vstore.hpp

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// Copyright 2021 Pierre Talbot
 
#ifndef LALA_CORE_VSTORE_HPP
#define LALA_CORE_VSTORE_HPP
 
#include "logic/logic.hpp"
#include "universes/arith_bound.hpp"
#include "abstract_deps.hpp"
#include <optional>
 
namespace lala {
 
  struct NonAtomicExtraction {
    static constexpr bool atoms = false;
  };
 
  struct AtomicExtraction {
    static constexpr bool atoms = true;
  };
 
/** The variable store abstract domain is a _domain transformer_ built on top of an abstract universe `U`.
Concretization function: \f$ \gamma(\rho) := \bigcap_{x \in \pi(\rho)} \gamma_{U_x}(\rho(x)) \f$.
The bot element is smashed and the equality between two stores is represented by the following equivalence relation, for two stores \f$ S \f$ and \f$ T \f$:
\f$ S \equiv T \Leftrightarrow \forall{x \in \mathit{Vars}},~S(x) = T(x) \lor \exists{x \in \mathit{dom}(S)},\exists{y \in \mathit{dom}(T)},~S(x) = \bot \land T(y) = \bot \f$.
Intuitively, it means that either all elements are equal or both stores have a bot element, in which case they "collapse" to the bot element, and are considered equal.
 
The top element is the element \f$ \langle \top, \ldots \rangle \f$, that is an infinite number of variables initialized to top.
In practice, we cannot represent infinite collections, so we represent top either as the empty collection or with a finite number of top elements.
Any finite store \f$ \langle x_1, \ldots, x_n \rangle \f$ should be seen as the concrete store \f$ \langle x_1, \ldots, x_n, \top, \ldots \rangle \f$.
 
This semantics has implication when joining or merging two elements.
For instance, \f$ \langle 1 \rangle.\mathit{meet}(\langle \bot, 4 \rangle) \f$ will be equal to bottom, in that case represented by \f$ \langle \bot \rangle \f$.
 
Template parameters:
  - `U` is the type of the abstract universe.
  - `Allocator` is the allocator of the underlying array of universes. */
template<class U, class Allocator>
class VStore {
public:
  using universe_type = U;
  using local_universe = typename universe_type::local_type;
  using allocator_type = Allocator;
  using this_type = VStore<universe_type, allocator_type>;
 
  template <class Alloc>
  struct var_dom {
    AVar avar;
    local_universe dom;
    var_dom() = default;
    var_dom(const var_dom<Alloc>&) = default;
    CUDA explicit var_dom(const Alloc&) {}
    CUDA var_dom(AVar avar, const local_universe& dom): avar(avar), dom(dom) {}
    template <class VarDom>
    CUDA var_dom(const VarDom& other): avar(other.avar), dom(other.dom) {}
  };
 
  template <class Alloc>
  using tell_type = battery::vector<var_dom<Alloc>, Alloc>;
 
  template <class Alloc>
  using ask_type = tell_type<Alloc>;
 
  template <class Alloc = allocator_type>
  using snapshot_type = battery::vector<local_universe, Alloc>;
 
  constexpr static const bool is_abstract_universe = false;
  constexpr static const bool sequential = universe_type::sequential;
  constexpr static const bool is_totally_ordered = false;
  constexpr static const bool preserve_bot = true;
  constexpr static const bool preserve_top = true;
  constexpr static const bool preserve_join = universe_type::preserve_join;
  constexpr static const bool preserve_meet = universe_type::preserve_meet;
  constexpr static const bool injective_concretization = universe_type::injective_concretization;
  constexpr static const bool preserve_concrete_covers = universe_type::preserve_concrete_covers;
  constexpr static const char* name = "VStore";
 
  template<class U2, class Alloc2>
  friend class VStore;
 
private:
  using store_type = battery::vector<universe_type, allocator_type>;
  using memory_type = typename universe_type::memory_type;
 
  AType atype;
  store_type data;
  B<memory_type> is_at_bot;
 
public:
  CUDA VStore(const this_type& other)
    : atype(other.atype), data(other.data), is_at_bot(other.is_at_bot)
  {}
 
