Re: [Qemu-devel] [PATCH v2 05/10] target-arm: optimize arm load/store multiple ops

[Aurelien Jarno <aurelien@aurel32.net>]

Juha.Riihimaki@nokia.com a écrit :
> On Oct 27, 2009, at 10:39, ext Aurelien Jarno wrote:
> 
>> On Sat, Oct 24, 2009 at 03:19:04PM +0300, juha.riihimaki@nokia.com  
>> wrote:
>>> From: Juha Riihimäki <juha.riihimaki@nokia.com>
>>>
>>> RM load/store multiple instructions can be slightly optimized by
>>> loading the register offset constant into a variable outside the
>>> register loop and using the preloaded variable inside the loop  
>>> instead
>>> of reloading the offset value to a temporary variable on each loop
>>> iteration. This causes less TCG ops to be generated for a ARM load/
>>> store multiple instruction if there are more than one register
>>> accessed, otherwise the number of generated TCG ops is the same.
>>>
>>> Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
>>> Acked-by: Laurent Desnogues <laurent.desnogues@gmail.com>
>> This patch breaks, the boot of an arm kernel, as tmp2 is used  
>> elsewhere
>> within this code path.
> 
> True, I just noticed that as well. This is because the resource leak  
> patch
> was refactored to utilize tmp2 inside the loop as well. I just sent a  
> new
> revision of this patch that uses tmp3 for th constant value.
> 
>> OTOH, while it reduce the number of TCG ops, that should not impact  
>> the
>> generated host asm code, as most (all ?) targets are able to add a
>> small constant value to a register in one instruction.
> 
> This is true, but I still think it provides a small speed gain as  
> there are
> less TCG ops to be processed when generating host code...?

It means less TCG ops, but one more temp variable, therefore if there is
a gain, I don't think it is something even measurable.

OTOH it makes the code a bit more complex to read. I am not really
opposed to this patch (and the other patches of the same kind), but I
will need some more arguments to convince me.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net