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

[Rob Herring]

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.

Rob