Updated mpfr_get_ld: removed useless and non-portable hacks, fixed

bugs, and slightly changed the behavior for the x87 extended format
(we shouldn't return long double's with more precision than really
supported, e.g. on FreeBSD and NetBSD).


git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/trunk@3722 280ebfd0-de03-0410-8827-d642c229c3f4

1 files changed, 22 insertions, 7 deletions

diff --git a/mpfr-impl.h b/mpfr-impl.h
index 509a00600..d2557f414 100644
--- a/mpfr-impl.h
+++ b/mpfr-impl.h
@@ -354,10 +354,25 @@ typedef union ieee_double_extract Ieee_double_extract;
  *************** Long double macros *******************
  ******************************************************/
 
-/* we only require that LDBL_MANT_DIG is a bound on the mantissa length
-   of the "long double" type */
-#define MPFR_LDBL_MANT_DIG   \
+/* We try to get the exact value of the precision of long double
+   (provided by the implementation) in order to provide correct
+   rounding in this case (not guaranteed if the C implementation
+   does not have an adequate long double arithmetic). Note that
+   it may be lower than the precision of some numbers that can
+   be represented in a long double; e.g. on FreeBSD/x86, it is
+   53 because the processor is configured to round in double
+   precision (even when using the long double type -- this is a
+   limitation of the x87 arithmetic), and on Mac OS X, it is 106
+   because the implementation is a double-double arithmetic.
+   Otherwise (e.g. in base 10), we get an upper bound of the
+   precision, and correct rounding isn't currently provided.
+*/
+#if LDBL_MANT_DIG && FLT_RADIX == 2
+# define MPFR_LDBL_MANT_DIG LDBL_MANT_DIG
+#else
+# define MPFR_LDBL_MANT_DIG \
   (sizeof(long double)*BITS_PER_MP_LIMB/sizeof(mp_limb_t))
+#endif
 #define MPFR_LIMBS_PER_LONG_DOUBLE \
   ((sizeof(long double)-1)/sizeof(mp_limb_t)+1)
 
@@ -368,7 +383,7 @@ typedef union ieee_double_extract Ieee_double_extract;
    happen only after other comparisons, not sure what's really going on.  In
    any case we can pick apart the bytes to identify a NaN.  */
 #ifdef HAVE_LDOUBLE_IEEE_QUAD_BIG
-# define LONGDOUBLE_NAN_ACTION(x, action)                        \
+# define LONGDOUBLE_NAN_ACTION(x, action)                       \
   do {                                                          \
     union {                                                     \
       long double    ld;                                        \
@@ -392,7 +407,7 @@ typedef union ieee_double_extract Ieee_double_extract;
    "volatile" here stops "cc" on mips64-sgi-irix6.5 from optimizing away
    x!=x. */
 #ifndef LONGDOUBLE_NAN_ACTION
-# define LONGDOUBLE_NAN_ACTION(x, action)                \
+# define LONGDOUBLE_NAN_ACTION(x, action)               \
   do {                                                  \
     volatile long double __x = LONGDOUBLE_VOLATILE (x); \
     if ((x) != __x)                                     \
@@ -428,9 +443,9 @@ typedef union {
   } s;
 } mpfr_long_double_t;
 
-#undef MPFR_LDBL_MANT_DIG
+/* #undef MPFR_LDBL_MANT_DIG */
 #undef MPFR_LIMBS_PER_LONG_DOUBLE
-#define MPFR_LDBL_MANT_DIG   64
+/* #define MPFR_LDBL_MANT_DIG   64 */
 #define MPFR_LIMBS_PER_LONG_DOUBLE ((64-1)/BITS_PER_MP_LIMB+1)
 
 #endif