Re: [PATCH] c++: Improve memory usage of subsumption [PR100828]

[Jason Merrill via Gcc-patches]

On 8/11/21 10:53 AM, Patrick Palka wrote:
On Wed, 11 Aug 2021, Jason Merrill wrote:
On 8/9/21 5:07 PM, Patrick Palka wrote:
On Wed, Jul 28, 2021 at 4:42 PM Jason Merrill <ja...@redhat.com> wrote:

On 7/19/21 6:05 PM, Patrick Palka wrote:
Constraint subsumption is implemented in two steps.  The first step
computes the disjunctive (or conjunctive) normal form of one of the
constraints, and the second step verifies that each clause in the
decomposed form implies the other constraint.   Performing these two
steps separately is problematic because in the first step the
disjunctive normal form can be exponentially larger than the original
constraint, and by computing it ahead of time we'd have to keep all of
it in memory.

This patch fixes this exponential blowup in memory usage by interleaving
these two steps, so that as soon as we decompose one clause we check
implication for it.  In turn, memory usage during subsumption is now
worst case linear in the size of the constraints rather than
exponential, and so we can safely remove the hard limit of 16 clauses
without introducing runaway memory usage on some inputs.  (Note the
_time_ complexity of subsumption is still exponential in the worst
case.)

In order for this to work we need formula::branch to prepend the copy
of the current clause directly after the current clause rather than
at the end of the list, so that we fully decompose a clause shortly
after creating it.  Otherwise we'd end up accumulating exponentially
many (partially decomposed) clauses in memory anyway.

Bootstrapped and regtested on x86_64-pc-linux-gnu, and also tested on
range-v3 and cmcstl2.  Does this look OK for trunk and perhaps 11?

OK for trunk.

Thanks a lot, patch committed to trunk as r12-2658.  Since this low
complexity limit was introduced in GCC 10, what do you think about
increasing the limit from 16 to say 128 in the 10/11 release branches
as a relatively safe stopgap?

Now that 11.2 is out, go ahead and apply this patch to the 11 branch.

Ah great, will do.


Won't a limit of 128 in GCC 10 lead to extremely long compile times for
affected code?  Is that more desirable than an error?

Potentially, though I think that'd be the case only if the original
(normalized) constraint is huge to begin with.  The comment for
max_problem_size says

  /* The largest number of clauses in CNF or DNF we accept as input
     for subsumption. This an upper bound of 2^16 expressions.  */
  static int max_problem_size = 16;

which implies increasing it to 128 would allow for at most 2^128
expressions (clearly unacceptable), but I'm not sure how this upper
bound was obtained.

FWIW I think another upper bound for the number of expressions in the
CNF/DNF is roughly 'max_problem_size * size_of_original_constraint',
since we allow at most 'max_problem_size' clauses in the decomposed form
and each clause is definitely no larger than the original constraint.
So according to this upper bound the dependence on max_problem_size as
it relates to worst-case compile time/memory usage of subsumption is
linear rather than exponential, contrary to the comment.  In that case
increasing the limit from 16 to 128 doesn't seem to be too bad.

Fair, though I would expect anyone writing new concepts code to use GCC 
11.  Up to you.

        PR c++/100828

gcc/cp/ChangeLog:

        * logic.cc (formula::formula): Use emplace_back.
        (formula::branch): Insert a copy of m_current in front of
        m_current instead of at the end of the list.
        (formula::erase): Define.
        (decompose_formula): Remove.
        (decompose_antecedents): Remove.
        (decompose_consequents): Remove.
        (derive_proofs): Remove.
        (max_problem_size): Remove.
        (diagnose_constraint_size): Remove.
        (subsumes_constraints_nonnull): Rewrite directly in terms of
        decompose_clause and derive_proof, interleaving decomposition
        with implication checking.  Use formula::erase to free the
        current clause before moving on to the next one.
---
    gcc/cp/logic.cc | 118
++++++++++++++----------------------------------
    1 file changed, 35 insertions(+), 83 deletions(-)

diff --git a/gcc/cp/logic.cc b/gcc/cp/logic.cc
index 142457e408a..3f872c11fe2 100644
--- a/gcc/cp/logic.cc
+++ b/gcc/cp/logic.cc
@@ -223,9 +223,7 @@ struct formula

      formula (tree t)
      {
-    /* This should call emplace_back(). There's an extra copy being
-       invoked by using push_back().  */
-    m_clauses.push_back (t);
+    m_clauses.emplace_back (t);
        m_current = m_clauses.begin ();
      }

