Re: arm memcpy of aligned data

[Mike Stump <mikestump@comcast.net>]

On Jun 15, 2015, at 7:30 AM, Kyrill Tkachov <kyrylo.tkachov@arm.com> wrote:
> 
> On 29/05/15 11:15, Kyrill Tkachov wrote:
>> On 29/05/15 10:08, Kyrill Tkachov wrote:
>>> Hi Mike,
>>> 
>>> On 28/05/15 22:15, Mike Stump wrote:
>>>> So, the arm memcpy code of aligned data isn’t as good as it can be.
>>>> 
>>>> void *memcpy(void *dest, const void *src, unsigned int n);
>>>> 
>>>> void foo(char *dst, int i) {
>>>>     memcpy (dst, &i, sizeof (i));
>>>> }
>>>> 
>>>> generates horrible code, but, it we are willing to notice the src or the destination are aligned, we can do much better:
>>>> 
>>>> $ ./cc1 -fschedule-fusion -fdump-tree-all-all -da -march=armv7ve -mcpu=cortex-m4 -fomit-frame-pointer -quiet -O2 /tmp/t.c -o t.s
>>>> $ cat t.s
>>>> [ … ]
>>>> foo:
>>>> 	@ args = 0, pretend = 0, frame = 4
>>>> 	@ frame_needed = 0, uses_anonymous_args = 0
>>>> 	@ link register save eliminated.
>>>> 	sub	sp, sp, #4
>>>> 	str	r1, [r0]	@ unaligned
>>>> 	add	sp, sp, #4
>>> I think there's something to do with cpu tuning here as well.
>> That being said, I do think this is a good idea.
>> I'll give it a test.
> 
> The patch passes bootstrap and testing ok and I've seen it
> improve codegen in a few places in SPEC.
> I've added a testcase all marked up.
> 
> Mike, I'll commit the attached patch in 24 hours unless somebody objects.

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