Re: GCC ARM: aligned access

[Peng Fan <van dot freenix at gmail dot com>]

On 09/02/2014 09:25 PM, Julian Brown wrote:
> 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.)

Thanks for the reply. I've tried memcpy and all is fine.

Regards,
Peng.
> 
> Julian
>