  /** Initialize an empty store. */
  CUDA VStore(AType atype, const allocator_type& alloc = allocator_type())
   : atype(atype), data(alloc), is_at_bot(false)
  {}
 
  CUDA VStore(AType atype, int size, const allocator_type& alloc = allocator_type())
   : atype(atype), data(size, alloc), is_at_bot(false)
  {}
 
  template<class R>
  CUDA VStore(const VStore<R, allocator_type>& other)
    : atype(other.atype), data(other.data), is_at_bot(other.is_at_bot)
  {}
 
  template<class R, class Alloc2>
  CUDA VStore(const VStore<R, Alloc2>& other, const allocator_type& alloc = allocator_type())
    : atype(other.atype), data(other.data, alloc), is_at_bot(other.is_at_bot)
  {}
 
  /** Copy the vstore `other` in the current element.
   *  `deps` can be empty and is not used besides to get the allocator (since this abstract domain does not have dependencies). */
  template<class R, class Alloc2, class... Allocators>
  CUDA VStore(const VStore<R, Alloc2>& other, const AbstractDeps<Allocators...>& deps)
   : VStore(other, deps.template get_allocator<allocator_type>()) {}
 
  CUDA VStore(this_type&& other):
    atype(other.atype), data(std::move(other.data)), is_at_bot(other.is_at_bot) {}
 
  CUDA allocator_type get_allocator() const {
    return data.get_allocator();
  }
 
  CUDA AType aty() const {
    return atype;
  }
 
  /** Returns the number of variables currently represented by this abstract element. */
  CUDA int vars() const {
    return data.size();
  }
 
  CUDA static this_type top(AType atype = UNTYPED,
    const allocator_type& alloc = allocator_type{})
  {
    return VStore(atype, alloc);
  }
 
  /** A special symbolic element representing top. */
  CUDA static this_type bot(AType atype = UNTYPED,
    const allocator_type& alloc = allocator_type{})
  {
    auto s = VStore{atype, alloc};
    s.meet_bot();
    return std::move(s);
  }
 
  template <class Env>
  CUDA static this_type bot(Env& env,
    const allocator_type& alloc = allocator_type{})
  {
    return bot(env.extends_abstract_dom(), alloc);
  }
 
  template <class Env>
  CUDA static this_type top(Env& env,
    const allocator_type& alloc = allocator_type{})
  {
    return top(env.extends_abstract_dom(), alloc);
  }
 
  /** \return `true` if at least one element is equal to bot in the store, `false` otherwise.
   * @parallel @order-preserving @increasing
  */
  CUDA local::B is_bot() const {
    return is_at_bot;
  }
 
  /** The bottom element of a store of `n` variables is when all variables are at bottom, or the store is empty.
   * We do not expect to use this operation a lot, so its complexity is linear in the number of variables.
   * @parallel @order-preserving @decreasing */
  CUDA local::B is_top() const {
    if(is_at_bot) { return false; }
    for(int i = 0; i < vars(); ++i) {
      if(!data[i].is_top()) {
        return false;
      }
    }
    return true;
  }
 
  /** Take a snapshot of the current variable store. */
  template <class Alloc = allocator_type>
  CUDA snapshot_type<Alloc> snapshot(const Alloc& alloc = Alloc()) const {
    return snapshot_type<Alloc>(data, alloc);
  }
 
  template <class Alloc>
  CUDA this_type& restore(const snapshot_type<Alloc>& snap) {
    while(snap.size() < data.size()) {
      data.pop_back();
    }
    is_at_bot = false;
    for(int i = 0; i < snap.size(); ++i) {
      data[i].join(snap[i]);
      is_at_bot.join(data[i].is_bot());
    }
    return *this;
  }
 
private:
  template <bool diagnose, class F, class Env, class Alloc2>
  CUDA NI bool interpret_existential(const F& f, Env& env, tell_type<Alloc2>& tell, IDiagnostics& diagnostics) const {
    assert(f.is(F::E));
    var_dom<Alloc2> k;
    if(local_universe::template interpret_tell<diagnose>(f, env, k.dom, diagnostics)) {
      if(env.interpret(f.map_atype(atype), k.avar, diagnostics)) {
        assert(k.avar.aty() == aty());
        tell.push_back(k);
        return true;
      }
    }
    return false;
  }
 
