[Bug analyzer/102671] -Wanalyzer-null-dereference false positive when compiling GNU Emacs

["eggert at cs dot ucla.edu" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102671

--- Comment #1 from eggert at cs dot ucla.edu ---
Created attachment 51582
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51582&action=edit
2nd test case illustrating the bug

I'm attaching a second test case, also taken from GNU Emacs, illustrating the
same bug or at least something similar. Compile it on the same platform with:

gcc -fanalyzer -O2 -S analyzer-null-dereference-2.i

and the output will be the following. The false alarm is incorrect here, too. I
plan to modify Emacs to disable the warnings in the two source-code files that
are generating these false alarms.

analyzer-null-dereference-2.i: In function 'Ftime_convert':
analyzer-null-dereference-2.i:36:13: warning: dereference of NULL 'time'
[CWE-476] [-Wanalyzer-null-dereference]
   36 |   return ((a->size & (PSEUDOVECTOR_FLAG | PVEC_TYPE_MASK))
      |            ~^~~~~~
  'Ftime_convert': events 1-4
    |
    |   72 | Ftime_convert (struct lisp *time)
    |      | ^~~~~~~~~~~~~
    |      | |
    |      | (1) entry to 'Ftime_convert'
    |   73 | {
    |   74 |   decode_time_components (time ? XCDR (time) : time);
    |      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |   |
    |      |   (2) following 'false' branch (when 'time' is NULL)...
    |      |   (3) ...to here
    |      |   (4) calling 'decode_time_components' from 'Ftime_convert'
    |
    +--> 'decode_time_components': events 5-7
           |
           |   43 |   if (! VECTORLIKEP (a))
           |      |      ~
           |      |      |
           |      |      (6) following 'true' branch...
           |......
           |   65 | decode_time_components (struct lisp *low)
           |      | ^~~~~~~~~~~~~~~~~~~~~~
           |      | |
           |      | (5) entry to 'decode_time_components'
           |......
           |   69 | }
           |      | ~
           |      | |
           |      | (7) ...to here
           |
    <------+
    |
  'Ftime_convert': events 8-11
    |
    |   36 |   return ((a->size & (PSEUDOVECTOR_FLAG | PVEC_TYPE_MASK))
    |      |            ~~~~~~~
    |      |             |
    |      |             (10) ...to here
    |      |             (11) dereference of NULL 'time'
    |......
    |   43 |   if (! VECTORLIKEP (a))
    |      |      ~
    |      |      |
    |      |      (9) following 'false' branch...
    |......
    |   74 |   decode_time_components (time ? XCDR (time) : time);
    |      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |   |
    |      |   (8) returning to 'Ftime_convert' from 'decode_time_components'
    |