[Bug analyzer/96894] New: Analyzer assumes pointer is NULL, even if pointer was tested to be non-null before

["vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96894

            Bug ID: 96894
           Summary: Analyzer assumes pointer is NULL, even if pointer was
                    tested to be non-null before
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Created attachment 49174
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49174&action=edit
fibheap.c, preprocessed version from gdb/binutils master

Using gcc-11 (SUSE Linux) 11.0.0 20200901 (experimental) [revision
b1850c617b14eedaf60b358f3b7d4707cff73b8a].

Invoked like this:
...
$ gcc-11 fibheap.c -fanalyzer -S
...

We have:
...
fibheap.c: In function ‘fibnode_remove’:
fibheap.c:3122:42: warning: dereference of NULL ‘*(node).parent’ [CWE-690]
[-Wanalyzer-null-dereference]
 3122 |                           && node->parent->child == node)
      |                              ~~~~~~~~~~~~^~~~~~~
...

Looking at the source code, we have:
...
  3118    if (node->parent !=
  3119
  3120                       ((void *)0)
  3121
  3122                            && node->parent->child == node)
  3123      node->parent->child = ret;
...

So, just before dereferencing node->parent, we check that it's non-null, so the
warning that node->parent is dereferenced while it's null makes no sense.