https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86693
--- Comment #6 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to H.J. Lu from comment #5)
> (In reply to Hongtao.liu from comment #4)
> > Change testcase a little bit, gcc now can generate lock btc
> >
> >
> > void func1();
> >
> > void func(unsigned long *counter)
> > {
> > if (__atomic_fetch_xor(counter, 1, __ATOMIC_ACQ_REL) & 1) {
> > func1();
> > }
> > }
> >
> >
> > func(unsigned long*):
> > lock btc QWORD PTR [rdi], 0
> > jc .L4
> > ret
> > .L4:
> > jmp func1()
>
> We should rewrite the original test to the canonical form, similar to
> r12-5102.
> Hongtao, can you do that?
The orginal testcase is not equal to btc, __atomic_fetch_xor(counter, 1,
__ATOMIC_ACQ_REL) == 1 require other bits of *counter is 0. And as #c1 said, we
don't have instructions to keep the old or new value of xor or ior in a
register.
I can't find a way to optimize off the exchangeloop.
