GNU bug report logs - #32463
27.0.50; (logior -1) => 4611686018427387903

Previous Next

Full log

Message #14 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Andy Moreton <andrewjmoreton <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: Re: bug#32463: 27.0.50; (logior -1) => 4611686018427387903
Date: Fri, 17 Aug 2018 10:27:25 +0100

On Fri 17 Aug 2018, Pip Cet wrote:

> Can you try the attached patch? It fixes a few things that I reported
> to Tom yesterday:
>
> ---
>
>
> None of these are necessarily bugs, but:
>
> * I find the behavior of `lsh', `logand', `logior', and `logxor', when
> given negative arguments, surprising. I think it would make most sense
> to treat negative numbers as the infinite bitstream consisting of all
> ones to the left of the specified value:
>
> (logand -1 -1) would be interpreted as ...1111111 & ...1111111 =
> ...1111111, so it would be -1 (rather than 2 * most-positive-fixnum +
> 1).
> (lsh (- (lsh -1 64) 1) -1) would be ...1110111...1111 shifted to the
> right by one digit, an odd number, rather than the even number
> currently produced. (I believe lsh and ash should behave identically.)

> @@ -3383,8 +3383,6 @@ ash_lsh_impl (Lisp_Object value, Lisp_Object count, bool lsh)
>        mpz_init (result);
>        if (XFIXNUM (count) >= 0)
>  	mpz_mul_2exp (result, XBIGNUM (value)->value, XFIXNUM (count));
> -      else if (lsh)
> -	mpz_tdiv_q_2exp (result, XBIGNUM (value)->value, - XFIXNUM (count));
>        else
>  	mpz_fdiv_q_2exp (result, XBIGNUM (value)->value, - XFIXNUM (count));
>        val = make_number (result);
> @@ -3401,14 +3399,7 @@ ash_lsh_impl (Lisp_Object value, Lisp_Object count, bool lsh)

Please add more test cases to test/src/data-tests.el to ensure that the
logical operations have the expected behaviour for positive and negative
arguments, and for both bignum and fixnum arguments. The tdiv/fdiv were
added to give expected results. Pay particular attention to values around
most-positive-fixnum and most-negative-fixnum.

>        if (XFIXNUM (count) >= 0)
>  	mpz_mul_2exp (result, result, XFIXNUM (count));
> -      else if (lsh)
> -	{
> -	  if (mpz_sgn (result) > 0)
> -	    mpz_fdiv_q_2exp (result, result, - XFIXNUM (count));
> -	  else
> -	    mpz_fdiv_q_2exp (result, result, - XFIXNUM (count));
> -	}
> -      else /* ash */
> +      else
>  	mpz_fdiv_q_2exp (result, result, - XFIXNUM (count));

This part is an obvious simplification and is fine (I should have
spotted it before sending my patch that Tom committed).

    AndyM

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.