analyzer: use __attribute__((nonnull)) at top level of analysis [PR106325]

analyzer: use __attribute__((nonnull)) at top level of analysis [PR106325]

PR analyzer/106325 reports false postives from
-Wanalyzer-null-dereference on code like this:

__attribute__((nonnull))
void foo_a (Foo *p)
{
  foo_b (p);

  switch (p->type)
    {
      /* ... */
    }
}

where foo_b (p) has a:

  g_return_if_fail (p);

that expands to:

  if (!p)
    {
      return;
    }

The analyzer "sees" the comparison against NULL in foo_b, and splits the
analysis into the NULL and not-NULL cases; later, back in foo_a,  at
  switch (p->type)
it complains that p is NULL.

Previously we were only using __attribute__((nonnull)) as something to
complain about when it was violated; we weren't using it as a source of
knowledge.

This patch fixes things by making the analyzer respect
__attribute__((nonnull)) at the top-level of the analysis: any such
params are now assumed to be non-NULL, so that the analyzer assumes the
g_return_if_fail inside foo_b doesn't fail when called from foo_a

Doing so fixes the false positives.

gcc/analyzer/ChangeLog:
	PR analyzer/106325
	* region-model-manager.cc
	(region_model_manager::get_or_create_null_ptr): New.
	* region-model-manager.h
	(region_model_manager::get_or_create_null_ptr): New decl.
	* region-model.cc (region_model::on_top_level_param): Add
	"nonnull" param and make use of it.
	(region_model::push_frame): When handling a top-level entrypoint
	to the analysis, determine which params __attribute__((nonnull))
	applies to, and pass to on_top_level_param.
	* region-model.h (region_model::on_top_level_param): Add "nonnull"
	param.

gcc/testsuite/ChangeLog:
	PR analyzer/106325
	* gcc.dg/analyzer/attr-nonnull-pr106325.c: New test.
	* gcc.dg/analyzer/attribute-nonnull.c (test_6): New.
	(test_7): New.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>