bug#41755: feature/native-comp (master?): temacs crash in GC during mark phase

["Nicolas Bértolo" <nicolasbertolo@gmail.com>]

> But you still have last_marked in your build, right? That would be a
> good starting point to find out which object was marked and what was
> actually on the stack there...

Yes, I'll take a look there. BTW, adding

#pragma GCC optimize ("-O0")

to the top of alloc.c does not prevent it from crashing, so debugging could be
easier.

> Is it always a symbol that's found on the stack by mark_maybe_*, though?

No, in this case it is a cons.

0x0000000400115a1a in cons_marked_p (c=0xfffffffc00, c@entry=0xbf07b0)
at alloc.c:3899
3899      return pdumper_object_p (c)
(gdb) bt
#0  0x0000000400115a1a in cons_marked_p (c=0xfffffffc00,
c@entry=0xbf07b0) at alloc.c:3899
#1  0x000000040011a567 in mark_object (arg=XIL(0xbf0890)) at alloc.c:6775
#2  0x00000004001125d9 in mark_interval_tree_1 (i=0x464a9b3,
dummy=0x0) at alloc.c:1468
#3  0x000000040018fde4 in traverse_intervals_noorder
(tree=tree@entry=0x464a9b3, function=function@entry=0x4001125b0
<mark_interval_tree_1>, arg=arg@entry=0x0) at intervals.c:234
#4  0x0000000400112619 in mark_interval_tree (i=0x464a9b3) at alloc.c:1477
#5  0x000000040011a2d4 in mark_object (arg=XIL(0x454c0b0)) at alloc.c:6629
#6  0x000000040011a5b2 in mark_object (arg=XIL(0x40061cf60),
arg@entry=XIL(0x4d14553)) at alloc.c:6786
#7  0x00000004001171dd in mark_maybe_pointer (p=p@entry=0x4d14553) at
alloc.c:4804
#8  0x0000000400117253 in mark_memory (start=0xbf0b30,
start@entry=0xbff990, end=0xbff990, end@entry=0xbf0b30) at
alloc.c:4854
#9  0x00000004001172b0 in mark_stack (bottom=0xbff990 "",
end=end@entry=0xbf0b30 "0\f\277") at alloc.c:5073
#10 0x00000004001a0a71 in mark_one_thread (thread=0x400559500
<main_thread>) at thread.c:630
#11 mark_threads_callback (ignore=ignore@entry=0x0) at thread.c:661
#12 0x00000004001172fe in flush_stack_call_func1
(func=func@entry=0x4001a0a30 <mark_threads_callback>,
arg=arg@entry=0x0) at alloc.c:5114
#13 0x00000004001a1c9c in flush_stack_call_func (arg=0x0,
func=0x4001a0a30 <mark_threads_callback>) at lisp.h:3825
#14 mark_threads () at thread.c:668
#15 0x0000000000000000 in ?? ()
Backtrace stopped: previous frame inner to this frame (corrupt stack?)