Re: [eigen] log10 support?

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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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