bignum.c: fix inexact estimation

* bignum.c (estimate_initial_sqrt): estimated square root is
  inexact if it is not equal to its ceil, needs Newton's method.
  [ruby-core:80696] [Bug #13440]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58366 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 3 insertions, 0 deletions

diff --git a/bignum.c b/bignum.c
index d1e83565c9..3207af9616 100644
--- a/bignum.c
+++ b/bignum.c
@@ -6824,6 +6824,9 @@ estimate_initial_sqrt(VALUE *xp, const size_t xn, const BDIGIT *nds, size_t len)
 	if (lowbits || (lowbits = !bary_zero_p(nds, len-dbl_per_bdig)))
 	    ++d;
     }
+    else {
+	lowbits = 1;
+    }
     rshift /= 2;
     rshift += (2-(len&1))*BITSPERDIG/2;
     if (rshift >= 0) {