* bignum.c (rb_big_pow): estimate result bit size more precisely.

[ruby-core:30735][Feature #3429]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@35081 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,8 @@
+Sun Mar 18 17:17:48 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* bignum.c (rb_big_pow): estimate result bit size more precisely.
+	  [ruby-core:30735][Feature #3429]
+
 Sun Mar 18 17:17:36 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* gc.c (free_method_entry_i): method entry may be in

bignum.c

@@ -3095,10 +3095,11 @@ rb_big_pow(VALUE x, VALUE y)
 	else {
 	    VALUE z = 0;
 	    SIGNED_VALUE mask;
-	    const long BIGLEN_LIMIT = 1024*1024 / SIZEOF_BDIGITS;
+	    const long xlen = RBIGNUM_LEN(x) - 1;
+	    const long xbits = ffs(RBIGNUM_DIGITS(x)[xlen]) + SIZEOF_BDIGITS*BITSPERDIG*xlen;
+	    const long BIGLEN_LIMIT = BITSPERDIG*1024*1024;
 
-	    if ((RBIGNUM_LEN(x) > BIGLEN_LIMIT) ||
-		(RBIGNUM_LEN(x) > BIGLEN_LIMIT / yy)) {
+	    if ((xbits > BIGLEN_LIMIT) || (xbits * yy > BIGLEN_LIMIT)) {
 		rb_warn("in a**b, b may be too big");
 		d = (double)yy;
 		break;

test/ruby/test_bignum.rb

@@ -326,6 +326,9 @@ class TestBignum < Test::Unit::TestCase
     ### rational changes the behavior of Bignum#**
     #assert_raise(TypeError) { T32**"foo" }
     assert_raise(TypeError, ArgumentError) { T32**"foo" }
+
+    feature3429 = '[ruby-core:30735]'
+    assert_instance_of(Bignum, (2 ** 7830457), feature3429)
   end
 
   def test_and