  /** Interpret a predicate without variables. */
  template <bool diagnose, class F, class Env, class Alloc2>
  CUDA NI bool interpret_zero_predicate(const F& f, const Env& env, tell_type<Alloc2>& tell, IDiagnostics& diagnostics) const {
    if(f.is_true()) {
      return true;
    }
    else if(f.is_false()) {
      tell.push_back(var_dom<Alloc2>(AVar{}, U::bot()));
      return true;
    }
    else {
      RETURN_INTERPRETATION_ERROR("Only `true` and `false` can be interpreted in the store without being named.");
    }
  }
 
  /** Interpret a predicate with a single variable occurrence. */
  template <IKind kind, bool diagnose, class F, class Env, class Alloc2>
  CUDA NI bool interpret_unary_predicate(const F& f, const Env& env, tell_type<Alloc2>& tell, IDiagnostics& diagnostics) const {
    local_universe u;
    bool res = local_universe::template interpret<kind, diagnose>(f, env, u, diagnostics);
    if(res) {
      const auto& varf = var_in(f);
      // When it is not necessary, we try to avoid using the environment.
      // This is for instance useful when deduction operators add new constraints but do not have access to the environment (e.g., split()), and to avoid passing the environment around everywhere.
      if(varf.is(F::V)) {
        if(varf.v().aty() == aty() || varf.v().aty() == UNTYPED) {
          tell.push_back(var_dom<Alloc2>(varf.v(), u));
        }
        else {
          RETURN_INTERPRETATION_ERROR("The variable was not declared in the current abstract element (but exists in other abstract elements).");
        }
      }
      else {
        auto var = var_in(f, env);
        if(!var.has_value()) {
          RETURN_INTERPRETATION_ERROR("Undeclared variable.");
        }
        auto avar = var->get().avar_of(atype);
        if(!avar.has_value()) {
          RETURN_INTERPRETATION_ERROR("The variable was not declared in the current abstract element (but exists in other abstract elements).");
        }
        assert(avar->aty() == aty());
        tell.push_back(var_dom<Alloc2>(*avar, u));
      }
      return true;
    }
    else {
      RETURN_INTERPRETATION_ERROR("Could not interpret a unary predicate in the underlying abstract universe.");
    }
  }
 
  template <IKind kind, bool diagnose, class F, class Env, class Alloc2>
  CUDA NI bool interpret_predicate(const F& f, Env& env, tell_type<Alloc2>& tell, IDiagnostics& diagnostics) const {
    if(f.type() != UNTYPED && f.type() != aty()) {
      RETURN_INTERPRETATION_ERROR("The abstract type of this predicate does not match the one of the current abstract element.");
    }
    if constexpr(kind == IKind::TELL) {
      if(f.is(F::E)) {
        return interpret_existential<diagnose>(f, env, tell, diagnostics);
      }
    }
    switch(num_vars(f)) {
      case 0: return interpret_zero_predicate<diagnose>(f, env, tell, diagnostics);
      case 1: return interpret_unary_predicate<kind, diagnose>(f, env, tell, diagnostics);
      default: RETURN_INTERPRETATION_ERROR("Interpretation of n-ary predicate is not supported in VStore.");
    }
  }
 
public:
  template <IKind kind, bool diagnose = false, class F, class Env, class I>
  CUDA NI bool interpret(const F& f, Env& env, I& intermediate, IDiagnostics& diagnostics) const {
    if(f.is_untyped() || f.type() == aty()) {
      return interpret_predicate<kind, diagnose>(f, env, intermediate, diagnostics);
    }
    else {
      RETURN_INTERPRETATION_ERROR("This abstract element cannot interpret a formula with a different type.");
    }
  }
 
  /** The store of variables lattice expects a formula with a single variable (including existential quantifiers) that can be handled by the abstract universe `U`.
   *
   * Variables must be existentially quantified before a formula containing variables can be interpreted.
   * Variables are immediately assigned to an index of `VStore` and initialized to \f$ \top_U \f$.
   * Shadowing/redeclaration of variables with existential quantifier is not supported.
   * The variable mapping is added to the environment only if the interpretation succeeds.
 
