On Mon, 19 Nov 2018, Vincent Lefevre wrote:
> On 2018-11-19 14:17:13 +0100, Niels Möller wrote:
> > You might want to add an entry for "Exponent range" to the concept index
> > in the manual.
>
> The concept index already has "Exponent", and the corresponding
> paragraph is:
>
> An “exponent” is a component of a regular floating-point number. Its
> C data type is ‘mpfr_exp_t’. Valid exponents are restricted to a subset
> of this type, and the exponent range can be changed globally as
> described in *note Exception Related Functions::. Special values do not
> have an exponent.
>
> I think that this is quite explicit.
>
> > I guess gcc should set things up similarly to the example on that page,
> > "This is an example of how to emulate binary double IEEE 754
> > arithmetic", with the caveat "This emulates a double IEEE 754 arithmetic
> > with correct rounding in the subnormal range, which may not be the case
> > for your hardware".
>
> AFAIK, GCC already changes the exponent range to the binary64 one
> (the lack of subnormals is not an issue, because in case of underflow,
> GCC ignores the computations done with MPFR).
GCC does ignore results in case of overflow or underflow but it
doesn't currently limit mpfrs exponent range. Or rather most
languages don't - I do see mpfr_set_e{min,max} calls being
done in the Fortran frontend only, but only temporarily and
seemingly only to call mpfr_subnormalize.
> But for its internal computations, I assume that mpc extends the
> exponent range (if it didn't you would have the same kind of problem,
> but much more often). Here it seems that the issue is that the maximum
> supported exponent range is not large enough for the chosen algorithm
> (which may not be the right one for this kind of input).
It happens that limiting the range makes the CPOW computation fast.
Richard.
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
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