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Re: Converting floor to rint


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.

       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. 

| 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.

-- Gaby


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