +                                state->ram_buf + state->ram_size);
>>>>>>> +                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>>>>
>>>>>>> Best regards
>>>>>>>
>>>>>>> Heinrich
>>>>>>>
>>>>>>>>
>>>>>>>> Cheers,
>>>>>>>>
>>>>>>>> Alex
>>>>>>>>
>>>>>>>> On 12/9/20 7:04 PM, Heinrich Schuchardt wrote:
>>>>>>>>> Hello Michael,
>>>>>>>>>
>>>>>>>>> function cacheflush() does not exist on many architectures.
>>>>>>>>>
>>>>>>>>> It would have saved me a lot of time if the man-page had
>>>>>>>>> referenced
>>>>>>>>> GCC's
>>>>>>>>>
>>>>>>>>> void __builtin___clear_cache(void *begin, void *end)
>>>>>>>>>
>>>>>>>>> Maybe you can add it to NOTES.
>>>>>>>>>
>>>>>>>>> Best regards
>>>>>>>>>
>>>>>>>>> heirnich
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
> 

-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/