Re: [eigen] log10 support?

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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
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.


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:
>> 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
>> ...

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