[Bug c/50521] -fstrict-volatile-bitfields is not strict

[kikairoya at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50521

--- Comment #13 from Tomohiro Kashiwada <kikairoya at gmail dot com> 2011-10-29 06:50:11 UTC ---
(In reply to comment #12)
> Regarding the double load. In a statement like a = b, both a & be should be
> individually accessed even if they refer to the same storage. So  
> bitfield.bits.a = bitfield.bits.c should load the bitfield variable twice, once
> for reading the rvalue and once for masking the lvalue assignment.
> 
> See 7.1.7.5 second and third paragraph and the note just after.

Is that means a statement
  a = b;
always should be treat as if
  tmp = b;
  a = tmp;
two individual statements?


> Regarding STRICT_ALIGNMENT, not strictly needed on ARM i think. Smaller
> accesses than the base type is acceptable, as long as it's aligned to the
> matching access size (8/16/32/64 bit) and on ARMv7 unaligned access is allowed,
> but at a performance penalty. And this change is technically unrelated to
> strict-volatile-bitfields even if there is overlap.

I think STRICT_ALIGNMENT is not only for ARM, but also MIPS, SH and others.
I'll create new ticket later about STRICT_ALIGNMENT.