| # stdalign.m4 |
| # serial 3 |
| dnl Copyright 2011-2026 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. |
| dnl This file is offered as-is, without any warranty. |
| |
| # 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="$gl_saved_CFLAGS -DINCLUDE_STDALIGN_H"]) |
| AC_COMPILE_IFELSE( |
| [AC_LANG_PROGRAM( |
| [[#include <stdint.h> |
| #ifdef INCLUDE_STDALIGN_H |
| #include <stdalign.h> |
| #endif |
| #include <stddef.h> |
| |
| /* Test that alignof yields a result consistent with offsetof. |
| This catches GCC bug 52023 |
| <https://gcc.gnu.org/PR52023>. */ |
| #ifdef __cplusplus |
| template <class t> struct alignof_helper { char a; t b; }; |
| # define ao(type) offsetof (alignof_helper<type>, b) |
| #else |
| # define ao(type) offsetof (struct { char a; type b; }, b) |
| #endif |
| char test_double[ao (double) % _Alignof (double) == 0 ? 1 : -1]; |
| char test_long[ao (long int) % _Alignof (long int) == 0 ? 1 : -1]; |
| char test_alignof[alignof (double) == _Alignof (double) ? 1 : -1]; |
| |
| /* Test alignas only on platforms where gnulib can help. */ |
| #if \ |
| ((defined __cplusplus && 201103 <= __cplusplus) \ |
| || (__TINYC__ && defined __attribute__) \ |
| || (defined __APPLE__ && defined __MACH__ \ |
| ? 4 < __GNUC__ + (1 <= __GNUC_MINOR__) \ |
| : __GNUC__) \ |
| || (__ia64 && (61200 <= __HP_cc || 61200 <= __HP_aCC)) \ |
| || __ICC || 0x590 <= __SUNPRO_C || 0x0600 <= __xlC__ \ |
| || 1300 <= _MSC_VER) |
| struct alignas_test { char c; char alignas (8) alignas_8; }; |
| char test_alignas[offsetof (struct alignas_test, alignas_8) == 8 |
| ? 1 : -1]; |
| #endif |
| ]])], |
| [gl_cv_header_working_stdalign_h=$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 |
| <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf>) |
| sections 6.2.8, 6.7.6. |
| C++11 (latest free draft |
| <https://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/PR52023>. |
| clang versions < 8.0.0 have the same bug. |
| IBM XL C V16.1.0 cc (non-clang) has the same bug. */ |
| # if (!defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 \ |
| || (defined __GNUC__ && __GNUC__ < 4 + (__GNUC_MINOR__ < 9) \ |
| && !defined __clang__) \ |
| || (defined __clang__ && __clang_major__ < 8) \ |
| || defined __xlC__) |
| # 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__ \ |
| && !defined __xlC__)) |
| # 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]) |
| ]) |
