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Fix PDD factor cache in case GC happens while factoring (#5170)

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* Add method to find largest power of two that is a divisor

* Binary resolve on PDDs

* Add unit tests for binary resolve

* Fix factor cache access in case GC happens while factoring

* Coding conventions

* Change to gc_generation
This commit is contained in:
Jakob Rath 2021-04-12 19:09:13 +02:00 • committed by Nikolaj Bjorner
parent 75c87a2869
commit 0e9fc4762a
3 changed files with 192 additions and 9 deletions
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126
src/math/dd/dd_pdd.cpp
View file

@ -696,8 +696,8 @@ namespace dd {
void pdd_manager::factor(pdd const& p, unsigned v, unsigned degree, pdd& lc, pdd& rest) {
unsigned level_v = m_var2level[v];
if (degree == 0) {
lc = p;
rest = zero();
lc = zero();
rest = p;
return;
}
if (level(p.root) < level_v) {
@ -707,12 +707,13 @@ namespace dd {
}
// Memoize nontrivial cases
auto* et = m_factor_cache.insert_if_not_there2({p.root, v, degree});
factor_entry& e = et->get_data();
if (e.is_valid()) {
lc = pdd(e.m_lc, this);
rest = pdd(e.m_rest, this);
factor_entry* e = &et->get_data();
if (e->is_valid()) {
lc = pdd(e->m_lc, this);
rest = pdd(e->m_rest, this);
return;
}
unsigned const gc_generation = m_gc_generation;
if (level(p.root) > level_v) {
pdd lc1 = zero(), rest1 = zero();
pdd vv = mk_var(p.var());
@ -743,10 +744,118 @@ namespace dd {
rest = p;
}
}
e.m_lc = lc.root;
e.m_rest = rest.root;
if (gc_generation != m_gc_generation) {
// Cache was reset while factoring (due to GC),
// which means the old entry has been removed and we need to insert it again.
auto* et = m_factor_cache.insert_if_not_there2({p.root, v, degree});
e = &et->get_data();
}
e->m_lc = lc.root;
e->m_rest = rest.root;
}
template <class Fn>
pdd pdd_manager::map_coefficients(pdd const& p, Fn f) {
if (p.is_val()) {
return mk_val(rational(f(p.val())));
} else {
pdd x = mk_var(p.var());
pdd lo = map_coefficients(p.lo(), f);
pdd hi = map_coefficients(p.hi(), f);
return x*hi + lo;
}
}
/**
* Perform S-polynomial reduction on p by q,
* treating monomial with v as leading.
*
* p = a v^l + b = a' 2^j v^l + b
* q = c v^m + d = c' 2^j v^m + d
* such that
* deg(v, p) = l, i.e., v does not divide a and there is no v^l in b
* deg(v, q) = m, i.e., v does not divide c and there is no v^m in d
* l >= m
* j maximal, i.e., not both of a', c' are divisible by 2
*
* Then we reduce p by q:
*
* r = c' p - a' v^(l-m) q
* = b c' - a' d v^(l-m)
*/
bool pdd_manager::resolve(unsigned v, pdd const& p, pdd const& q, pdd& r) {
unsigned const l = p.degree(v);
unsigned const m = q.degree(v);
if (l < m) {
// no reduction
return false;
}
if (m == 0) {
// no reduction (result would still contain v^l)
return false;
}
pdd a = zero();
pdd b = zero();
pdd c = zero();
pdd d = zero();
p.factor(v, l, a, b);
q.factor(v, m, c, d);
unsigned const j = std::min(max_pow2_divisor(a), max_pow2_divisor(c));
SASSERT(j != UINT_MAX); // should only be possible when both l and m are 0
rational const pow2j = rational::power_of_two(j);
auto div_pow2j = [&pow2j](rational const& r) -> rational {
rational result = r / pow2j;
SASSERT(result.is_int());
return result;
};
pdd aa = map_coefficients(a, div_pow2j);
pdd cc = map_coefficients(c, div_pow2j);
pdd vv = one();
for (unsigned deg = l - m; deg-- > 0; ) {
vv *= mk_var(v);
}
r = b * cc - aa * d * vv;
return true;
}
/**
* Returns the largest j such that 2^j divides p.
*/
unsigned pdd_manager::max_pow2_divisor(PDD p) {
init_mark();
unsigned min_j = UINT_MAX;
m_todo.push_back(p);
while (!m_todo.empty()) {
PDD r = m_todo.back();
m_todo.pop_back();
if (is_marked(r)) {
continue;
}
set_mark(r);
if (is_zero(r)) {
// skip
}
else if (is_val(r)) {
rational const& c = val(r);
if (c.is_odd()) {
m_todo.reset();
return 0;
} else {
unsigned j = c.trailing_zeros();
min_j = std::min(j, min_j);
}
}
else {
m_todo.push_back(lo(r));
m_todo.push_back(hi(r));
}
}
return min_j;
}
unsigned pdd_manager::max_pow2_divisor(pdd const& p) {
return max_pow2_divisor(p.root);
}
bool pdd_manager::is_linear(pdd const& p) {
return is_linear(p.root);
@ -1142,6 +1251,7 @@ namespace dd {
}
void pdd_manager::gc() {
m_gc_generation++;
init_dmark();
m_free_nodes.reset();
SASSERT(well_formed());

6
src/math/dd/dd_pdd.h
View file

@ -200,6 +200,7 @@ namespace dd {
rational m_freeze_value;
rational m_mod2N;
unsigned m_power_of_2 { 0 };
unsigned m_gc_generation { 0 }; ///< will be incremented on each GC
void reset_op_cache();
void init_nodes(unsigned_vector const& l2v);
@ -261,6 +262,9 @@ namespace dd {
unsigned degree(pdd const& p) const;
unsigned degree(PDD p) const;
unsigned degree(PDD p, unsigned v);
unsigned max_pow2_divisor(PDD p);
unsigned max_pow2_divisor(pdd const& p);
template <class Fn> pdd map_coefficients(pdd const& p, Fn f);
void factor(pdd const& p, unsigned v, unsigned degree, pdd& lc, pdd& rest);
@ -320,6 +324,7 @@ namespace dd {
pdd reduce(pdd const& a, pdd const& b);
pdd subst_val(pdd const& a, vector<std::pair<unsigned, rational>> const& s);
pdd subst_val(pdd const& a, unsigned v, rational const& val);
bool resolve(unsigned v, pdd const& p, pdd const& q, pdd& r);
bool is_linear(PDD p) { return degree(p) == 1; }
bool is_linear(pdd const& p);
@ -411,6 +416,7 @@ namespace dd {
double tree_size() const { return m.tree_size(*this); }
unsigned degree() const { return m.degree(*this); }
unsigned degree(unsigned v) const { return m.degree(root, v); }
unsigned max_pow2_divisor() const { return m.max_pow2_divisor(root); }
unsigned_vector const& free_vars() const { return m.free_vars(*this); }