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

[Måns Rullgård <mans.rullgard@xxxxxxxxxx>]

Rob Herring <robherring2@xxxxxxxxx> writes:

> 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.

The Cortex-A9 TRM insists ldrd and ldm should have the same timing.
However, measuring it suggests that ldm is in fact faster, at least in
some cases.

The Cortex-A8 TRM is a bit unclear, but measuring gives the same speed
for both.

The manuals for older cores suggest equivalent timing, but I don't have
any nearby to test.

-- 
Måns Rullgård
mans@xxxxxxxxx


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