@@ -248,8 +246,7 @@ struct formula
      clause& branch ()
      {
        gcc_assert (!done ());
-    m_clauses.push_back (*m_current);
-    return m_clauses.back ();
+    return *m_clauses.insert (std::next (m_current), *m_current);
      }

      /* Returns the position of the current clause.  */
@@ -287,6 +284,14 @@ struct formula
        return m_clauses.end ();
      }

+  /* Remove the specified clause.  */
+
+  void erase (iterator i)
+  {
+    gcc_assert (i != m_current);
+    m_clauses.erase (i);
+  }
+
      std::list<clause> m_clauses; /* The list of clauses.  */
      iterator m_current; /* The current clause.  */
    };
@@ -659,39 +664,6 @@ decompose_clause (formula& f, clause& c, rules r)
      f.advance ();
    }

-/* Decompose the logical formula F according to the logical
-   rules determined by R.  The result is a formula containing
-   clauses that contain only atomic terms.  */
-
-void
-decompose_formula (formula& f, rules r)
-{
-  while (!f.done ())
-    decompose_clause (f, *f.current (), r);
-}
-
-/* Fully decomposing T into a list of sequents, each comprised of
-   a list of atomic constraints, as if T were an antecedent.  */
-
-static formula
-decompose_antecedents (tree t)
-{
-  formula f (t);
-  decompose_formula (f, left);
-  return f;
-}
-
-/* Fully decomposing T into a list of sequents, each comprised of
-   a list of atomic constraints, as if T were a consequent.  */
-
-static formula
-decompose_consequents (tree t)
-{
-  formula f (t);
-  decompose_formula (f, right);
-  return f;
-}
-
    static bool derive_proof (clause&, tree, rules);

    /* Derive a proof of both operands of T.  */
@@ -744,28 +716,6 @@ derive_proof (clause& c, tree t, rules r)
      }
    }

-/* Derive a proof of T from disjunctive clauses in F.  */
-
-static bool
-derive_proofs (formula& f, tree t, rules r)
-{
-  for (formula::iterator i = f.begin(); i != f.end(); ++i)
-    if (!derive_proof (*i, t, r))
-      return false;
-  return true;
-}
-
-/* The largest number of clauses in CNF or DNF we accept as input
-   for subsumption. This an upper bound of 2^16 expressions.  */
-static int max_problem_size = 16;
-
-static inline bool
-diagnose_constraint_size (tree t)
-{
-  error_at (input_location, "%qE exceeds the maximum constraint
complexity", t);
-  return false;
-}
-
    /* Key/value pair for caching subsumption results. This associates a
pair of
       constraints with a boolean value indicating the result.  */

@@ -845,31 +795,33 @@ subsumes_constraints_nonnull (tree lhs, tree rhs)
      if (bool *b = lookup_subsumption(lhs, rhs))
        return *b;

-  int n1 = dnf_size (lhs);
-  int n2 = cnf_size (rhs);
-
-  /* Make sure we haven't exceeded the largest acceptable problem.  */
-  if (std::min (n1, n2) >= max_problem_size)
-    {
-      if (n1 < n2)
-        diagnose_constraint_size (lhs);
-      else
-     diagnose_constraint_size (rhs);
-      return false;
-    }
-
-  /* Decompose the smaller of the two formulas, and recursively
-     check for implication of the larger.  */
-  bool result;
-  if (n1 <= n2)
-    {
-      formula dnf = decompose_antecedents (lhs);
-      result = derive_proofs (dnf, rhs, left);
-    }
+  tree x, y;
+  rules r;
+  if (dnf_size (lhs) <= cnf_size (rhs))
+    /* When LHS looks simpler than RHS, we'll determine subsumption by
+       decomposing LHS into its disjunctive normal form and checking
that
+       each (conjunctive) clause implies RHS.  */
+    x = lhs, y = rhs, r = left;
      else
+    /* Otherwise, we'll determine subsumption by decomposing RHS into
its
+       conjunctive normal form and checking that each (disjunctive)
clause
+       implies LHS.  */
+    x = rhs, y = lhs, r = right;
+
+  /* Decompose X into a list of sequents according to R, and
recursively
+     check for implication of Y.  */
+  bool result = true;
+  formula f (x);
+  while (!f.done ())
        {
-      formula cnf = decompose_consequents (rhs);
-      result = derive_proofs (cnf, lhs, right);
+      auto i = f.current ();
+      decompose_clause (f, *i, r);
+      if (!derive_proof (*i, y, r))
+     {
+       result = false;
+       break;
+     }
+      f.erase (i);
        }

      return save_subsumption (lhs, rhs, result);