From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: [Q] Portability of <= and >= with real operands
Date: 1996/12/03
Date: 1996-12-03T00:00:00+00:00 [thread overview]
Message-ID: <dewar.849654327@merv> (raw)
In-Reply-To: E1u0nL.L0I@thomsoft.com
Keith says
"Suppose you've computed two quantities, X and Y, such that X is
mathematically known to be less than Y, but it may be arbitrarily close.
Since floating-point is of finite precision, the representations of X
and Y may be equal. (For example, X = 0.0, Y = some tiny value which
underflows to 0.0). Then X <= Y may reflect the relationship more
accurately than X < Y.
"
Well first, surely you are not meaning to say that X<=Y is different
from not (X>Y), which you are arguing is somehow less portable than
X>=Y, so this argument is not self consistent!
In fact if your analysis is based on a view of the mathemaytical
proper values of variables, then this argument is almost certainly
bogus, since approximations may lead X > Y to be true, even if
X < Y mathematically.
You cannot substitute rules like this for proper analysis. Both > and
>= have perfectly well defined portable meanings for a given model
of floating-point arithmetic, and you have to analyze your code with
respect to a particular model.
next prev parent reply other threads:[~1996-12-03 0:00 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
1996-11-29 0:00 [Q] Portability of <= and >= with real operands JP Thornley
1996-11-29 0:00 ` Robert Dewar
1996-12-03 0:00 ` Keith Thompson
1996-12-03 0:00 ` Thomas Koenig
1996-12-03 0:00 ` Tucker Taft
1996-12-03 0:00 ` Larry Kilgallen
1996-12-03 0:00 ` Robert Dewar
1996-12-04 0:00 ` Keith Thompson
1996-12-03 0:00 ` Robert Dewar [this message]
1996-12-01 0:00 ` Tucker Taft
1996-12-02 0:00 ` Norman H. Cohen
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