   * There is a small quirk: different stores might be produced if quantifiers do not appear in the same order.
   * This is because we attribute the first available index to variables when interpreting the quantifier.
   * In that case, the store will only be equivalent modulo the `env` structure.
  */
  template <bool diagnose = false, class F, class Env, class Alloc2>
  CUDA NI bool interpret_tell(const F& f, Env& env, tell_type<Alloc2>& tell, IDiagnostics& diagnostics) const {
    return interpret<IKind::TELL, diagnose>(f, env, tell, diagnostics);
  }
 
  /** Similar to `interpret_tell` but do not support existential quantifier and therefore leaves `env` unchanged. */
  template <bool diagnose = false, class F, class Env, class Alloc2>
  CUDA NI bool interpret_ask(const F& f, const Env& env, ask_type<Alloc2>& ask, IDiagnostics& diagnostics) const {
    return const_cast<this_type*>(this)->interpret<IKind::ASK, diagnose>(f, const_cast<Env&>(env), ask, diagnostics);
  }
 
  template <class Group, class Store>
  CUDA void copy_to(Group& group, Store& store) const {
    assert(vars() == store.vars());
    if(group.thread_rank() == 0) {
      store.is_at_bot = is_at_bot;
    }
    if(is_at_bot) {
      return;
    }
    for (int i = group.thread_rank(); i < store.vars(); i += group.num_threads()) {
      store.data[i] = data[i];
    }
  }
 
  template <class Store>
  CUDA void copy_to(Store& store) const {
    assert(vars() == store.vars());
    store.is_at_bot = is_at_bot;
    if(is_at_bot) {
      return;
    }
    for (int i = 0; i < store.vars(); ++i) {
      store.data[i] = data[i];
    }
  }
 
#ifdef __CUDACC__
  void prefetch(int dstDevice) const {
    if(!is_at_bot) {
      cudaMemPrefetchAsync(data.data(), data.size() * sizeof(universe_type), dstDevice);
    }
  }
#endif
 
  /** Change the allocator of the underlying data, and reallocate the memory without copying the old data. */
  CUDA void reset_data(allocator_type alloc) {
    data = store_type(data.size(), alloc);
  }
 
  template <class Univ>
  CUDA void project(AVar x, Univ& u) const {
    u.meet(project(x));
  }
 
  CUDA const universe_type& project(AVar x) const {
    assert(x.aty() == aty());
    assert(x.vid() < data.size());
    return data[x.vid()];
  }
 
  /** This projection must stay const, otherwise the user might tell new information in the universe, but we need to know in case we reach `top`. */
  CUDA const universe_type& operator[](int x) const {
    return data[x];
  }
 
  CUDA const universe_type& operator[](AVar x) const {
    return project(x);
  }
 
  CUDA void meet_bot() {
    is_at_bot.join_top();
  }
 
  /** Given an abstract variable `v`, `embed(VID(v), dom)` will update the domain of this variable with the new information `dom`.
   * This `embed` method follows PCCP's model, but the variable `x` must already be initialized in the store.
   * @parallel @order-preserving @increasing
  */
  CUDA bool embed(int x, const universe_type& dom) {
    assert(x < data.size());
    bool has_changed = data[x].meet(dom);
    if(has_changed && data[x].is_bot()) {
      is_at_bot.join_top();
    }
    return has_changed;
  }
 
  /** This `embed` method follows PCCP's model, but the variable `x` must already be initialized in the store. */
  CUDA bool embed(AVar x, const universe_type& dom) {
    assert(x.aty() == aty());
    return embed(x.vid(), dom);
  }
 
