akr	2013-06-03 23:49:33 +0900 (Mon, 03 Jun 2013)

  New Revision: 41050

  http://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=rev&revision=41050

  Log:
    * bignum.c (bitlength_bdigit): New function.
      (rb_big_pow): Use bitlength_bdigit instead of ffs.

  Modified files:
    trunk/ChangeLog
    trunk/bignum.c

Index: ChangeLog
===================================================================
--- ChangeLog	(revision 41049)
+++ ChangeLog	(revision 41050)
@@ -1,3 +1,8 @@ https://github.com/ruby/ruby/blob/trunk/ChangeLog#L1
+Mon Jun  3 23:47:55 2013  Tanaka Akira  <akr@f...>
+
+	* bignum.c (bitlength_bdigit): New function.
+	  (rb_big_pow): Use bitlength_bdigit instead of ffs.
+
 Mon Jun  3 23:11:19 2013  Ayumu AIZAWA  <ayumu.aizawa@g...>
 
 	* lib/fileutils.rb: fix behavior when mkdir/mkdir_p accepted "/".
Index: bignum.c
===================================================================
--- bignum.c	(revision 41049)
+++ bignum.c	(revision 41050)
@@ -3178,6 +3178,24 @@ bigsqr(VALUE x) https://github.com/ruby/ruby/blob/trunk/bignum.c#L3178
     return bigtrunc(bigmul0(x, x));
 }
 
+static int
+bitlength_bdigit(BDIGIT v)
+{
+    if (v == 0)
+        return 0;
+    v |= v >> 1;
+    v |= v >> 2;
+    v |= v >> 4;
+    v |= v >> 8;
+#if 2 < SIZEOF_BDIGITS
+    v |= v >> 16;
+#endif
+    v++;
+    if (v == 0)
+        return SIZEOF_BDIGITS*CHAR_BIT;
+    return ffs(v); /* assumption: sizeof(BDIGIT) <= sizeof(int) */
+}
+
 /*
  *  call-seq:
  *     big ** exponent   -> numeric
@@ -3219,7 +3237,7 @@ rb_big_pow(VALUE x, VALUE y) https://github.com/ruby/ruby/blob/trunk/bignum.c#L3237
 	    VALUE z = 0;
 	    SIGNED_VALUE mask;
 	    const long xlen = RBIGNUM_LEN(x) - 1;
-	    const long xbits = ffs(RBIGNUM_DIGITS(x)[xlen]) + SIZEOF_BDIGITS*BITSPERDIG*xlen;
+	    const long xbits = bitlength_bdigit(RBIGNUM_DIGITS(x)[xlen]) + SIZEOF_BDIGITS*BITSPERDIG*xlen;
 	    const long BIGLEN_LIMIT = BITSPERDIG*1024*1024;
 
 	    if ((xbits > BIGLEN_LIMIT) || (xbits * yy > BIGLEN_LIMIT)) {

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