https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114597
Xi Ruoyao <xry111 at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |xry111 at gcc dot gnu.org
--- Comment #4 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to wierton from comment #3)
> Thanks a lot, I have checked it out.
>
> I'm still somewhat confused. Does the difference in compilation stem from
> GCC interpreting "=m" and "=r" differently, leading it to assume that once
> "n.p" is declared as an output operand without matched input declaration
> (eg. "=m" expects "m", "=r" expects "r"), its value is expected to change?
>
> ```
> asm("":"=m"(n.p):"r"(n.p)); // n.p is expected to change
> asm("":"=r"(n.p):"r"(n.p)); // n.p can retain its value
> asm("":"=m"(n.p):"m"(n.p)); // n.p can retain its value
> ```
I believe the second form is still incorrect and it just works by luck. To
ensure it work you have to do:
asm("mov %1,%0":"=r"(n.p):"r"(n.p));
Or
asm("":"+r"(n.p));
Not so sure about the third form.
