bug#20783: 25.0.50; [PATCH] byte-to-position has internal off-by-one bug

[Eli Zaretskii <eliz@gnu.org>]

> From: Wolfgang Jenkner <wjenkner@inode.at>
> Cc: 20783@debbugs.gnu.org
> Date: Tue, 16 Jun 2015 17:40:38 +0200
> 
> +      while (!CHAR_HEAD_P (*chp))
> +	{
> +	  pos_byte--;
> +	  /* There's no buffer gap in the middle of a character.  */
> +	  chp--;
> +	}

Thanks, but I'd prefer we didn't have code that manipulated pointers
to buffer text directly.  E.g., if we ever have some kind of
multi-threading, or even if at some point someone adds a non-trivial
function call to this loop, this code will be a subtle bug waiting to
bite.  It's fundamentally not safe to do this, and not only due to the
gap considerations, but also because in general BEG_ADDR might change
under certain circumstances behind your back.  (Buffer text and string
data are implemented with double indirection for good reasons.)

For some very tight loops, it might be justified to take these
shortcuts (with WARNING COMMENTS CRYING BLOODY MURDER all around), but
this function doesn't belong to those cases.

So I prefer the previous variant, even though it will lose that
benchmark.