[Bug middle-end/115346] New: [15] Volatile load elimination with packed struct bitfields at -O2

[Bug middle-end/115346] New: [15] Volatile load elimination with packed struct bitfields at -O2

[patrick at rivosinc dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115346

            Bug ID: 115346
           Summary: [15] Volatile load elimination with packed struct
                    bitfields at -O2
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: patrick at rivosinc dot com
  Target Milestone: ---

Testcase:
#pragma pack(1)
struct a {
  unsigned b;
  long c : 33;
  unsigned d : 31;
};
volatile struct a h[40];
int main() {
  for (int i = 0; i < 40; i++) {
    h[0];
  }
}

-O3 asm output:
main:
        xor     eax, eax
        ret
h:
        .zero   480

Removing the pack pragma causes the loads to be emitted.

Godbolt: https://godbolt.org/z/3YfrbzdPz
GCC 4.0.4 does not eliminate the loads: https://godbolt.org/z/frsP8o7YT

[Bug middle-end/115346] [15] Volatile load elimination with packed struct bitfields

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115346

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>GCC 4.0.4 does not eliminate the loads: https://godbolt.org/z/frsP8o7YT

But 3.4.6 did not emit them either.

[Bug middle-end/115346] [15] Volatile load elimination with packed struct bitfields

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115346

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note this is not even emitted at -O0, the gimplifier removes it for some reason
...

[Bug middle-end/115346] [15] Volatile load elimination with packed struct bitfields

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115346

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> Note this is not even emitted at -O0, the gimplifier removes it for some
> reason ...

Oh see PR 99258 for analysis of the gimplifier (I think). with `#pragma
pack(1)`, the struct becomes BLKmode.

[Bug middle-end/115346] [15] Volatile load elimination with packed struct bitfields

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115346

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
duplicate

*** This bug has been marked as a duplicate of bug 99258 ***