fsqrt

[Kumar Gala]

On Jun 7, 2013, at 7:14 AM, Benjamin Herrenschmidt wrote:

> On Fri, 2013-06-07 at 11:48 +0100, David Laight wrote:
>>> For those interested, this is the Quake3 sqrt from Carmack ...
>> there's
>>> plenty of literature about it one or two google clicks away :-)
>> 
>> I guess that is a rough enough approximation for graphics.
>> 
>> However it will be miscompiled unless i and y are put in a union.
> 
> It won't in the kernel disables strict aliasing :-)
> 
> Anyway, that was just a hack and plenty enough to get anaconda going,
> the bloody thing only uses fsqrt because it's python crappola does
> something like exp(1.0) / sqrt(2.0) as part of its random number stuff.
> 
> Honestly I could have made it just return 1.0 and it would probably have
> worked :-)
> 
> However, my point remains, it would be very much worthwhile for the
> kernel to have some reasonable emulation of those missing instructions
> (afaik only a handful) like we have for isel, popcnt* etc... especially
> since distros seem to be keen on enabling the use of them in their
> toolchain.
> 
> I don't personally have the bandwidth to do a clean implementation (that
> handles FP exceptions, NaNs, FPSCR, etc...) but I believe it would be
> valuable if somebody else did (hint hint hint :-) since without this,
> Fedora ppc64 is basically going to be a non-started on those chips.
> 
> BTW. Did you guys (ie. FSL) finally add fsqrt to e6500 or it's still
> out ? 
> 
> Cheers,
> Ben.

Pretty sure fsqrt is still out of e6500.

- k