[Bug rtl-optimization/92151] Spurious register copying

[Bug rtl-optimization/92151] Spurious register copying

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92151

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Alexander Monakov from comment #3)
> 
> Here's a manually reduced variant that exhibits a similar issue at -O1:
> 
> void foo(int num, int c) {
>     asm("# %0" : "+r"(num));
>     while (--c)
>         asm goto("# %0" :: "r"(num) :: l2);
> l2:
>     asm("# %0" :: "r"(num));
> }

Note the DEAD_REG is updated in IRA anyways so that is not the issue.

This reduced testcase seems to be fixed in GCC 11+, maybe r11-209
(https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545442.html).

The original testcase did fix on mov issue.
But 
  __asm__("\t mov   %q[w], (%q[d])\n"   // write
                                    : "=m" (*d) : [d] "r" (d), [w] "r" (w)) ;
  __asm__("\t lea   8(%q[d]), %q[d]\n"
                                    : [d] "+r" (d)) ;

Still causes an extra move to happen.

Note there was one improvement in GCC 11 in the original testcase too.

I think TWITCH difference is due to the way register variables are represented
in gimple ...