Re: [PATCH] Don't call double rint from float powf.

[Corinna Vinschen <vinschen at redhat dot com>]

On Dec 11 19:37, Craig Howland wrote:
> On 12/11/2017 07:08 PM, Jim Wilson wrote:
> > This is the first patch of two to fix problems with the powf code.  I haven't
> > written the other patch yet.
> > 
> > For targets that have only single float support, such as RISC-V rv32imafc, a
> > program using powf ends up pulling in soft-float adddf3 and subdf3 routines.
> > These come from the double rint call, which should be float rintf instead.
> > This makes a significant difference in the size of the resulting program.
> > 
> > I tested this with a simple testcase, with an rint function that calls abort
> > to verify that I'm testing the right code path.  Compiling with
> > "gcc -fno-builtin" I get for the patched and unpatched newlib
> > 
> > gamma02:2195$ ls -lt a.out*
> > -rwxrwxr-x 1 jimw jimw 47012 Dec 11 15:01 a.out
> > -rwxrwxr-x 1 jimw jimw 75680 Dec 11 14:46 a.out.unpatched
> > gamma02:2196$
> > 
> > So the result with the patch is about 60% of the size without the patch.  The
> > test program gives the correct result with the patch.  I also ran the newlib
> > make check, which passed with no regression, but I see it doesn't do much
> > useful.
> > 
> > 	newlib/
> > 	* libm/math/wf_pow.c (powf): Call rintf instead of rint.
> > ---
> >   newlib/libm/math/wf_pow.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/newlib/libm/math/wf_pow.c b/newlib/libm/math/wf_pow.c
> > index be453558b..4ca0dc79d 100644
> > --- a/newlib/libm/math/wf_pow.c
> > +++ b/newlib/libm/math/wf_pow.c
> > @@ -127,11 +127,11 @@
> >   		    if (_LIB_VERSION == _SVID_) {
> >   		       exc.retval = HUGE;
> >   		       y *= 0.5;
> > -		       if(x<0.0&&rint(y)!=y) exc.retval = -HUGE;
> > +		       if(x<0.0&&rintf(y)!=y) exc.retval = -HUGE;
> >   		    } else {
> >   		       exc.retval = HUGE_VAL;
> >                          y *= 0.5;
> > -		       if(x<0.0&&rint(y)!=y) exc.retval = -HUGE_VAL;
> > +		       if(x<0.0&&rintf(y)!=y) exc.retval = -HUGE_VAL;
> >   		    }
> >   		    if (_LIB_VERSION == _POSIX_)
> >   		        errno = ERANGE;
> To be most rigorous, "0.0" on the same lines as the rintf() should be "0.0F"
> (as otherwise the comparison strictly should be done in double).  (Not
> addressing the exact change you are making, but is the same class of fix and
> are on the same line of source code.)

Also, HUGE_VAL is double, so it should better be replaced with HUGE_VALF.


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
Red Hat

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