GNU bug report logs - #41357
28.0.50; GC may miss to mark calle safe register content

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Message #56 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: bug-gnu-emacs <at> gnu.org, akrl <at> sdf.org
Subject: Re: 28.0.50; GC may miss to mark calle safe register content
Date: Sun, 17 May 2020 12:05:25 -0700

On 5/17/20 10:24 AM, Eli Zaretskii wrote:
> I struggle to see
> how a Lisp object could appear in a register without leaving any trace
> on the stack

Quite easily. It happens all the time. If I do something like this:

    Lisp_Object a = Fcons (b, c);
    f (x, y);
    return a;

The compiler might put 'a' into a callee-save register R, which means that while
f is running there's no trace of 'a' on the stack (unless f's code itself
decides to use R for whatever reason, but let's suppose it doesn't). This
situation can persist even if f calls g which calls h which calls the garbage
collector, and the garbage collector will then think the cons is garbage even
though it's not.

The proposed fix is harmless except it may execute a handful more instructions
per GC. So the cost of applying the fix is tiny, whereas the potential
reliability benefit is large.

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