das at FreeBSD.ORG
Sun May 9 23:45:53 PDT 2004
On Mon, May 10, 2004, Bruce Evans wrote:
> On Sun, 9 May 2004, I wrote:
> > ...
> > % @@ -43,11 +43,27 @@
> > % subl $4,%esp
> > %
> > % fldl 8(%ebp)
> > % + fxam
> > % + fnstsw %ax
> > % + sahf
> > This is the main runtime overhead. I think it can mostly be avoided by
> > checking the return value. ilogb() can only be INT_MIN after overflow
> > or other conversion errors (check this). There 3 cases:
> > - logb(0) = -Inf; fistpl(-Inf) = INT_MIN + IE
> > - logb(Inf) = Inf; fistpl(-Inf) = INT_MIN + IE
> > - logb(NaN) = same NaN; fistpl(NaN) = INT_MIN + IE
> > After finding one of these rare cases, the exact case still needs to be
> > determined by looking at the original value or the result of fxtract.
> > Then fucom with 0 should be a faster way to do the classification. A
> > full classification is not needed sice denormals are not special here
> > and unsupported formats are unsupported here too.
> Another thing to decide is whether the exception flags should be set (or
> not) to indicate overflow. I think they should be. Checking after doing
> the operation sets them; checking before does not. The standard is not
> clear. It says that the result is equivalent to (int)logb() for the
> non-overflowing cases but has special rules with unusual wording for
> the overflowing cases. It says "If x is zero they [the ilogb functions]
> compute the value FP_ILOGB0. ..." Computation a constant value is
> different from returning it, so this can be interpreted as saying that
> the exception flags may be set. I think there's a meta-rule that math
> functions set execption flags where appropriate.
ilogb() isn't part of IEEE-754 and it is poorly specified in C99,
but it is a proposed addition to the next revision of 754, namely
IEEE-754R. In that proposal, ilogb() signals no exceptions,
except for the automatic invalid exception you get from a
signalling NaN. See http://754r.ucbtest.org.
(This seems strange, given that logb(0) signals a divide-by-zero
exception, but the proposal is quite specific.)
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