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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.

llvm-svn: 201729
This commit is contained in:
Chandler Carruth 2014-02-19 22:35:01 +00:00
parent bcee91805e
commit e813984b43
2 changed files with 14 additions and 0 deletions
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10
clang/lib/Headers/stddef.h
View File

@ -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 */

4
clang/test/Headers/c11.c
View File

@ -22,6 +22,10 @@ _Static_assert(__alignof(c) == 4, "");
#define __STDC_WANT_LIB_EXT1__ 1
#include <stddef.h>
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).