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analyzer: fix constraint explosion on many-cased switch [PR96653]

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PR analyzer/96653 reports a CPU-time and memory explosion in -fanalyzer
seen in Linux 5.9-rc1:drivers/media/v4l2-core/v4l2-ctrls.c on a switch
statement with many cases.

The issue is some old code in constraint_manager::get_or_add_equiv_class
for ensuring that comparisons between equivalence classes containing
constants work correctly.  The old code added constraints for every
pair of ECs containing constants, leading to O(N^2) constraints (for
N constants).  Given that get_or_add_equiv_class also involves O(N)
comparisons, this led to at least O(N^3) CPU time, and O(N^2) memory
consumption when handling the "default" case, where N is the number of
other cases in the switch statement.

The state rewrite of r11-2694-g808f4dfeb3a95f50f15e71148e5c1067f90a126d
added checking for comparisons between constants, making these explicit
constraints redundant, but failed to remove the code mentioned above.

This patch removes it, fixing the blow-up of constraints in the default
case.

gcc/analyzer/ChangeLog:
	PR analyzer/96653
	* constraint-manager.cc
	(constraint_manager::get_or_add_equiv_class): Don't accumulate
	transitive closure of all constraints on constants.

gcc/testsuite/ChangeLog:
	PR analyzer/96653
	* gcc.dg/analyzer/pr96653.c: New test.
This commit is contained in:
David Malcolm 2020-09-10 21:23:38 -04:00
parent 00adddd656
commit 799dd4e100
2 changed files with 1105 additions and 33 deletions
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33
gcc/analyzer/constraint-manager.cc
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@ -1160,39 +1160,6 @@ constraint_manager::get_or_add_equiv_class (const svalue *sval)
equiv_class_id new_id (m_equiv_classes.length () - 1);
if (sval->maybe_get_constant ())
{
/* If we have a new EC for a constant, add constraints comparing this
to other constants we may have (so that we accumulate the transitive
closure of all constraints on constants as the constants are
added). */
for (equiv_class_id other_id (0); other_id.m_idx < new_id.m_idx;
other_id.m_idx++)
{
const equiv_class &other_ec = other_id.get_obj (*this);
if (other_ec.m_constant
&& types_compatible_p (TREE_TYPE (new_ec->m_constant),
TREE_TYPE (other_ec.m_constant)))
{
/* If we have two ECs, both with constants, the constants must be
non-equal (or they would be in the same EC).
Determine the direction of the inequality, and record that
fact. */
tree lt
= fold_binary (LT_EXPR, boolean_type_node,
new_ec->m_constant, other_ec.m_constant);
if (lt == boolean_true_node)
add_constraint_internal (new_id, CONSTRAINT_LT, other_id);
else if (lt == boolean_false_node)
add_constraint_internal (other_id, CONSTRAINT_LT, new_id);
/* Refresh new_id, in case ECs were merged. SVAL should always
be present by now, so this should never lead to a
recursion. */
new_id = get_or_add_equiv_class (sval);
}
}
}
return new_id;
}

1105
gcc/testsuite/gcc.dg/analyzer/pr96653.c Normal file
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