[Bug libstdc++/65147] alignment of std::atomic object is not correct

[alexey.lapshin at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65147

--- Comment #7 from Alexey Lapshin <alexey.lapshin at oracle dot com> ---
(In reply to Jonathan Wakely from comment #5)
> Author: redi
> Date: Thu Mar 26 19:27:02 2015
> New Revision: 221703
> 
> URL: https://gcc.gnu.org/viewcvs?rev=221703&root=gcc&view=rev
> Log:
>       PR libstdc++/62259
>       PR libstdc++/65147
>       * include/std/atomic (atomic<T>): Increase alignment for types with
>       the same size as one of the integral types.
>       * testsuite/29_atomics/atomic/60695.cc: Adjust dg-error line number.
>       * testsuite/29_atomics/atomic/62259.cc: New.
> 
> Added:
>     trunk/libstdc++-v3/testsuite/29_atomics/atomic/62259.cc
>       - copied, changed from r221701,
> trunk/libstdc++-v3/testsuite/29_atomics/atomic/60695.cc
> Modified:
>     trunk/libstdc++-v3/ChangeLog
>     trunk/libstdc++-v3/include/std/atomic
>     trunk/libstdc++-v3/testsuite/29_atomics/atomic/60695.cc


It looks like this fix makes alignment of atomic object to be the same as
alignment of integral non-atomic object of the same size.

The gcc behavior is different it makes alignment of atomic objects of sizes
1,2,4,8,16 to match with size :

G++ :
$ cat all.cc

#include <atomic>
#include <stdio.h>

typedef struct {
   char c[16];
} S16;

int main ( void ) {
   std::atomic<char>      ac;
   std::atomic<short>     as;
   std::atomic<long>      al;
   std::atomic<long long> all;
   std::atomic<S16>       a16;

   printf("\n sizeof(ac) %d alignof(ac) %d",  sizeof(ac), alignof(ac) );
   printf("\n sizeof(as) %d alignof(as) %d",  sizeof(as), alignof(as) );
   printf("\n sizeof(al) %d alignof(al) %d",  sizeof(al), alignof(al) );
   printf("\n sizeof(all) %d alignof(all) %d",  sizeof(all), alignof(all) );
   printf("\n sizeof(a16) %d alignof(a16) %d",  sizeof(a16), alignof(a16) );
   printf("\n");
}
$g++ -latomic -std=c++11 -m32 all.cc 
$./a.out

 sizeof(ac) 1 alignof(ac) 1
 sizeof(as) 2 alignof(as) 2
 sizeof(al) 4 alignof(al) 4
 sizeof(all) 8 alignof(all) 4
 sizeof(a16) 16 alignof(a16) 1


gcc : 

$ cat all.c

#include <stdatomic.h>
#include <stdio.h>

typedef struct {
   char c[16];
} S16;

int main ( void ) {
   _Atomic char      ac;
   _Atomic short     as;
   _Atomic long      al;
   _Atomic long long all;
   _Atomic S16       a16;

   printf("\n sizeof(ac) %d alignof(ac) %d",  sizeof(ac), __alignof__(ac) );
   printf("\n sizeof(as) %d alignof(as) %d",  sizeof(as), __alignof__(as) );
   printf("\n sizeof(al) %d alignof(al) %d",  sizeof(al), __alignof__(al) );
   printf("\n sizeof(all) %d alignof(all) %d",  sizeof(all), __alignof__(all)
);
   printf("\n sizeof(a16) %d alignof(a16) %d",  sizeof(a16), __alignof__(a16)
);

   printf("\n");
}
$ gcc -latomic -std=c11 -m32 all.c 
$ ./a.out

 sizeof(ac) 1 alignof(ac) 1
 sizeof(as) 2 alignof(as) 2
 sizeof(al) 4 alignof(al) 4
 sizeof(all) 8 alignof(all) 8
 sizeof(a16) 16 alignof(a16) 16
avl@ficus:~/atomic_test$ 

Note 8-bytes and 16-bytes objects aligned at their size at -m32.