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Amend #if and comments for cmp_d and get_d about unreleased gmp, don't

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know when or as what version number it will come out.
This commit is contained in:
Kevin Ryde 2004-08-06 01:26:26 +00:00
parent 8f094b9720
commit f8a8200bc0
1 changed files with 27 additions and 22 deletions
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49
libguile/numbers.c
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@ -116,10 +116,10 @@ isinf (double x)
#endif
/* mpz_cmp_d only recognises infinities in gmp 4.2 and up.
For prior versions use an explicit check here. */
#if __GNU_MP_VERSION < 4 \
|| (__GNU_MP_VERSION == 4 && __GNU_MP_VERSION_MINOR < 2)
/* mpz_cmp_d in gmp 4.1.3 doesn't recognise infinities, so xmpz_cmp_d uses
an explicit check. In some future gmp (don't know what version number),
mpz_cmp_d is supposed to do this itself. */
#if 1
#define xmpz_cmp_d(z, d) \
(xisinf (d) ? (d < 0.0 ? 1 : -1) : mpz_cmp_d (z, d))
#else
@ -244,23 +244,29 @@ scm_i_dbl2num (double u)
with R5RS exact->inexact.
The approach is to use mpz_get_d to pick out the high DBL_MANT_DIG bits
(ie. it truncates towards zero), then adjust to get the closest double by
examining the next lower bit and adding 1 if necessary.
(ie. truncate towards zero), then adjust to get the closest double by
examining the next lower bit and adding 1 (to the absolute value) if
necessary.
Note that bignums exactly half way between representable doubles are
rounded to the next higher absolute value (ie. away from zero). This
seems like an adequate interpretation of R5RS "numerically closest", and
it's easier and faster than a full "nearest-even" style.
Bignums exactly half way between representable doubles are rounded to the
next higher absolute value (ie. away from zero). This seems like an
adequate interpretation of R5RS "numerically closest", and it's easier
and faster than a full "nearest-even" style.
The bit test is done on the absolute value of the mpz_t, which means we
must use mpz_getlimbn. mpz_tstbit is not right, it treats negatives as
twos complement.
The bit test must be done on the absolute value of the mpz_t, which means
we need to use mpz_getlimbn. mpz_tstbit is not right, it treats
negatives as twos complement.
Prior to GMP 4.2, the rounding done by mpz_get_d was unspecified. It
happened to follow the hardware rounding mode, but on the absolute value
of its operand. This is not what we want, so we put the high
DBL_MANT_DIG bits into a temporary. This extra init/clear is a slowdown,
but doesn't matter too much since it's only for older GMP. */
In current gmp 4.1.3, mpz_get_d rounding is unspecified. It ends up
following the hardware rounding mode, but applied to the absolute value
of the mpz_t operand. This is not what we want so we put the high
DBL_MANT_DIG bits into a temporary. In some future gmp, don't know when,
mpz_get_d is supposed to always truncate towards zero.
ENHANCE-ME: The temporary init+clear to force the rounding in gmp 4.1.3
is a slowdown. It'd be faster to pick out the relevant high bits with
mpz_getlimbn if we could be bothered coding that, and if the new
truncating gmp doesn't come out. */
double
scm_i_big2dbl (SCM b)
@ -270,10 +276,9 @@ scm_i_big2dbl (SCM b)
bits = mpz_sizeinbase (SCM_I_BIG_MPZ (b), 2);
#if __GNU_MP_VERSION < 4 \
|| (__GNU_MP_VERSION == 4 && __GNU_MP_VERSION_MINOR < 2)
#if 1
{
/* GMP prior to 4.2, force truncate towards zero */
/* Current GMP, eg. 4.1.3, force truncation towards zero */
mpz_t tmp;
if (bits > DBL_MANT_DIG)
{
@ -289,7 +294,7 @@ scm_i_big2dbl (SCM b)
}
}
#else
/* GMP 4.2 and up */
/* Future GMP */
result = mpz_get_d (SCM_I_BIG_MPZ (b));
#endif