[Bug c/113106] Missing CSE with cast to volatile

[ubizjak at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113106

--- Comment #5 from Uroš Bizjak  ---
The issue in comment #2 happens in a couple of places when compiling linux
kernel (with named address spaces enabled). However, the issue is not specific
to named AS, I was just more attentive to moves from %gs: prefixed locations.

[Bug c/113106] Missing CSE with cast to volatile

[ubizjak at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113106

--- Comment #4 from Uroš Bizjak  ---
(In reply to Richard Biener from comment #3)
> The situation with address-spaces isn't valid as we need to preserve the
> second load because it's volatile.  I think we simply refuse to combine
> volatile loads out of caution in the first case.

int __seg_gs b;
return *(volatile __seg_gs int *)  + b;

But the above is the same w.r.t to volatile as:

int a;
return *(volatile int *)  + a;

?

BTW: I also checked with clang, and it creates expected code in all cases.

[Bug c/113106] Missing CSE with cast to volatile

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113106

Richard Biener  changed:

   What|Removed |Added

 CC||rguenth at gcc dot gnu.org

--- Comment #3 from Richard Biener  ---
The situation with address-spaces isn't valid as we need to preserve the second
load because it's volatile.  I think we simply refuse to combine
volatile loads out of caution in the first case.

[Bug c/113106] Missing CSE with cast to volatile

[ubizjak at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113106

--- Comment #2 from Uroš Bizjak  ---
For reference, the same optimization should be applied with address spaces:

--cut here--
int __seg_gs b;

int bar(void)
{
  return *(volatile __seg_gs int *)  + b;
}
--cut here--

the above testcase currently compiles to:

movl%gs:b(%rip), %eax
addl%gs:b(%rip), %eax
ret

but can be compiled to:

movl%gs:b(%rip), %eax
addl%eax, %eax
ret

[Bug c/113106] Missing CSE with cast to volatile

[ubizjak at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113106

--- Comment #1 from Uroš Bizjak  ---
Perhaps related,

--cut here--
int a;

int foo(void)
{
  return *(volatile int *)  + *(volatile int *) 
}
--cut here--

compiles with -O2 to:

movla(%rip), %eax
movla(%rip), %edx
addl%edx, %eax
ret

but may be compiled to:

movla(%rip), %eax
addla(%rip), %eax
ret

(the memory read may propagate to the insn)