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# stdalign.m4
# serial 1
dnl Copyright 2011-2024 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
dnl with or without modifications, as long as this notice is preserved.
# Check for alignas and alignof that conform to C23.
dnl Written by Paul Eggert and Bruno Haible.
# Prepare for substituting <stdalign.h> if it is not supported.
AC_DEFUN([gl_ALIGNASOF],
[
AC_CACHE_CHECK([for alignas and alignof],
[gl_cv_header_working_stdalign_h],
[gl_saved_CFLAGS=$CFLAGS
for gl_working in "yes, keywords" "yes, <stdalign.h> macros"; do
AS_CASE([$gl_working],
[*stdalign.h*], [CFLAGS="$gl_saved_CFLAGS -DINCLUDE_STDALIGN_H"])
AC_COMPILE_IFELSE(
[AC_LANG_PROGRAM(
[[#include <stdint.h>
#ifdef INCLUDE_STDALIGN_H
#include <stdalign.h>
#endif
#include <stddef.h>
/* Test that alignof yields a result consistent with offsetof.
This catches GCC bug 52023
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023>. */
#ifdef __cplusplus
template <class t> struct alignof_helper { char a; t b; };
# define ao(type) offsetof (alignof_helper<type>, b)
#else
# define ao(type) offsetof (struct { char a; type b; }, b)
#endif
char test_double[ao (double) % _Alignof (double) == 0 ? 1 : -1];
char test_long[ao (long int) % _Alignof (long int) == 0 ? 1 : -1];
char test_alignof[alignof (double) == _Alignof (double) ? 1 : -1];
/* Test alignas only on platforms where gnulib can help. */
#if \
((defined __cplusplus && 201103 <= __cplusplus) \
|| (__TINYC__ && defined __attribute__) \
|| (defined __APPLE__ && defined __MACH__ \
? 4 < __GNUC__ + (1 <= __GNUC_MINOR__) \
: __GNUC__) \
|| (__ia64 && (61200 <= __HP_cc || 61200 <= __HP_aCC)) \
|| __ICC || 0x590 <= __SUNPRO_C || 0x0600 <= __xlC__ \
|| 1300 <= _MSC_VER)
struct alignas_test { char c; char alignas (8) alignas_8; };
char test_alignas[offsetof (struct alignas_test, alignas_8) == 8
? 1 : -1];
#endif
]])],
[gl_cv_header_working_stdalign_h=$gl_working],
[gl_cv_header_working_stdalign_h=no])
CFLAGS=$gl_saved_CFLAGS
test "$gl_cv_header_working_stdalign_h" != no && break
done])
AS_CASE([$gl_cv_header_working_stdalign_h],
[yes*keyword*],
[AC_DEFINE([HAVE_C_ALIGNASOF], [1],
[Define to 1 if the alignas and alignof keywords work.])])
dnl The "zz" puts this toward config.h's end, to avoid potential
dnl collisions with other definitions.
AH_VERBATIM([zzalignas],
[#if !defined HAVE_C_ALIGNASOF \
&& !(defined __cplusplus && 201103 <= __cplusplus) \
&& !defined alignof
# if defined HAVE_STDALIGN_H
# include <stdalign.h>
# endif
/* ISO C23 alignas and alignof for platforms that lack it.
References:
ISO C23 (latest free draft
<http://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf>)
sections 6.5.3.4, 6.7.5, 7.15.
C++11 (latest free draft
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf>)
section 18.10. */
/* alignof (TYPE), also known as _Alignof (TYPE), yields the alignment
requirement of a structure member (i.e., slot or field) that is of
type TYPE, as an integer constant expression.
This differs from GCC's and clang's __alignof__ operator, which can
yield a better-performing alignment for an object of that type. For
example, on x86 with GCC and on Linux/x86 with clang,
__alignof__ (double) and __alignof__ (long long) are 8, whereas
alignof (double) and alignof (long long) are 4 unless the option
'-malign-double' is used.
