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

From: Paul Eggert
Subject: bug#30855: 25.3; temacs fails with bus error during garbage collection
Date: Tue, 20 Mar 2018 14:42:15 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

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.

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