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

[Paul Eggert]

On 5/26/20 8:17 AM, Eli Zaretskii wrote:
>>  static bool
>>  maybe_lisp_pointer (void *p)
>>  {
>> -  return (uintptr_t) p % LISP_ALIGNMENT == 0;
>> +  return (uintptr_t) p % GCALIGNMENT == 0;
>>  }
> On non-USE_LSB_TAG systems, GCALIGNMENT is 1, so this doesn't look
> right (or maybe I'm missing something).

Good point; I'd neglected that. I.e., on !USE_LSB_TAG systems the proposed
emacs-27 patch is overly-conservative, as it causes maybe_lisp_pointer to always
return true. Although this hurts GC performance it doesn't affect correctness
and the patch does fix a crash on USE_LSB_TAG systems, so it (or something like
it) is needed for emacs-27.

I installed the attached patch into master to fix the !USE_LSB_TAG performance
issue you raised.  This patch does not fix crashes; it's merely a performance 
tweak.

I am planning on looking into related crashes for Lisp_Symbol next. Perhaps we
should wait on that before worrying about what exact patch should go into 
emacs-27.

0001-Tweak-GC-performance-if-USE_LSB_TAG.patch
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