This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: movmemm pattern
On Tue, Oct 26, 2010 at 1:12 PM, Paul Koning <Paul_Koning@dell.com> wrote:
>
> On Oct 26, 2010, at 1:27 PM, Dave Korn wrote:
>
>> On 26/10/2010 17:16, Paul Koning wrote:
>>> On Oct 25, 2010, at 9:28 PM, Dave Korn wrote:
>>>> ...
>>>>
>>>> What happens if you dereference i and j before the memcpy in foo? ?Do you
>>>> then get int-sized shared alignment in movmemM?
>>>>
>>>> extern int *i, *j; void foo (void) { *i; *j; memcpy (i, j, 10); }
>>>
>>> That doesn't make any difference; I still get alignment 1.
>>>
>>
>> ?That sounds like a missed optimisation opportunity to me, but maybe there's
>> some reason the compiler can't infer a larger alignment. ?(Or maybe the reads
>> got discarded, you might need volatile to avoid that?)
>
> No, they weren't discarded, I can see them in the output assembler code. ?Actually, I wrote *i = 20; *j = 10;
Alignment propagation (bit-CCP) doesn't (yet) exploit information from
such a load (on x86 it couldn't - that architecture can handle unaligned
loads just fine).
Richard.
> ? ? ? ?paul
>
>
>