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

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)
 #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++" {