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Date: Thu, 07 Nov 2002 01:21:00 -0000
From: Jan Hubicka <jh@suse.cz>
To: Gabriel Dos Reis <gdr@integrable-solutions.net>
Cc: Jan Hubicka <jh@suse.cz>, Richard Henderson <rth@redhat.com>,
	gcc-patches@gcc.gnu.org, aj@suse.de
Subject: Re: Converting floor to rint
Message-ID: <20021107092112.GF7964@kam.mff.cuni.cz>
References: <20021105171400.GX14655@kam.mff.cuni.cz> <20021105173800.GD20534@redhat.com> <20021106092310.GE22059@kam.mff.cuni.cz> <20021106175441.GZ22059@kam.mff.cuni.cz> <20021106180930.GA22066@redhat.com> <20021106211059.GB1316@atrey.karlin.mff.cuni.cz> <m365va2xfl.fsf@soliton.integrable-solutions.net> <20021106222922.GH1316@atrey.karlin.mff.cuni.cz> <m365va1f5b.fsf@soliton.integrable-solutions.net>
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> 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.

I can builtinize nearbyint too if it makes sense.  I am bit confused by
rint.  Does it imply that rint will raise exception for any non-integral
arugment?

Honza
> 
> I don't think that I really like that transformation, even under
> -funsafe-math.  But I would like to hear other people opinons.
> 
> -- Gaby