bug#30855: 25.3; temacs fails with bus error during garbage collection

[Paul Eggert <eggert@cs.ucla.edu>]

On 03/20/2018 02:19 PM, Ulrich Mueller wrote:
> Also, doesn't this bug affect other architectures too, and at least
> cause a performance penalty for unaligned access?

It might cause crashes on other (presumably less-common) architectures, 
which is worrisome.

The performance penalty is not something we'd want to worry about in the 
emacs-26 branch, since we don't want to assume that the compiler is 
storing Lisp words on fast-aligned boundaries; this is why we're using 
alignof instead of __alignof__. So the patch does not address the issue 
of optimizing for compilers that use only fast-aligned Lisp words (and I 
doubt whether it's worth worrying about, even in the master branch).

Although 32-bit sparc64 is rare compared to x86-64, I wouldn't consider 
it to be "extremely rare" in an absolute sense (at least, not for the 
next several years; ask me again after 2038 :-). Perhaps I'm biased by 
the fact that one of our department's production servers is still 
regularly used that way, but whatever. I backported the patch to the 
emacs-26 branch and am closing Bug#30855.