Re: Model specific optimizations?

[Rich Felker <dalias@libc.org>]

On Sat, Oct 01, 2016 at 10:52:14AM +0200, Markus Wichmann wrote:
> On Sat, Oct 01, 2016 at 01:50:23AM -0400, Rich Felker wrote:
> > I don't think this works at all. sqrt() is required to be
> > correctly-rounded; that's the whole reason sqrt.c is costly.
> 
> It's an approximation, at least, which was rather my point.
> 
> As I've come to realize over the course of this discussion, the fsqrt
> instruction is useless here and pretty much everywhere out there:

I don't think that conclusion is correct. It certainly makes sense for
libc to use it in targets that have it, assuming it safely produces
correct results, and for compilers to generate it in place of a call
to sqrt.

> - If you are looking for accuracy over speed, the standard C library has
>   got you covered.

Yes.

> - If you are looking for speed over accuracy, you can code up the
>   Babylonian method inside five minutes. You can even tune it to suit
> your needs to an extent (mainly, number of rounds and method of first
> approximation). This method is also portable to other architectures, and
> can be done entirely in C (requiring IEEE floating point, but then, most
> serious FP code does that).

This is not going to give you speed. If you want fast sqrt
approximations, there are lots out there that are actually fast. And
if the final result you need is 1/sqrt there are even faster ones.

> Also, at least according to Apple, which were the only ones actually
> looking at the thing, such as I could find, it was only ever supported
> by the 970 and the 970FX cores, released in 2002 and 2004, respectively.
> I highly doubt they'll have much relevance. Chalk up my suspicions from
> the OP to not having researched enough.

Do you mean these are the only non-POWER line models that have fsqrt?

> In closing: Nice discussion, but I'm sorry for the noise.

I don't think it's noise. It's been informative. And it does suggest
that we should add static, compile-time support for using fsqrt on
POWER and perhaps on these specific models that have it. That's useful
information for making it a better-supported target under musl.

Rich