[PATCH] ARM: lib: use LDRD/STRD for data copy

[Michael Hope]

On 20 March 2012 05:36, Rob Herring <robherring2 at gmail.com> wrote:
> On 03/19/2012 10:41 AM, Russell King - ARM Linux wrote:
>> On Mon, Mar 19, 2012 at 09:36:41AM -0500, Rob Herring wrote:
>>> On 03/19/2012 03:55 AM, Russell King - ARM Linux wrote:
>>>> On Mon, Mar 19, 2012 at 04:02:48PM +0900, Boojin Kim wrote:
>>>>> This patch uses LDRD/STRD that loads and stores data as DWORD unit
>>>>> for the copy of 8-words data.
>>>>> It brings better performance than LDRM/STRM that was used originally.
>>>>
>>>> And what about CPUs that don't have ldrd/strd ?
>>>>
>>>
>>> And what about CPUs that do have ldrd/strd but is slower than ldm/stm?
>>> I'm pretty sure that is almost everything currently out there.
>>
>> The double-word load/stores were introduced in ARMv6.  Some Intel based
>> CPUs prior to this have the support as well.  Everything else doesn't.
>>
>> So taht's nowhere close to 'almost everything'.
>
> I meant of all platforms that support both instructions, ldm/stm will be
> faster than ldrd/strd on almost all of them AFAIK. I don't think the
> claim about being faster is true for an CortexA9 or anything prior.
> Linaro folks have done some benchmarking in this area and would be
> better to comment.

My understanding is that the A15 does well with LDRD and poorly with
LDM, all other cores do well with LDM, and the A9 at least does poorly
with LDRD.  I don't have numbers at hand to back it up.

FYI, here's ARM's Cortex-A15 LDRD based memcpy implementation:
 http://sourceware.org/ml/newlib/2011/msg00469.html

-- Michael