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

[Juha.Riihimaki]

On Oct 27, 2009, at 11:00, ext Aurelien Jarno wrote:

> address@hidden a écrit :
>> On Oct 27, 2009, at 10:39, ext Aurelien Jarno wrote:
>>
>>> On Sat, Oct 24, 2009 at 03:19:04PM +0300, address@hidden
>>> wrote:
>>>> From: Juha Riihimäki <address@hidden>
>>>>
>>>> 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 <address@hidden>
>>>> Acked-by: Laurent Desnogues <address@hidden>
>>> 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.

Shouldn't the amount of temp variables stay the same? tcg_gen_addi_i32  
will internally allocate a temporary variable to hold the integer  
parameter. The only difference is that the temporary stays alive  
during the loop instead of being allocated/deallocated during each  
iteration. But I agree, the performance gain is probably quite small.


Regards,
Juha