Re: git: 43703bc489ec - main - stdlib.h: Fix qsort_r compatibility with GCC 12.
Date: Thu, 19 Jan 2023 23:36:05 UTC
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...
Jess
> Jess
>
>> #endif
>>
>> /*
>>
>> Does that work instead for you after reverting this commit?
>>
>> Jess
>>
>>> Reviewed by: emaste
>>> Differential Revision: https://reviews.freebsd.org/D37410
>>> ---
>>> include/stdlib.h | 6 ++++++
>>> 1 file changed, 6 insertions(+)
>>>
>>> diff --git a/include/stdlib.h b/include/stdlib.h
>>> index 754e8f5f5fd4..30d24aea1c10 100644
>>> --- a/include/stdlib.h
>>> +++ b/include/stdlib.h
>>> @@ -352,9 +352,15 @@ void __qsort_r_compat(void *, size_t, size_t, void *,
>>> __sym_compat(qsort_r, __qsort_r_compat, FBSD_1.0);
>>> #endif
>>> #if defined(__generic) && !defined(__cplusplus)
>>> +#if __GNUC__ == 12
>>> +#define qsort_r(base, nel, width, arg4, arg5) \
>>> + __generic(arg5, int (void *, const void *, const void *), \
>>> + __qsort_r_compat, qsort_r)(base, nel, width, arg4, arg5)
>>> +#else
>>> #define qsort_r(base, nel, width, arg4, arg5) \
>>> __generic(arg5, int (*)(void *, const void *, const void *), \
>>> __qsort_r_compat, qsort_r)(base, nel, width, arg4, arg5)
>>> +#endif
>>> #elif defined(__cplusplus)
>>> __END_DECLS
>>> extern "C++" {