Re: GCC ARM: aligned access

[Julian Brown <julian at codesourcery dot com>]

On Mon, 1 Sep 2014 09:14:31 +0800
Peng Fan <van.freenix@gmail.com> wrote:

> On 09/01/2014 08:09 AM, Matt Thomas wrote:
> > 
> > On Aug 31, 2014, at 11:32 AM, Joel Sherrill
> > <joel.sherrill@OARcorp.com> wrote:
> >> I think this is totally expected. You were passed a u8 pointer
> >> which is aligned for that type (no restrictions likely). You cast
> >> it to a type with stricter alignment requirements. The code is
> >> just flawed. Some CPUs handle unaligned accesses but not your ARM.
> > 
> armv7 and armv6 arch except armv6-m support unaligned access. a u8
> pointer is casted to u32 pointer, should gcc take the align problem
> into consideration to avoid possible errors? because
> -mno-unaligned-access.

Using -munaligned-access (or its inverse) isn't enough to make GCC
generate code that can perform arbitrary unaligned accesses, because
several instructions (e.g. VFP loads/stores or load/store multiple
instructions IIRC) must still act on naturally-aligned data even when
the hardware flag to enable unaligned accesses is on, and those
instructions will still be generated by GCC when they are considered
safe, i.e. when not doing explicitly-unaligned accesses in packed
structures or similar.

It would be *possible* to add an option to the backend to allow
arbitrary alignment for any access, I think, but it's not at all clear
that it's a good idea, and would certainly negatively affect
performance.

(If you need unaligned accesses, you can use e.g. memcpy, and that will
probably generate good inline code.)

Julian