  /** This deduce method can grow the store if required, and therefore do not satisfy the PCCP model.
   * @sequential @order-preserving @increasing
  */
  template <class Alloc2>
  CUDA bool deduce(const tell_type<Alloc2>& t) {
    if(t.size() == 0) {
      return false;
    }
    int largest_vid = 0;
    for(int i = 0; i < t.size(); ++i) {
      if(t[i].avar == AVar{}) {
        return is_at_bot.join(local::B(true));
      }
      largest_vid = battery::max(largest_vid, t[i].avar.vid());
    }
    if(largest_vid >= data.size()) {
      data.resize(largest_vid+1);
    }
    bool has_changed = false;
    for(int i = 0; i < t.size(); ++i) {
      has_changed |= embed(t[i].avar, t[i].dom);
    }
    return has_changed;
  }
 
  /** Precondition: `other` must be smaller or equal in size than the current store. */
  template <class U2, class Alloc2>
  CUDA bool meet(const VStore<U2, Alloc2>& other) {
    bool has_changed = is_at_bot.join(other.is_at_bot);
    int min_size = battery::min(vars(), other.vars());
    for(int i = 0; i < min_size; ++i) {
      has_changed |= data[i].meet(other[i]);
    }
    for(int i = min_size; i < other.vars(); ++i) {
      assert(other[i].is_top()); // the size of the current store cannot be modified.
    }
    return has_changed;
  }
 
  CUDA void join_top() {
    is_at_bot.meet_bot();
    for(int i = 0; i < data.size(); ++i) {
      data[i].join_top();
    }
  }
 
  /** Precondition: `other` must be smaller or equal in size than the current store. */
  template <class U2, class Alloc2>
  CUDA bool join(const VStore<U2, Alloc2>& other)  {
    if(other.is_bot()) {
      return false;
    }
    int min_size = battery::min(vars(), other.vars());
    bool has_changed = is_at_bot.meet(other.is_at_bot);
    for(int i = 0; i < min_size; ++i) {
      has_changed |= data[i].join(other[i]);
    }
    for(int i = min_size; i < vars(); ++i) {
      has_changed |= data[i].join(U::top());
    }
    for(int i = min_size; i < other.vars(); ++i) {
      assert(other[i].is_top());
    }
    return has_changed;
  }
 
  /** \return `true` when we can deduce the content of `t` from the current domain.
   * For instance, if we have in the store `x = [0..10]`, we can deduce `x = [-1..11]` but we cannot deduce `x = [5..8]`.
   * @parallel @order-preserving @decreasing */
  template <class Alloc2>
  CUDA local::B ask(const ask_type<Alloc2>& t) const {
    for(int i = 0; i < t.size(); ++i) {
      if(!(data[t[i].avar.vid()] <= t[i].dom)) {
        return false;
      }
    }
    return true;
  }
 
  CUDA int num_deductions() const { return 0; }
  CUDA local::B deduce(int) const { assert(false); return false; }
 
  /**  An abstract element is extractable when it is not equal to bot.
   * If the strategy is `atoms`, we check the domains are singleton.
   */
  template<class ExtractionStrategy = NonAtomicExtraction>
  CUDA bool is_extractable(const ExtractionStrategy& strategy = ExtractionStrategy()) const {
    if(is_bot()) {
      return false;
    }
    if constexpr(ExtractionStrategy::atoms) {
      for(int i = 0; i < data.size(); ++i) {
        if(data[i].lb().value() != data[i].ub().value()) {
          return false;
        }
      }
    }
    return true;
  }
 
#ifdef __CUDACC__
  template<class ExtractionStrategy = NonAtomicExtraction>
  __device__ bool is_extractable(auto& group, const ExtractionStrategy& strategy = ExtractionStrategy()) const {
    if(is_bot()) {
      return false;
    }
    if constexpr(ExtractionStrategy::atoms) {
      __shared__ bool res;
      if(group.thread_rank() == 0) {
        res = true;
      }
      group.sync();
      for(int i = group.thread_rank(); i < data.size(); i += group.num_threads()) {
        if(data[i].lb().value() != data[i].ub().value()) {
          res = false;
        }
      }
      group.sync();
      return res;
    }
    else {
      return true;
    }
  }
#endif
 
