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From: Eliot Moss <moss AT cs DOT umass DOT edu>
Subject: gcc and 128-bit compare/exchange
Message-ID: <ab69ca04-06a2-eeb9-4771-e37432b59a77@cs.umass.edu>
Date: Sun, 8 Mar 2020 22:29:11 -0400
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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, 
__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 ...

Regards - Eliot Moss
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