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From: "thiago at kde dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/102566] [i386] GCC should emit LOCK BTS for simple
 bit-test-and-set operations with std::atomic
Date: Wed, 06 Oct 2021 15:40:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102566

--- Comment #20 from Thiago Macieira <thiago at kde dot org> ---
And:

$ cat /tmp/test.cpp=20
#include <atomic>
bool tbit(std::atomic<long> &i)
{
  return i.fetch_xor(CONSTANT, std::memory_order_relaxed) & (CONSTANT);
}
$ ~/dev/gcc/bin/gcc "-DCONSTANT=3D(1LL<<63)" -S -o - -O2 /tmp/test.cpp | sed
'1,/startproc/d;/endproc/,$d'
        lock btcq       $63, (%rdi)
        setc    %al
        ret

Nice!=