  /** Whenever `this` is different from `bot`, we extract its data into `ua`.
   * \pre `is_extractable()` must be `true`.
   * For now, we suppose VStore is only used to store under-approximation, I'm not sure yet how we would interact with over-approximation. */
  template<class U2, class Alloc2>
  CUDA void extract(VStore<U2, Alloc2>& ua) const {
    if((void*)&ua != (void*)this) {
      ua.data = data;
      ua.is_at_bot.meet_bot();
    }
  }
 
private:
  template<class U2, class Env, class Allocator2>
  CUDA TFormula<typename Env::allocator_type> deinterpret(AVar avar, const U2& dom, const Env& env, const Allocator2& allocator) const {
    auto f = dom.deinterpret(avar, env, allocator);
    f.type_as(aty());
    map_avar_to_lvar(f, env);
    return std::move(f);
  }
 
public:
  template<class Env, class Allocator2 = typename Env::allocator_type>
  CUDA NI TFormula<Allocator2> deinterpret(const Env& env, const Allocator2& allocator = Allocator2()) const {
    using F = TFormula<Allocator2>;
    if(data.size() == 0) {
      return is_bot() ? F::make_false() : F::make_true();
    }
    typename F::Sequence seq{allocator};
    for(int i = 0; i < data.size(); ++i) {
      AVar v(aty(), i);
      seq.push_back(F::make_exists(aty(), env.name_of(v), env.sort_of(v)));
      seq.push_back(deinterpret(AVar(aty(), i), data[i], env, allocator));
    }
    return F::make_nary(AND, std::move(seq), aty());
  }
 
  template<class I, class Env, class Allocator2 = typename Env::allocator_type>
  CUDA NI TFormula<Allocator2> deinterpret(const I& intermediate, const Env& env, const Allocator2& allocator = Allocator2()) const {
    using F = TFormula<Allocator2>;
    if(intermediate.size() == 0) {
      return F::make_true();
    }
    else if(intermediate.size() == 1) {
      return deinterpret(intermediate[0].avar, intermediate[0].dom, env, allocator);
    }
    else {
      typename F::Sequence seq{allocator};
      for(int i = 0; i < intermediate.size(); ++i) {
        seq.push_back(deinterpret(intermediate[i].avar, intermediate[i].dom, env, allocator));
      }
      return F::make_nary(AND, std::move(seq), aty());
    }
  }
 
  CUDA void print() const {
    if(is_bot()) {
      printf("\u22A5 | ");
    }
    printf("<");
    for(int i = 0; i < vars(); ++i) {
      data[i].print();
      printf("%s", (i+1 == vars() ? "" : ", "));
    }
    printf(">\n");
  }
};
 
// Lattice operations.
// Note that we do not consider the logical names.
// These operations are only considering the indices of the elements.
 
template<class L, class K, class Alloc>
CUDA auto fmeet(const VStore<L, Alloc>& a, const VStore<K, Alloc>& b)
{
  using U = decltype(fmeet(a[0], b[0]));
  if(a.is_bot() || b.is_bot()) {
    return VStore<U, Alloc>::bot(UNTYPED, a.get_allocator());
  }
  int max_size = battery::max(a.vars(), b.vars());
  int min_size = battery::min(a.vars(), b.vars());
  VStore<U, Alloc> res(UNTYPED, max_size, a.get_allocator());
  for(int i = 0; i < min_size; ++i) {
    res.embed(i, fmeet(a[i], b[i]));
  }
  for(int i = min_size; i < a.vars(); ++i) {
    res.embed(i, a[i]);
  }
  for(int i = min_size; i < b.vars(); ++i) {
    res.embed(i, b[i]);
  }
  return res;
}
 
