analyzer: new warning: -Wanalyzer-deref-before-check [PR99671]

This patch implements a new -Wanalyzer-deref-before-check within
-fanalyzer.  It complains about code paths in which a pointer is checked
for NULL after it has already been dereferenced.

For example, for the testcase in PR 77432 the diagnostic emits:
deref-before-check-1.c: In function 'test_from_pr77432':
deref-before-check-1.c:6:8: warning: check of 'a' for NULL after already dereferencing it [-Wanalyzer-deref-before-check]
    6 |     if (a)
      |        ^
  'test_from_pr77432': events 1-2
    |
    |    5 |     int b = *a;
    |      |         ^
    |      |         |
    |      |         (1) pointer 'a' is dereferenced here
    |    6 |     if (a)
    |      |        ~
    |      |        |
    |      |        (2) pointer 'a' is checked for NULL here but it was already dereferenced at (1)
    |

and in PR 77425 we had an instance of this hidden behind a
macro, which the diagnostic complains about as follows:

deref-before-check-pr77425.c: In function 'get_odr_type':
deref-before-check-pr77425.c:35:10: warning: check of 'odr_types_ptr' for NULL after already dereferencing it [-Wanalyzer-deref-before-check]
   35 |       if (odr_types_ptr)
      |          ^
  'get_odr_type': events 1-3
    |
    |   27 |   if (cond)
    |      |      ^
    |      |      |
    |      |      (1) following 'false' branch...
    |......
    |   31 |   else if (other_cond)
    |      |           ~~~~~~~~~~~
    |      |           ||
    |      |           |(2) ...to here
    |      |           (3) following 'true' branch...
    |
  'get_odr_type': event 4
    |
    |   11 | #define odr_types (*odr_types_ptr)
    |      |                   ~^~~~~~~~~~~~~~~
    |      |                    |
    |      |                    (4) ...to here
deref-before-check-pr77425.c:33:7: note: in expansion of macro 'odr_types'
    |   33 |       odr_types[val->id] = 0;
    |      |       ^~~~~~~~~
    |
  'get_odr_type': event 5
    |
    |   11 | #define odr_types (*odr_types_ptr)
    |      |                   ~^~~~~~~~~~~~~~~
    |      |                    |
    |      |                    (5) pointer 'odr_types_ptr' is dereferenced here
deref-before-check-pr77425.c:33:7: note: in expansion of macro 'odr_types'
    |   33 |       odr_types[val->id] = 0;
    |      |       ^~~~~~~~~
    |
  'get_odr_type': event 6
    |
    |   35 |       if (odr_types_ptr)
    |      |          ^
    |      |          |
    |      |          (6) pointer 'odr_types_ptr' is checked for NULL here but it was already dereferenced at (5)
    |

gcc/analyzer/ChangeLog:
	PR analyzer/99671
	* analyzer.opt (Wanalyzer-deref-before-check): New warning.
	* diagnostic-manager.cc
	(null_assignment_sm_context::set_next_state): Only add state
	change events for transition to "null" state.
	(null_assignment_sm_context::is_transition_to_null): New.
	* engine.cc (impl_region_model_context::on_pop_frame): New.
	* exploded-graph.h (impl_region_model_context::on_pop_frame): New
	decl.
	* program-state.cc (sm_state_map::clear_any_state): New.
	(sm_state_map::can_merge_with_p): New.
	(program_state::can_merge_with_p): Replace requirement that
	sm-states be equal in favor of an attempt to merge them.
	* program-state.h (sm_state_map::clear_any_state): New decl.
	(sm_state_map::can_merge_with_p): New decl.
	* region-model.cc (region_model::eval_condition): Make const.
	(region_model::pop_frame): Call ctxt->on_pop_frame.
	* region-model.h (region_model::eval_condition): Make const.
	(region_model_context::on_pop_frame): New vfunc.
	(noop_region_model_context::on_pop_frame): New.
	(region_model_context_decorator::on_pop_frame): New.
	* sm-malloc.cc (enum resource_state): Add RS_ASSUMED_NON_NULL.
	(allocation_state::dump_to_pp): Drop "final".
	(struct assumed_non_null_state): New subclass.
	(malloc_state_machine::m_assumed_non_null): New.
	(assumed_non_null_p): New.
	(class deref_before_check): New.
	(assumed_non_null_state::dump_to_pp): New.
	(malloc_state_machine::get_or_create_assumed_non_null_state_for_frame):
	New.
	(malloc_state_machine::maybe_assume_non_null): New.
	(malloc_state_machine::on_stmt): Transition from start state to
	"assumed-non-null" state for pointers passed to
	__attribute__((nonnull)) arguments, and for pointers explicitly
	dereferenced.  Call maybe_complain_about_deref_before_check for
	pointers explicitly compared against NULL.
	(malloc_state_machine::maybe_complain_about_deref_before_check):
	New.
	(malloc_state_machine::on_deallocator_call): Also transition
	"assumed-non-null" states to "freed".
	(malloc_state_machine::on_pop_frame): New.
	(malloc_state_machine::maybe_get_merged_states_nonequal): New.
	* sm-malloc.dot: Update for changes to sm-malloc.cc.
	* sm.h (state_machine::on_pop_frame): New.
	(state_machine::maybe_get_merged_state): New.
	(state_machine::maybe_get_merged_states_nonequal): New.

gcc/ChangeLog:
	* doc/gcc/gcc-command-options/options-that-control-static-analysis.rst:
	Add -Wanalyzer-deref-before-check.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/deref-before-check-1.c: New test.
	* gcc.dg/analyzer/deref-before-check-2.c: New test.
	* gcc.dg/analyzer/deref-before-check-pr77425.c: New test.
	* gcc.dg/analyzer/malloc-1.c (test_51): New test.

gcc/ChangeLog:
	PR analyzer/99671
	* tristate.h (tristate::is_unknown): New.

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

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