The result cannot be used as a value for an 'enum' constant, if you
want to be portable to HP-UX 10.20 cc and AIX 3.2.5 xlc. */
/* GCC releases before GCC 4.9 had a bug in _Alignof. See GCC bug 52023
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023>.
clang versions < 8.0.0 have the same bug. */
# if (!defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 \
|| (defined __GNUC__ && __GNUC__ < 4 + (__GNUC_MINOR__ < 9) \
&& !defined __clang__) \
|| (defined __clang__ && __clang_major__ < 8))
# undef/**/_Alignof
# ifdef __cplusplus
# if (201103 <= __cplusplus || defined _MSC_VER)
# define _Alignof(type) alignof (type)
# else
template <class __t> struct __alignof_helper { char __a; __t __b; };
# if (defined __GNUC__ && 4 <= __GNUC__) || defined __clang__
# define _Alignof(type) __builtin_offsetof (__alignof_helper<type>, __b)
# else
# define _Alignof(type) offsetof (__alignof_helper<type>, __b)
# endif
# define _GL_STDALIGN_NEEDS_STDDEF 1
# endif
# else
# if (defined __GNUC__ && 4 <= __GNUC__) || defined __clang__
# define _Alignof(type) __builtin_offsetof (struct { char __a; type __b; }, __b)
# else
# define _Alignof(type) offsetof (struct { char __a; type __b; }, __b)
# define _GL_STDALIGN_NEEDS_STDDEF 1
# endif
# endif
# endif
# if ! (defined __cplusplus && (201103 <= __cplusplus || defined _MSC_VER))
# undef/**/alignof
# define alignof _Alignof
# endif
/* alignas (A), also known as _Alignas (A), aligns a variable or type
to the alignment A, where A is an integer constant expression. For
example:
int alignas (8) foo;
struct s { int a; int alignas (8) bar; };
aligns the address of FOO and the offset of BAR to be multiples of 8.
A should be a power of two that is at least the type's alignment
and at most the implementation's alignment limit. This limit is
2**28 on typical GNUish hosts, and 2**13 on MSVC. To be portable
to MSVC through at least version 10.0, A should be an integer
constant, as MSVC does not support expressions such as 1 << 3.
To be portable to Sun C 5.11, do not align auto variables to
anything stricter than their default alignment.
The following C23 requirements are not supported here:
- If A is zero, alignas has no effect.
- alignas can be used multiple times; the strictest one wins.
- alignas (TYPE) is equivalent to alignas (alignof (TYPE)).
*/
# if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112
# if defined __cplusplus && (201103 <= __cplusplus || defined _MSC_VER)
# define _Alignas(a) alignas (a)
# elif (!defined __attribute__ \
&& ((defined __APPLE__ && defined __MACH__ \
? 4 < __GNUC__ + (1 <= __GNUC_MINOR__) \
: __GNUC__ && !defined __ibmxl__) \
|| (4 <= __clang_major__) \
|| (__ia64 && (61200 <= __HP_cc || 61200 <= __HP_aCC)) \
|| __ICC || 0x590 <= __SUNPRO_C || 0x0600 <= __xlC__))
# define _Alignas(a) __attribute__ ((__aligned__ (a)))
# elif 1300 <= _MSC_VER
# define _Alignas(a) __declspec (align (a))
# endif
# endif
# if !defined HAVE_STDALIGN_H
# if ((defined _Alignas \
&& !(defined __cplusplus \
&& (201103 <= __cplusplus || defined _MSC_VER))) \
|| (defined __STDC_VERSION__ && 201112 <= __STDC_VERSION__))
# define alignas _Alignas
# endif
# endif
# if defined _GL_STDALIGN_NEEDS_STDDEF
# include <stddef.h>
# endif
#endif])
])
AC_DEFUN([gl_STDALIGN_H],
[
AC_REQUIRE([gl_ALIGNASOF])
if test "$gl_cv_header_working_stdalign_h" = no; then
GL_GENERATE_STDALIGN_H=true
else
GL_GENERATE_STDALIGN_H=false
fi
gl_CHECK_NEXT_HEADERS([stdalign.h])
if test $ac_cv_header_stdalign_h = yes; then
HAVE_STDALIGN_H=1
else
HAVE_STDALIGN_H=0
fi
AC_SUBST([HAVE_STDALIGN_H])
])
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