Re: git: 43703bc489ec - main - stdlib.h: Fix qsort_r compatibility with GCC 12.

On 1/19/23 3:36 PM, Jessica Clarke wrote:
> On 19 Jan 2023, at 23:31, Jessica Clarke <jrtc27@FreeBSD.org> wrote:
>>
>> On 19 Jan 2023, at 23:11, Jessica Clarke <jrtc27@FreeBSD.org> wrote:
>>>
>>> On 19 Jan 2023, at 22:49, John Baldwin <jhb@FreeBSD.org> wrote:
>>>>
>>>> The branch main has been updated by jhb:
>>>>
>>>> URL: https://cgit.FreeBSD.org/src/commit/?id=43703bc489ec504b947b869045c492ed38c1a69c
>>>>
>>>> commit 43703bc489ec504b947b869045c492ed38c1a69c
>>>> Author:     John Baldwin <jhb@FreeBSD.org>
>>>> AuthorDate: 2023-01-19 22:48:52 +0000
>>>> Commit:     John Baldwin <jhb@FreeBSD.org>
>>>> CommitDate: 2023-01-19 22:48:52 +0000
>>>>
>>>>   stdlib.h: Fix qsort_r compatibility with GCC 12.
>>>>
>>>>   GCC 12 (unlike GCC 9) does not match a function argument passed to the
>>>>   old qsort_r() API (as is used in the qsort_r_compat test) to a
>>>>   function pointer type via __generic.  It treats the function type as a
>>>>   distinct type from a function pointer.  As a workaround, add a second
>>>>   definition of qsort_r for GCC 12 which uses the bare function type.
>>>
>>> As far as I can tell both versions of GCC behave the same. The
>>> difference is whether __generic is using _Generic or
>>> __builtin_choose_expr with __builtin_types_compatible_p, since function
>>> and function pointer types are not compatible. Clang will take the
>>> __has_extension path, but GCC will take the builtins path, and so Clang
>>> works but GCC doesn’t.
>>>
>>> As a result of this change you’ve likely broken code that does
>>> qsort_r(..., &f) as that will have the opposite problem. The right fix
>>> is to force arg5 to decay, such as by (ab)using the comma operator with
>>> __generic((0, arg5), ...). I guess that probably belongs in the
>>> fallback implementation of __generic though, not here, which would give
>>> the following real fix:
>>>
>>> diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
>>> index 83ba7584e5b9..f7eff4768151 100644
>>> --- a/sys/sys/cdefs.h
>>> +++ b/sys/sys/cdefs.h
>>> @@ -312,6 +312,9 @@
>>> * __generic().  Unlike _Generic(), this macro can only distinguish
>>> * between a single type, so it requires nested invocations to
>>> * distinguish multiple cases.
>>> + *
>>> + * Note that the comma operator is used to force expr to decay in order to
>>> + * match _Generic.
>>> */
>>>
>>> #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) || \
>>> @@ -321,7 +324,7 @@
>>> #elif __GNUC_PREREQ__(3, 1) && !defined(__cplusplus)
>>> #define	__generic(expr, t, yes, no)					\
>>> 	__builtin_choose_expr(						\
>>> -	    __builtin_types_compatible_p(__typeof(expr), t), yes, no)
>>> +	    __builtin_types_compatible_p(__typeof((0, expr)), t), yes, no)
>>
>> With (expr) instead of expr, of course...
> 
> And as for why GCC 9 works:
> 
> It doesn’t. The tests just aren’t built because MK_CXX=no disables
> MK_TESTS. GCC 12 only hits it because it’s new enough to be able to
> build libc++ and not force MK_CXX=no.
> 
> It would be nice to unpick that...

Hmm, ok I thought I had managed to build this test with GCC 9 previously, but
perhaps not.

I will give the cdefs.h change a go locally though and see what happens.

-- 
John Baldwin