bug#41321: 27.0.91; Emacs aborts due to invalid pseudovector objects

[Paul Eggert]

On 5/30/20 11:16 AM, Eli Zaretskii wrote:

> Please explain in comments why we are marking one more pointer in the
> loop.

Sure. I'm attaching the revised patch proposed for emacs-27. This is 
very similar to what Pip Cet just proposed in Bug#41321#353, but the 
code is simpler with fewer casts (and I like my comment better :-).

> I don't think I understand why this solves all of the
> problems we were discussing; is this in addition to another patch that
> you propose for emacs-27?

This replaces all the patches that I proposed for emacs-27 in this 
thread. Although this patch doesn't solve all the problems we have been 
discussing, it does solve the urgent ones:

* The problem you observed on MinGW for markers; it can also occur for 
many other object types. This problem can cause the GC to incorrectly 
reclaim storage for objects, causing the usual disasters.

* The similar problem that Pip Cet noted for symbols.

The patch does not solve less-urgent problems we've talked about, such 
as over-alignment of Lisp objects on MinGW (this is a relatively minor 
performance issue), or the more-obscure and unlikely GC bugs that we've 
been living with for a while (which I haven't had the time to think 
through entirely).

0001-Be-more-aggressive-in-marking-objects-during-GC.patch
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