r201729 - Teach Clang to provide ::max_align_t in C11 and C++11 modes.

Chandler Carruth chandlerc at gmail.com
Wed Feb 19 14:35:02 PST 2014

Author: chandlerc
Date: Wed Feb 19 16:35:01 2014
New Revision: 201729

URL: http://llvm.org/viewvc/llvm-project?rev=201729&view=rev
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.


Modified: cfe/trunk/lib/Headers/stddef.h
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Headers/stddef.h?rev=201729&r1=201728&r2=201729&view=diff
--- cfe/trunk/lib/Headers/stddef.h (original)
+++ cfe/trunk/lib/Headers/stddef.h Wed Feb 19 16:35:01 2014
@@ -84,6 +84,16 @@ using ::std::nullptr_t;
+#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 offsetof(t, d) __builtin_offsetof(t, d)
 #endif /* __STDDEF_H */

Modified: cfe/trunk/test/Headers/c11.c
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Headers/c11.c?rev=201729&r1=201728&r2=201729&view=diff
--- cfe/trunk/test/Headers/c11.c (original)
+++ cfe/trunk/test/Headers/c11.c Wed Feb 19 16:35:01 2014
@@ -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).

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