gcc and 128-bit compare/exchange

Eliot Moss moss@cs.umass.edu
Mon Mar 9 02:59:51 GMT 2020

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, __ATOMIC_SEQ_CST, 
> 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.

Regards - EM

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