[tests/tset_ld.c] added note

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/trunk@10840 280ebfd0-de03-0410-8827-d642c229c3f4

1 files changed, 8 insertions, 0 deletions

diff --git a/tests/tset_ld.c b/tests/tset_ld.c
index eae857f24..b1468d786 100644
--- a/tests/tset_ld.c
+++ b/tests/tset_ld.c
@@ -119,6 +119,14 @@ print_binary (long double d, int flag)
     printf ("0.");
   if (flag == 2) printf ("3: d=%.36Le e=%.36Le prec=%ld\n", d, e,
                          (long) prec);
+  /* Note: the method we use here to extract the bits of r is the following,
+     to deal with the case where the rounding precision is less than the
+     precision of r:
+     (1) we accumulate the upper bits of r into f
+     (2) when accumulating a new bit into f is not exact, we subtract
+         f from r and reset f to 0
+     This is guaranteed to work only when the rounding precision is at least
+     half the precision of r, since otherwise r-f might not be exact. */
   f = 0.0; /* will hold accumulated powers of 2 */
   r = d;   /* invariant: r = d - f */
   while (r > (long double) 0.0)