Re: USE_LSB_TAG not supported on this platform

[Colin Baxter <m43cap@yandex.com>]

>> Cc: herbert@mailbox.org, emacs-devel@gnu.org, m43cap@yandex.com
>> From: Paul Eggert <eggert@cs.ucla.edu>
>> Date: Tue, 9 Feb 2016 09:37:15 -0800
>> 
>> On 02/09/2016 08:59 AM, Eli Zaretskii wrote:
>> > Thanks.  Out of curiosity: which hosts can behave like that?
>> 
>> 16-bit PDP-11s running 7th Edition Unix. malloc returned only a multiple 
>> of 2 there. :-)
>
> There's only one 16-bit platform supported by Emacs: MS-DOS.  And
> there the problem doesn't exist, because the library switches the CPU
> to 32-bit mode.
>
>> > Also, why are we sure that the loops will end at some point on those
>> > hosts?  Shouldn't we perhaps set a limit to the loop iterations, to be
>> > sure we don't infloop there?
>> 
>> We're not absolutely sure. Certainly the C standard doesn't guarantee 
>> it; malloc can return a pointer that is always odd, on weird platforms 
>> where alignof always returns 1. I view this as almost purely theoretical 
>> though, due to the practical performance benefit of alignment to at 
>> least sizeof(double). It's conceivable (though very unlikely) that Emacs 
>> will infloop on some truly oddball platform that does not care about 
>> performance; but if that happens it'll be OK, as the infloop would 
>> almost surely happen during a build and the builder would then send us a 
>> bug report and we can deal with it then. I think adding a counter would 
>> complicate the code (and possibly introduce bugs, in code that's never 
>> really exercised) for not enough benefit.
>
> Maybe these considerations are worth having as comments near the loop,
> so that whoever bumps into the problem will know faster and better
> what to do.
>
> Thanks.

You might like to know that the latest git-pull builds satisfactorily
for me, giving Emacs 25.1.50.1 (i686-pc-linux-gnu, GTK+ Version 3.4.2)
of 2016-02-10

Best wishes,

Colin.