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Re: The state of glibc libm
On Thu, 2012-03-15 at 03:07 +0100, Vincent Lefevre wrote:
> On 2012-03-14 14:40:06 +0000, Joseph S. Myers wrote:
> > On Wed, 14 Mar 2012, Vincent Lefevre wrote:
> >
> > > For double-double (IBM long double), I don't think the notion of
> > > correct rounding makes much sense anyway. Actually the double-double
> > > arithmetic is mainly useful for the basic operations in order to be
> > > able to implement elementary functions accurately (first step in
> > > Ziv's strategy, possibly a second step as well). IMHO, on such a
> > > platform, if expl() (for instance) just calls exp(), this is OK.
> >
Why would that be OK? If we have higher precision long double then the
libm should deliver that higher precision.
> > expl just calling exp - losing 53 bits of precision - seems rather
> > extreme. But I'd think it would be fine to say: when asked to compute
> > f(x), take x' within 10ulp of x, and return a number within 10ulp of
> > f(x'), where ulp is interpreted as if the mantissa were a fixed 106 bits
> > (fewer bits for subnormals, of course). (And as a consequence, accurate
> > range reduction for large arguments would be considered not to matter for
> > IBM long double; sin and cos could return any value in the range [-1, 1]
> > for sufficiently large arguments.)
>
> After thinking about this, you could assume that you have a 106-bit
> floating-point system (BTW, LDBL_MANT_DIG = 106) and use the same
> method to generate code that provides an accurate implementation
> (if the code generator doesn't assume an IEEE 754 compatible FP
> system). Concerning sin and cos, I think there should be a minimum
> of specification and some consistency (such as sin(x)Â + cos(x)Â
> being close to 1).
>
actually back in 2007 I overrode slowexp and slowpow for powerpc/power4
(and later) to use expl, powl on the slow path of exp/pow, instead of
the mpa.h implementation. This provide a nice performance improvement
but does imply some rounding mode issues.