[Bug target/32392] New: Support using -mrecip w/o additional Newton-Raphson run

[Bug target/32392] New: Support using -mrecip w/o additional Newton-Raphson run

[burnus at gcc dot gnu dot org]

Paolo Bonzini wrote:
>> That said, there is a whole bunch of applications that would kill for -mrecip, 
> even for 11bit ones. Games are one of them, for sure ;)
> What about -mrecip=0/1/2 for the number of NR steps? Or would two steps be 
> slower than divss?
>
> I was thinking of adding this as a follow-up patch ;) Just look how the 
> operations are grouped together.

As Richard pointed out: Having two NR does not make sense. For some cases doing
with out Newton-Raphson is enough. (Example: Games -- or SPEC CPU 2006:
http://www.hpcwire.com/hpc/1556972.html)

Other compilers have this option, e.g. Pathscale's -OPT:rsqrt=2 [yes, this is
used for SPEC runs ;-)]


-- 
           Summary: Support using -mrecip w/o additional Newton-Raphson run
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: burnus at gcc dot gnu dot org
GCC target triplet: x86_64-*-* i686-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32392

[Bug target/32392] Support using -mrecip w/o additional Newton-Raphson run

[burnus at gcc dot gnu dot org]

------- Comment #1 from burnus at gcc dot gnu dot org  2007-06-18 15:03 -------
Initial suggestion, see:
http://gcc.gnu.org/ml/gcc-patches/2007-06/msg01068.html

Richard's remark:
http://gcc.gnu.org/ml/gcc-patches/2007-06/msg01224.html
> Two NR steps don't make sense, they wouldn't improve accuracy because of the
> extra roundings we get for the NR.  And of course it would be slower.

(However, two NR are said to be enough for double precision. I don't know
whether doing rsqrt+(2x NR) is faster than 1/sqrt() for double or not.)

Related - closed - PRs:
PR 31723 - Use reciprocal and reciprocal square root with -ffast-math (FIXED)
PR 32352 - Using rsqrt, Polyhedron's aermod test crashes (WONTFIX)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32392

[Bug target/32392] Support using -mrecip w/o additional Newton-Raphson run

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2007-06-19 09:15 -------
Confirmed.  For 2 NR steps to reach double precision (we'd miss it by some more
ulps than the 2.5 for float precision) we would need to do at least the second
NR in double precision.  Note that this would make sense only for double
precision input values that are exactly representable in float precision
(otherwise, why the extra precision?).  So practically not worth it.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-06-19 09:15:09
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32392