Re: [eigen] log10 support?

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

2010/6/7 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> ok so time for another answer :)
>
> The only thing that I really don't want is for us to add
> SSE/AltiVec/whatever code for log10. We must reuse the existing code
> for log there.
>
> Then there are two ways:
> a) the way of your patch: add a new custom functor for log10
> b) the way i suggested: just let log10 return a product expression.
>
> Your way a) has the advantage of sparing one multiplication (at least
> in the non vectorized case), but b) means less extra code.
>
> Now Hauke raises a good point, that I overlooked: even with a) you can
> still reuse our vectorization code for log10. So actually, I'm OK with
> your approach, provided that we do that. You can do it generically for
> all types at once (Hauke showed the way for float but there's no
> reason to special-case it): edit Core/GenericPacketMath.h and
> implement ei_plog10 there like the other functions, returning
>  ei_pmul(
>    ei_pset1(log(Scalar(0.1))),

woooooops!!

replace log(Scalar(0.1)) by Scalar(1) / log(Scalar(10))

and sorry Hauke, I saw too late that you already wrote that. Though I
guess there's value in insisting that it's faster to multiply by a
constant, than to divide by a constant.

Benoit

>    ei_plog(x)
>  )
> The next problem is to cache the result of ei_pset1(log(Scalar(0.1))),
> probably a static constant, maybe have a look at what's done in
> ei_plog itself in SSE/. If that sounds obscure, leave that for later.
>
> In any case, we don't need a new HasLog10. Since we'll be using
> ei_plog, all we need is the existing HasLog.
>
> Benoit
>
> 2010/6/7 Trevor Irons <trevorirons@xxxxxxxxx>:
>> OK, thanks. I'm on a really old machine and can't check sse here. I'll test
>> your code and generate another patch. Unless I hear that is is unwanted.
>>
>> (This just goes in eigenNes/Eigen/src/Core/arch/SSE/MathFunctions.h) right?
>>
>> -Trevor
>>
>> On 7 June 2010 15:46, Hauke Heibel <hauke.heibel@xxxxxxxxxxxxxx> wrote:
>>>
>>> Let me try to answer for Benoit. :)
>>>
>>> On Mon, Jun 7, 2010 at 11:20 PM, Trevor Irons <trevorirons@xxxxxxxxx>
>>> wrote:
>>> > Sure, I can do this. I prefer store a minimum of constants. But if it
>>> > will
>>> > be faster, then sure.
>>> >
>>> > Why won't calls to std::log10 be vectorized? No sse instructions?
>>>
>>> This is because std::log is not right away available as an SSE
>>> intrinsic and we have a special version based on the code from here:
>>> http://gruntthepeon.free.fr/ssemath/
>>>
>>> Nonetheless I think your patch is the right way to go. We could
>>> achieve easily what Benoit suggests by offering a version of ei_plog10
>>> such as
>>>
>>> Packet4f ei_plog10(Packet4f x)
>>> {
>>>  const Packet4f ei_p4f_log10 = ei_plog(ei_pset1(10.0f));
>>>  return ei_pdiv(ei_plog(x), ei_p4f_log10);
>>> }
>>>
>>> which uses "Packet4f ei_plog(Packet4f x)". The constant would probably
>>> be taken care of by a decent compiler, i.e. I don't think we need
>>> manual caching.
>>>
>>> > Also, is there any desire to have log10 defined your way in Eigen, for
>>> > convenience? Or are you just suggesting I do this locally?
>>>
>>> I am pretty sure he meant that it would be nice to really have this in
>>> Eigen and not just in a local copy... though I am wildly guessing.
>>>
>>> - Hauke
>>>
>>> p.s. I am pretty tired an hope that I picked the right conversion factor
>>> ...
>>>
>>>
>>
>>
>