Re: [PATCH] PPC faster sqrt functions

[Steve Munroe <sjmunroe at us dot ibm dot com>]

libc-alpha-owner@sources.redhat.com wrote on 05/25/2004 11:48:40 AM:

> 
> > +/* The method is based on a description in
> > +   Computation of elementary functions on the IBM RISC 
> System/6000 processor,
> > +   P. W. Markstein, IBM J. Res. Develop, 34(1) 1990.
> > +   Basically, it consists of two interleaved Newton-Rhapson 
approximations,
> > +   one to find the actual square root, and one to find its reciprocal
> > +   without the expense of a division operation.   The tricky bit here
> > +   is the use of the POWER/PowerPC multiply-add operation to get the
> > +   required accuracy with high speed.
> > +
> > + ...
> 
> What's all that crap?  There is already a generic implementation and
> this must be used.  If the generic implementation can be improved this
> is another thing but nothing of any concern here.
> 

This is nothing new. This implementation has been in place for over 4 year 
(as far back as cvsweb goes).

This is just the implementation from w_sqrt.c copied to e_sqrt.c because I 
was told to: http://sources.redhat.com/ml/libc-alpha/2004-05/msg00004.html

Other architecture override e_sqrt so I don't understand why PPC should be 
forced to use the generic e_sqrt implementation. This implementation, 
while not as fast as the fsqrt instruction available on PPC64, is faster 
than the generic implementation because it can take advantage of the 
hardware multiply-add instructions and multiple independent pipelines of 
PPC. 

If other architectures have hardware multiply-add instructions and wish to 
leverage this implementation they can do so by #include.

Steven J. Munroe
Linux on Power Toolchain Architect
IBM Corporation, Linux Technology Center