Re: Converting floor to rint

[Jan Hubicka <jh@suse.cz>]

> Jan Hubicka <jh@suse.cz> writes:
> 
> | > Jan Hubicka <jh@suse.cz> writes:
> | > 
> | > | On the related note.  How bad would you consider converting
> | > | floor(x) into rint(x-0.5) in the fast-math mode?
> | > 
> | > If you think you do want to do that transformation, then I would
> | > prefer this
> | > 
> | >   floor(x) -> nearbyint(x-0.5)
> | > 
> | > | That transformation would do a miracles for i386, where rint is faster
> | > | than floor by quite a lot and should suffice for 3D application in
> | > | reliablility.  I can imagine it to fail only for very large numbers...
> | > 
> | > Not really.  floor(1) == 1 and rint(1 - 0.5) maybe be 0 or 1 depending
> | > on the current rounding mode.
> | Hmm, is rint really expected to be dependent on the rouding mode?
> | Man page claims:
> |        The nearbyint functions round their argument to an integer value
> |        in floating point format, using the current
> |        rounding direction and without raising the inexact
> |        exception.
> | 
> |        The rint functions do the same, but will raise the
> |        inexact exception when the result differs in  value
> |        from the argument.
> 
> The C definition says:
> 
>        7.12.9.3  The nearbyint functions
> 
>        Synopsis
> 
>        [#1]
> 
>                #include <math.h>
>                double nearbyint(double x);
>                float nearbyintf(float x);
>                long double nearbyintl(long double x);
> 
>        Description
> 
>        [#2]  The  nearbyint  functions  round  their argument to an
>        integer value in floating-point format,  using  the  current
>        rounding  direction  and  without  raising  the  ``inexact''
>        floating-point exception.
> 
>        Returns
> 
>        [#3] The nearbyint  functions  return  the  rounded  integer
>        value.
> 
>        7.12.9.4  The rint functions
> 
>        Synopsis
> 
>        [#1]
> 
>                #include <math.h>
>                double rint(double x);
>                float rintf(float x);
>                long double rintl(long double x);
> 
>        Description
> 
>        [#2]  The rint functions differ from the nearbyint functions
>        (7.12.9.3) only in that the rint  functions  may  raise  the
>        ``inexact''  floating-point  exception if the result differs
>        in value from the argument.

I see, that is quite different than what I have in manpage.  I will
check the standard for other differences
> 
>        Returns
> 
>        [#3] The rint functions return the rounded integer value.
> 
> | I can builtinize nearbyint too if it makes sense.
> 
> Do we have framework to deal correctly with current rounding mode?
> I not, I would say, leave it as is until we have the appropriate
> machinery. 

RTL truncating expressions are underfined for nonintegral FP values, so
we don't have problems with the compiler, only we can't fold at compile
time for non-integral constats, since the results depends on runtime
conditions.
> 
> | I am bit confused by
> | rint.  Does it imply that rint will raise exception for any non-integral
> | arugment?
> 
> And depending on the rounding mode.
Bad :(
I will check whether frndint will do the exception.  If not, I guess the
current glibc rint builtin is wrong and it really should be rint
builtin.  The other functions (floor and friends) are not throwing
inexact I guess, but we can safely mask this out of rounding mode of
course.

Honza
> 
> -- Gaby