template<class L, class K, class Alloc>
CUDA auto fjoin(const VStore<L, Alloc>& a, const VStore<K, Alloc>& b)
{
  using U = decltype(fjoin(a[0], b[0]));
  if(a.is_bot()) {
    if(b.is_bot()) {
      return VStore<U, Alloc>::bot(UNTYPED, a.get_allocator());
    }
    else {
      return VStore<U, Alloc>(b);
    }
  }
  else if(b.is_bot()) {
    return VStore<U, Alloc>(a);
  }
  else {
    int min_size = battery::min(a.vars(), b.vars());
    VStore<U, Alloc> res(UNTYPED, min_size, a.get_allocator());
    for(int i = 0; i < min_size; ++i) {
      res.embed(i, fjoin(a[i], b[i]));
    }
    return res;
  }
}
 
template<class L, class K, class Alloc1, class Alloc2>
CUDA bool operator<=(const VStore<L, Alloc1>& a, const VStore<K, Alloc2>& b)
{
  if(b.is_top()) {
    return true;
  }
  else {
    int min_size = battery::min(a.vars(), b.vars());
    for(int i = 0; i < min_size; ++i) {
      if(!(a[i] <= b[i])) {
        return false;
      }
    }
    for(int i = min_size; i < b.vars(); ++i) {
      if(!b[i].is_top()) {
        return false;
      }
    }
  }
  return true;
}
 
template<class L, class K, class Alloc1, class Alloc2>
CUDA bool operator<(const VStore<L, Alloc1>& a, const VStore<K, Alloc2>& b)
{
  if(a.is_bot()) {
    return !b.is_bot();
  }
  else {
    int min_size = battery::min(a.vars(), b.vars());
    bool strict = false;
    for(int i = 0; i < a.vars(); ++i) {
      if(i < b.vars()) {
        if(a[i] < b[i]) {
          strict = true;
        }
        else if(!(a[i] <= b[i])) {
          return false;
        }
      }
      else if(!a[i].is_top()) {
        strict = true;
        break;
      }
    }
    for(int i = min_size; i < b.vars(); ++i) {
      if(!b[i].is_top()) {
        return false;
      }
    }
    return strict;
  }
}
 
template<class L, class K, class Alloc1, class Alloc2>
CUDA bool operator>=(const VStore<L, Alloc1>& a, const VStore<K, Alloc2>& b)
{
  return b <= a;
}
 
template<class L, class K, class Alloc1, class Alloc2>
CUDA bool operator>(const VStore<L, Alloc1>& a, const VStore<K, Alloc2>& b)
{
  return b < a;
}
 
template<class L, class K, class Alloc1, class Alloc2>
CUDA bool operator==(const VStore<L, Alloc1>& a, const VStore<K, Alloc2>& b)
{
  if(a.is_bot()) {
    return b.is_bot();
  }
  else if(b.is_bot()) {
    return false;
  }
  else {
    int min_size = battery::min(a.vars(), b.vars());
    for(int i = 0; i < min_size; ++i) {
      if(a[i] != b[i]) {
        return false;
      }
    }
    for(int i = min_size; i < a.vars(); ++i) {
      if(!a[i].is_top()) {
        return false;
      }
    }
    for(int i = min_size; i < b.vars(); ++i) {
      if(!b[i].is_top()) {
        return false;
      }
    }
  }
  return true;
}
 
template<class L, class K, class Alloc1, class Alloc2>
CUDA bool operator!=(const VStore<L, Alloc1>& a, const VStore<K, Alloc2>& b)
{
  return !(a == b);
}
 
template<class L, class Alloc>
std::ostream& operator<<(std::ostream &s, const VStore<L, Alloc> &vstore) {
  if(vstore.is_bot()) {
    s << "\u22A5: ";
  }
  else {
    s << "<";
    for(int i = 0; i < vstore.vars(); ++i) {
      s << vstore[i] << (i+1 == vstore.vars() ? "" : ", ");
    }
    s << ">";
  }
  return s;
}
 
} // namespace lala
 
#endif