analyzer: handle __attribute__((const)) [PR104434]

analyzer: handle __attribute__((const)) [PR104434]

When testing -fanalyzer on openblas-0.3, I noticed slightly over 2000
false positives from -Wanalyzer-malloc-leak on code like this:

        if( LAPACKE_lsame( vect, 'b' ) || LAPACKE_lsame( vect, 'p' ) ) {
            pt_t = (lapack_complex_float*)
                LAPACKE_malloc( sizeof(lapack_complex_float) *
                                ldpt_t * MAX(1,n) );
            [...snip...]
        }

        [...snip lots of code...]

        if( LAPACKE_lsame( vect, 'b' ) || LAPACKE_lsame( vect, 'q' ) ) {
            LAPACKE_free( pt_t );
        }

where LAPACKE_lsame is a char-comparison function implemented in a
different TU.
The analyzer naively considers the execution path where:
  LAPACKE_lsame( vect, 'b' ) || LAPACKE_lsame( vect, 'p' )
is true at the malloc guard, but then false at the free guard, which
is thus a memory leak.

This patch makes -fanalyer respect __attribute__((const)), so that the
analyzer treats such functions as returning the same value when given
the same inputs.

I've filed https://github.com/xianyi/OpenBLAS/issues/3543 suggesting that
LAPACKE_lsame be annotated with __attribute__((const)); with that, and
with this patch, the false positives seem to be fixed.

gcc/analyzer/ChangeLog:
	PR analyzer/104434
	* analyzer.h (class const_fn_result_svalue): New decl.
	* region-model-impl-calls.cc (call_details::get_manager): New.
	* region-model-manager.cc
	(region_model_manager::get_or_create_const_fn_result_svalue): New.
	(region_model_manager::log_stats): Log
	m_const_fn_result_values_map.
	* region-model.cc (const_fn_p): New.
	(maybe_get_const_fn_result): New.
	(region_model::on_call_pre): Handle fndecls with
	__attribute__((const)) by calling the above rather than making
	a conjured_svalue.
	* region-model.h (visitor::visit_const_fn_result_svalue): New.
	(region_model_manager::get_or_create_const_fn_result_svalue): New
	decl.
	(region_model_manager::const_fn_result_values_map_t): New typedef.
	(region_model_manager::m_const_fn_result_values_map): New field.
	(call_details::get_manager): New decl.
	* svalue.cc (svalue::cmp_ptr): Handle SK_CONST_FN_RESULT.
	(const_fn_result_svalue::dump_to_pp): New.
	(const_fn_result_svalue::dump_input): New.
	(const_fn_result_svalue::accept): New.
	* svalue.h (enum svalue_kind): Add SK_CONST_FN_RESULT.
	(svalue::dyn_cast_const_fn_result_svalue): New.
	(class const_fn_result_svalue): New.
	(is_a_helper <const const_fn_result_svalue *>::test): New.
	(template <> struct default_hash_traits<const_fn_result_svalue::key_t>):
	New.

gcc/testsuite/ChangeLog:
	PR analyzer/104434
	* gcc.dg/analyzer/attr-const-1.c: New test.
	* gcc.dg/analyzer/attr-const-2.c: New test.
	* gcc.dg/analyzer/attr-const-3.c: New test.
	* gcc.dg/analyzer/pr104434-const.c: New test.
	* gcc.dg/analyzer/pr104434-nonconst.c: New test.
	* gcc.dg/analyzer/pr104434.h: New test.

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