Re: HAVE_FAST_UNALIGNED_ACCESS

[Eli Zaretskii <eliz@gnu.org>]

> From: Arsen Arsenović <arsen@aarsen.me>
> Cc: Po Lu <luangruo@yahoo.com>, mattias.engdegard@gmail.com,
>  vibhavp@gmail.com, rpluim@gmail.com, emacs-devel@gnu.org
> Date: Sat, 01 Apr 2023 14:59:53 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > I'm still unconvinced, and I said already what will have a chance of
> > convincing me: a specific report about a problem this particular code
> > causes on a specific existing platform we support in Emacs 29 and with
> > a specific compiler.
> 
> Similar (but not exactly the same) loops as this one have been shown to
> generate incorrect code in this thread.  It's not a large leap for it to
> happen to this one, introducing subtle errors for a bit of code that is
> completely unnecessary (as demonstrated by it being optional),
> especially at higher optimization levels, where the compiler could
> easily produce better code than the assumption of a 'mov' would.
> 
> Is the following trivial enough for 29?

You are again trying to push for a change without showing any actual
bug with the existing code.  Please humor me, and please show me an
actual bug due to the existing code before suggesting a solution.  See
above for the description of the details I'd like to know about such
actual bug.

> .. or something similar to it, assuming I made an error, which is likely
> given the circumstances.  This does pass the testsuite, anyway.  It
> should just expand deferences into explicit memcpys.
> 
> No actual memcpy calls are produced, and this is at least functional on
> a superset of compilers, and I suspect replacing the whole thing with a
> naive-looking while (*(w1++) != *(w2++)); loop would be even better (but
> I can settle for that being too experimental).

Sorry, I don't want to risk any errors, and I would like to avoid any
experiments with the release branch.  Which is why I'm asking for hard
evidence.  It isn't that I don't understand what you and others are
saying, or don't believe you.  It's just that we need to see the
problems before we can judge the solutions that must be safe on this
branch.