Converting floor to rint
Gabriel Dos Reis
gdr@integrable-solutions.net
Thu Nov 7 01:33:00 GMT 2002
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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