fix description for current implementation

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57534 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

internal.h

@@ -360,12 +360,24 @@ rb_fix_plus_fix(VALUE x, VALUE y)
 #ifdef HAVE_BUILTIN___BUILTIN_ADD_OVERFLOW
     long lz;
     /* NOTE
-     * (1) Fixnum's LSB is always 1.
+     * (1) `LONG2FIX(FIX2LONG(x)+FIX2LONG(y))`
+     +     = `((lx*2+1)/2 + (ly*2+1)/2)*2+1`
+     +     = `lx*2 + ly*2 + 1`
+     +     = `(lx*2+1) + (ly*2+1) - 1`
+     +     = `x + y - 1`
+     * (2) Fixnum's LSB is always 1.
      *     It means you can always run `x - 1` without overflow.
-     * (2) Of course `z = x + (y-1)` may overflow.
-     *     Now z's LSB is always 1, and the MSB of true result is also 1.
-     *     You can get true result in long as `(1<<63)|(z>>1)`,
-     *     and it equals to `(z<<63)|(z>>1)` == `ror(z)`.
+     * (3) Of course `z = x + (y-1)` may overflow.
+     *     At that time true value is
+     *     * positive: 0b0 1xxx...1, and z = 0b1xxx...1
+     *     * nevative: 0b1 0xxx...1, and z = 0b0xxx...1
+     *     To convert this true value to long,
+     *     (a) Use arithmetic shift
+     *         * positive: 0b11xxx...
+     *         * negative: 0b00xxx...
+     *     (b) invert MSB
+     *         * positive: 0b01xxx...
+     *         * negative: 0b10xxx...
      */
     if (__builtin_add_overflow((long)x, (long)y-1, &lz)) {
 	return rb_int2big(rb_overflowed_fix_to_int(lz));