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

[Pip Cet <pipcet@gmail.com>]

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On Fri, May 22, 2020 at 7:22 AM Eli Zaretskii <eliz@gnu.org> wrote:
>   #0  PSEUDOVECTORP (code=<optimized out>, a=<optimized out>) at lisp.h:1720
>   #1  MARKERP (x=<optimized out>) at lisp.h:2618
>   #2  CHECK_MARKER (x=XIL(0xa000000018ac0518)) at marker.c:133
>   #3  0x010f073c in Fmarker_position (marker=XIL(0xa000000018ac0518))
>       at marker.c:452

I think I've worked it out: it's this mingw bug:
https://sourceforge.net/p/mingw-w64/bugs/778/

On mingw, if <stdint.h> is included before/instead of stddef.h,
alignof (max_align_t) == 16. However, as can be seen by the backtrace
above, Eli's malloc only returned an 8-byte-aligned block. That's not
normally a problem, because mark_maybe_object doesn't care about
alignment; but in conjunction with the gcc behavior change, we rely or
mark_maybe_pointer to mark the pointer, and it doesn't, because the
pointer is not aligned to a LISP_ALIGNMENT = 16-byte boundary.

Brute-force patch attached until we can work out how to fix this properly.

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From abb79bf33622b4e8407565ab8e82771b6a35945e Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@gmail.com>
Date: Sat, 23 May 2020 23:51:55 +0000
Subject: [PATCH] Accept unaligned pointers in maybe_lisp_pointer

* src/alloc.c (maybe_lisp_pointer): Don't require pointers be aligned
  to a LISP_ALIGNMENT boundary, as this is false on mingw builds.
---
 src/alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/alloc.c b/src/alloc.c
index 1c6b664b22..86e81cd1f6 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -4594,7 +4594,7 @@ mark_maybe_objects (Lisp_Object const *array, ptrdiff_t nelts)
 static bool
 maybe_lisp_pointer (void *p)
 {
-  return (uintptr_t) p % LISP_ALIGNMENT == 0;
+  return (uintptr_t) p % GC_ALIGNMENT == 0;
 }
 
 /* If P points to Lisp data, mark that as live if it isn't already
-- 
2.27.0.rc0