[Bug middle-end/63991] redundant read when write to volatile member of packed structure

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63991

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |arm-none-eabi
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-11-20
                 CC|                            |ebotcazou at gcc dot gnu.org
          Component|c                           |middle-end
     Ever confirmed|0                           |1
      Known to fail|                            |5.0

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
That's indeed a bug as if the structure is unaligned (which is what happens
with -fpack-struct).  Correct code looks like

        mov     r2, #32
        mov     r3, #0
        ldrb    r1, [r0, #20]   @ zero_extendqisi2
        strb    r2, [r0, #20]
        ldrb    r2, [r0, #21]   @ zero_extendqisi2
        strb    r3, [r0, #21]
        ldrb    r2, [r0, #22]   @ zero_extendqisi2
        strb    r3, [r0, #22]
        ldrb    r2, [r0, #23]   @ zero_extendqisi2
        strb    r3, [r0, #23]

but still for some reason we create volatile unused loads at expansion time
(and those prevail and don't get DCEd, obviously).

Probably happens on all strict-alignment targets.