[Bug #17037] Improve accuracy of division near precision limits

When dividing near the precision limit of `double`, use Bignum
division to get rid of rounding errors.

numeric.c

@@ -4096,7 +4096,13 @@ static double
 fix_fdiv_double(VALUE x, VALUE y)
 {
     if (FIXNUM_P(y)) {
-        return double_div_double(FIX2LONG(x), FIX2LONG(y));
+        long iy = FIX2LONG(y);
+#if SIZEOF_LONG * CHAR_BIT > DBL_MANT_DIG
+        if ((iy < 0 ? -iy : iy) >= (1L << DBL_MANT_DIG)) {
+            return rb_big_fdiv_double(rb_int2big(FIX2LONG(x)), rb_int2big(iy));
+        }
+#endif
+        return double_div_double(FIX2LONG(x), iy);
     }
     else if (RB_BIGNUM_TYPE_P(y)) {
         return rb_big_fdiv_double(rb_int2big(FIX2LONG(x)), y);
@@ -4114,7 +4120,7 @@ rb_int_fdiv_double(VALUE x, VALUE y)
 {
     if (RB_INTEGER_TYPE_P(y) && !FIXNUM_ZERO_P(y)) {
         VALUE gcd = rb_gcd(x, y);
-        if (!FIXNUM_ZERO_P(gcd)) {
+        if (!FIXNUM_ZERO_P(gcd) && gcd != INT2FIX(1)) {
             x = rb_int_idiv(x, gcd);
             y = rb_int_idiv(y, gcd);
         }

test/ruby/test_integer.rb

@@ -704,6 +704,14 @@ class TestInteger < Test::Unit::TestCase
   def test_fdiv
     assert_equal(1.0, 1.fdiv(1))
     assert_equal(0.5, 1.fdiv(2))
+
+    m = 50 << Float::MANT_DIG
+    prev = 1.0
+    (1..100).each do |i|
+      val = (m + i).fdiv(m)
+      assert_operator val, :>=, prev, "1+epsilon*(#{i}/100)"
+      prev = val
+    end
   end
 
   def test_obj_fdiv