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Update comments regarding GMP earlier than 4.2.

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* libguile/numbers.c: Update comments regarding GMP earlier than 4.2.
  Remove speculations about versions of GMP that had not yet been
  released when the comments were written.  Replace them with facts that
  are now known about the changes made in GMP 4.2.
This commit is contained in:
Mark H Weaver 2011-02-22 21:39:37 -05:00
parent 7d185392ce
commit 0655c9eb29
1 changed files with 18 additions and 14 deletions
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32
libguile/numbers.c
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@ -134,9 +134,9 @@ static double acosh (double x) { return log (x + sqrt (x * x - 1)); }
static double atanh (double x) { return 0.5 * log ((1 + x) / (1 - x)); }
#endif
/* 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. */
/* mpz_cmp_d in GMP before 4.2 didn't recognise infinities, so
xmpz_cmp_d uses an explicit check. Starting with GMP 4.2 (released
in March 2006), mpz_cmp_d now handles infinities properly. */
#if 1
#define xmpz_cmp_d(z, d) \
(isinf (d) ? (d < 0.0 ? 1 : -1) : mpz_cmp_d (z, d))
@ -316,16 +316,15 @@ scm_i_dbl2num (double u)
we need to use mpz_getlimbn. mpz_tstbit is not right, it treats
negatives as twos complement.
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.
In GMP before 4.2, mpz_get_d rounding was unspecified. It ended 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. Starting with GMP 4.2
(released in March 2006) mpz_get_d now always truncates 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. */
ENHANCE-ME: The temporary init+clear to force the rounding in GMP
before 4.2 is a slowdown. It'd be faster to pick out the relevant
high bits with mpz_getlimbn. */
double
scm_i_big2dbl (SCM b)
@ -337,7 +336,12 @@ scm_i_big2dbl (SCM b)
#if 1
{
/* Current GMP, eg. 4.1.3, force truncation towards zero */
/* For GMP earlier than 4.2, force truncation towards zero */
/* FIXME: DBL_MANT_DIG is the number of base-`FLT_RADIX' digits,
_not_ the number of bits, so this code will break badly on a
system with non-binary doubles. */
mpz_t tmp;
if (bits > DBL_MANT_DIG)
{
@ -353,7 +357,7 @@ scm_i_big2dbl (SCM b)
}
}
#else
/* Future GMP */
/* GMP 4.2 or later */
result = mpz_get_d (SCM_I_BIG_MPZ (b));
#endif