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

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

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On 5/28/20 11:27 AM, Eli Zaretskii wrote:
>> Cc: pipcet@gmail.com, 41321@debbugs.gnu.org, monnier@iro.umontreal.ca
>> From: Paul Eggert <eggert@cs.ucla.edu>
>> Date: Wed, 27 May 2020 09:58:11 -0700
>>
>> we can then talk about what to do with emacs-27.
> 
> After thinking about this some, I think the only sensible thing to do
> on emacs-27 is to return to 8-byte alignment test in GC for 32-bit
> MinGW builds.  That is, replace max_align_t with just 8 in the
> definition of LISP_ALIGNMENT in that case.

Exactly the same problem can occur for other x86 platforms (e.g., GNU/Linux, GCC
7-and-later, glibc 2.25-and-earlier), because these other platforms also have
the bug that malloc can return a pointer that is 8 modulo 16 even though alignof
(max_align_t) is 16.  so I suggest doing the replacement for those platforms
too, as in the attached patch.

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From b3501c978f315d980f7a26481989725d63953558 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Thu, 28 May 2020 12:27:27 -0700
Subject: [PATCH] Fix aborts due to GC losing pseudovectors

Problem reported by Eli Zaretskii (Bug#41321).
* src/alloc.c (maybe_lisp_pointer): Modulo GCALIGNMENT,
not modulo LISP_ALIGNMENT.  Master has a more-elaborate fix.
Do not merge to master.
---
 src/alloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/alloc.c b/src/alloc.c
index 1c6b664b22..c7a4a3ee86 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -4589,12 +4589,12 @@ mark_maybe_objects (Lisp_Object const *array, ptrdiff_t nelts)
    collected, and false otherwise (i.e., false if it is easy to see
    that P cannot point to Lisp data that can be garbage collected).
    Symbols are implemented via offsets not pointers, but the offsets
-   are also multiples of LISP_ALIGNMENT.  */
+   are also multiples of GCALIGNMENT.  */
 
 static bool
 maybe_lisp_pointer (void *p)
 {
-  return (uintptr_t) p % LISP_ALIGNMENT == 0;
+  return (uintptr_t) p % GCALIGNMENT == 0;
 }
 
 /* If P points to Lisp data, mark that as live if it isn't already
-- 
2.17.1