__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 not defined on aarch64 (correction)

[Toebs Douglass]

On 26/06/17 09:37, Toebs Douglass wrote:
> Hej all.
> 
> __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 is defined on x86_64 if and only if
> -mcx16 is given.
> 
> gcc version 4.9.2 (Debian 4.9.2-10)
> winterflaw@localhost:~$ gcc -march=native -dM -E - < /dev/null | sort |
> grep SYNC
> #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
> #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 1
> #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1
> #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 1
> #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 1

Stunning, eh?  what happened was; without -march=native, there's no
SYNC_16.  This isn't too surprising, I think, because looking over the
list of predefines (and configure options) the compiler actually seems
to have no default arch - I mean, it must, but there are no defines
indicating it.  Lord knows what it's assuming, since the i586 introduced
DWCAS on Intel.

Even with that seeming lack of arch, with -mcx16, we do of course then
get SYNC_16, since we've told the compiler it exists.

If I then move to arch native, I get SYNC_16 even without mcx16; this on
4.9.2.  That surprised me (and is the cause of the mistake in the
original post).

Anyways, this wasn't the main point - it was only there to point out
SYNC_16 is set on x86_64 (usually).

> __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 is not defined at all on aarch64, as
> far as I can tell.
> 
> gcc version 4.9.2 (Debian/Linaro 4.9.2-10)
> debian@pine64:~$ gcc -march=armv8-a+crypto+crc -dM -E - < /dev/null |
> sort | grep SYNC
> #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
> #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1
> #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 1
> #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 1

This being the main point.