# Check for alignas and alignof that conform to C23. 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. dnl Written by Paul Eggert and Bruno Haible. # Prepare for substituting 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=$CFLAGS for gl_working in "yes, keywords" "yes, macros"; do AS_CASE([$gl_working], [*stdalign.h*], [CFLAGS="$gl_saved_CFLAGS -DINCLUDE_STDALIGN_H"]) AC_COMPILE_IFELSE( [AC_LANG_PROGRAM( [[#include #ifdef INCLUDE_STDALIGN_H #include #endif #include /* Test that alignof yields a result consistent with offsetof. This catches GCC bug 52023 . */ #ifdef __cplusplus template struct alignof_helper { char a; t b; }; # define ao(type) offsetof (alignof_helper, b) #else # define ao(type) offsetof (struct { char a; type b; }, b) #endif char test_double[ao (double) % _Alignof (double) == 0 ? 1 : -1]; char test_long[ao (long int) % _Alignof (long int) == 0 ? 1 : -1]; char test_alignof[alignof (double) == _Alignof (double) ? 1 : -1]; /* Test alignas only on platforms where gnulib can help. */ #if \ ((defined __cplusplus && 201103 <= __cplusplus) \ || (__TINYC__ && defined __attribute__) \ || (defined __APPLE__ && defined __MACH__ \ ? 4 < __GNUC__ + (1 <= __GNUC_MINOR__) \ : __GNUC__) \ || (__ia64 && (61200 <= __HP_cc || 61200 <= __HP_aCC)) \ || __ICC || 0x590 <= __SUNPRO_C || 0x0600 <= __xlC__ \ || 1300 <= _MSC_VER) struct alignas_test { char c; char alignas (8) alignas_8; }; char test_alignas[offsetof (struct alignas_test, alignas_8) == 8 ? 1 : -1]; #endif ]])], [gl_cv_header_working_stdalign_h=$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 # endif /* ISO C23 alignas and alignof for platforms that lack it. References: ISO C23 (latest free draft ) sections 6.5.3.4, 6.7.5, 7.15. C++11 (latest free draft ) 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 . 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 struct __alignof_helper { char __a; __t __b; }; # if (defined __GNUC__ && 4 <= __GNUC__) || defined __clang__ # define _Alignof(type) __builtin_offsetof (__alignof_helper, __b) # else # define _Alignof(type) offsetof (__alignof_helper, __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 # 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]) ])