gcc and 128-bit compare/exchange

Eliot Moss moss@cs.umass.edu
Tue Mar 10 21:13:56 GMT 2020

On 3/10/2020 4:35 PM, Brian Inglis wrote:
> On 2020-03-08 20:59, Eliot Moss wrote:
>> On 3/8/2020 10:29 PM, Eliot Moss wrote:
>>> This is probably to the gcc maintainer ...
>>> I am running on a processor that has compare/exchange 128-bit (cx16 capability),
>>> and I compiler with -mcx16 and -latomic.  I'm on the latest release cygwin gcc
>>> (9.2.0-3, I believe) and the corresponding libatomic.  I have a program with
>>> this in it:
>>> __atomic_compare_exchange((__int128 *)&s1, (__int128 *)&z, (__int128 *)&s2, 0,
>>> This compiles to a call (nice if it would inline, but ...) to
>>> __atomic_compare_exchange_16, which uses mutex's, not the CMPXCHG16B
>>> instruction I was hoping for.  Note I am doing dynamic linking,
>>> which on at least one other platform results in dynamic selection
>>> of a lib_at implementation of the compare/exchange, which does use
>>> the desired instruction.
>>> Is this a limitation of cygwin gcc, or should I be doing something
>>> different to achieve the desired effect?
>>> Obviously it would be best not to going an asm inline if I can avoid it,
>>> but I suppose I can dig into the libatomic source to get the right
>>> incantation for it if need be ...
>> A quick followup: I was able to get __sync_val_compare_and_swap_16 to work
>> (and its bool form).  That will do for now, though of course it's deprecated.
> You just needed to go to the next info page:
> 	$ info gcc __atomic
> and use:
> 	#include <stdatomic.h>
> 	extern bool __atomic_compare_exchange ( TYPE *ptr,
> 						TYPE *expected,
> 						TYPE *desired,
> 						bool weak,
> 						int success_memorder,
> 						int failure_memorder);
> or higher level macro:
> 	atomic_compare_exchange_strong(PTR, VAL, DES)
> defined in /lib/gcc/x86_64-pc-cygwin/*/include/stdatomic.h

That is what I was already doing :-) .... it compiles and runs,
landing at libat_compare_exchange_16, which uses mutexes, not the
16-byte compare-exchange instruction.  So how do I get the compare-
exchange instruction to be used in the library?

Regards - EM

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