From e0c3dac5c8c5e7789438380f2ad1341055a10360 Mon Sep 17 00:00:00 2001 From: Tom Stellard Date: Mon, 12 May 2014 17:32:29 +0000 Subject: Merging r201729: ------------------------------------------------------------------------ r201729 | chandlerc | 2014-02-19 17:35:01 -0500 (Wed, 19 Feb 2014) | 21 lines Teach Clang to provide ::max_align_t in C11 and C++11 modes. This definition is not chosen idly. There is an unfortunate reality with max_align_t -- the specific nature of its definition leaks into the ABI almost immediately. Because it is part of C11 and C++11 it becomes essential for it to match with other systems on that ABI. There is an effort to discourage any further use of this construct as a consequence -- using max_align_t introduces an immediate ABI problem. We can never update it to have larger alignment even as the microarchitecture changes to necessitate higher alignment. =/ The particular definition here exactly matches the ABI of GCC's chosen ::max_align_t definition, for better or worse. This was written with the help of Richard Smith who was decoding the exact ABI implications of the selected definition in GCC. Notably, in-register arguments are impacted by the particular definition chosen. =/ No one is under the illusion that this is a "good" or "useful" definition of max_align_t, and we are working with the standards committee to specify a more useful interface to address this need. ------------------------------------------------------------------------ git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_34@208600 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Headers/stddef.h | 10 ++++++++++ test/Headers/c11.c | 4 ++++ 2 files changed, 14 insertions(+) diff --git a/lib/Headers/stddef.h b/lib/Headers/stddef.h index 6a64d6d32a..b6926942ef 100644 --- a/lib/Headers/stddef.h +++ b/lib/Headers/stddef.h @@ -84,6 +84,16 @@ using ::std::nullptr_t; #endif #endif +#if __STDC_VERSION__ >= 201112L || __cplusplus >= 201103L +typedef struct { + long long __clang_max_align_nonce1 + __attribute__((__aligned__(__alignof__(long long)))); + long double __clang_max_align_nonce2 + __attribute__((__aligned__(__alignof__(long double)))); +} max_align_t; +#define __CLANG_MAX_ALIGN_T_DEFINED +#endif + #define offsetof(t, d) __builtin_offsetof(t, d) #endif /* __STDDEF_H */ diff --git a/test/Headers/c11.c b/test/Headers/c11.c index 11bec19544..e335d0cebc 100644 --- a/test/Headers/c11.c +++ b/test/Headers/c11.c @@ -22,6 +22,10 @@ _Static_assert(__alignof(c) == 4, ""); #define __STDC_WANT_LIB_EXT1__ 1 #include rsize_t x = 0; +_Static_assert(sizeof(max_align_t) >= sizeof(long long), ""); +_Static_assert(alignof(max_align_t) >= alignof(long long), ""); +_Static_assert(sizeof(max_align_t) >= sizeof(long double), ""); +_Static_assert(alignof(max_align_t) >= alignof(long double), ""); // If we are freestanding, then also check RSIZE_MAX (in a hosted implementation // we will use the host stdint.h, which may not yet have C11 support). -- cgit v1.2.3