[Bug middle-end/109856] New: -Wnull-dereference false positive in Emacs itree.c (regression from GCC 12)

[Bug middle-end/109856] New: -Wnull-dereference false positive in Emacs itree.c (regression from GCC 12)

[eggert at cs dot ucla.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109856

            Bug ID: 109856
           Summary: -Wnull-dereference false positive in Emacs itree.c
                    (regression from GCC 12)
           Product: gcc
           Version: 13.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at cs dot ucla.edu
  Target Milestone: ---

Created attachment 55083
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55083&action=edit
compile with 'gcc -O2 -Wnull-dereference' to reproduce the bug

This is GCC 13.1.1 20230426 (Red Hat 13.1.1-1) on x86-64, and this is a
regression from GCC (Ubuntu 12.2.0-17ubuntu1) 12.2.0. I ran into this problem
when compiling GNU Emacs src/itree.c. This is a stripped-down version of the
original problem (the code makes no sense by itself).

Compile the attached program with:

gcc -O2 -S -Wnull-dereference v2.i

The output is:

v2.i: In function ‘itree_remove_fix’:
v2.i:28:16: warning: potential null pointer dereference [-Wnull-dereference]
   28 |       if (other->left)
      |           ~~~~~^~~~~~

First, the diagnostic is confusing, because it highlights an entire expression,
which cannot itself be a null pointer dereference.

Second, there's no possible null pointer dereference here. The previous two
lines are "if (!other)__builtin_unreachable ();" so OTHER itself cannot be
null.

[Bug middle-end/109856] -Wnull-dereference false positive in Emacs itree.c (regression from GCC 12)

[eggert at cs dot ucla.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109856

--- Comment #1 from Paul Eggert <eggert at cs dot ucla.edu> ---
Created attachment 55337
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55337&action=edit
Another illustration of the bug, taken from GNU tar

Here is another example of the bug, taken from GNU tar. Compile it with:

gcc -O2 -Wnull-dereference -S w.i

The output is:

w.i: In function ‘_write_volume_label.part.0’:
w.i:9290:30: warning: potential null pointer dereference [-Wnull-dereference]
 9290 |       label->header.typeflag = 'V';
      |       ~~~~~~~~~~~~~~~~~~~~~~~^~~~~
w.i:9290:30: warning: potential null pointer dereference [-Wnull-dereference]

This is a false positive, since the local variable 'label' cannot be null in
line 9290, given that line 9284 has checked that it is not null. Also, the two
copies of the same diagnostic are suspicious.