[analysis][NFC] Move powerset lattices to their own header (#6028)

Move the powerset lattices out of lattice.h, which now only contains the Lattice
concept, to their own dedicated header in a new analysis/lattices directory.

2 files changed, 264 insertions, 0 deletions

diff --git a/src/analysis/lattices/powerset-impl.h b/src/analysis/lattices/powerset-impl.h
new file mode 100644
index 000000000..3390934d3
--- /dev/null
+++ b/src/analysis/lattices/powerset-impl.h
@@ -0,0 +1,106 @@
+/*
+ * Copyright 2023 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_analysis_lattices_powerset_impl_h
+#define wasm_analysis_lattices_powerset_impl_h
+
+#include <iostream>
+
+#include "powerset.h"
+
+namespace wasm::analysis {
+
+inline LatticeComparison FiniteIntPowersetLattice::compare(
+  const FiniteIntPowersetLattice::Element& left,
+  const FiniteIntPowersetLattice::Element& right) const noexcept {
+  // Both must be from the powerset lattice of the same set.
+  assert(left.bitvector.size() == right.bitvector.size());
+
+  // True in left, false in right.
+  bool leftNotRight = false;
+
+  // True in right, false in left.
+  bool rightNotLeft = false;
+
+  size_t size = left.bitvector.size();
+
+  for (size_t i = 0; i < size; ++i) {
+    leftNotRight |= (left.bitvector[i] && !right.bitvector[i]);
+    rightNotLeft |= (right.bitvector[i] && !left.bitvector[i]);
+
+    // We can end early if we know neither is a subset of the other.
+    if (leftNotRight && rightNotLeft) {
+      return NO_RELATION;
+    }
+  }
+
+  if (!leftNotRight) {
+    if (!rightNotLeft) {
+      return EQUAL;
+    }
+    return LESS;
+  } else if (!rightNotLeft) {
+    return GREATER;
+  }
+
+  return NO_RELATION;
+}
+
+inline FiniteIntPowersetLattice::Element
+FiniteIntPowersetLattice::getBottom() const noexcept {
+  FiniteIntPowersetLattice::Element result(setSize);
+  return result;
+}
+
+// We count the number of element members present in the element by counting the
+// trues in the bitvector.
+inline size_t FiniteIntPowersetLattice::Element::count() const {
+  size_t count = 0;
+  for (auto it : bitvector) {
+    count += it;
+  }
+  return count;
+}
+
+// Least upper bound is implemented as a logical OR between the bitvectors on
+// both sides. We return true if a bit is flipped in-place on the left so the
+// worklist algorithm will know if when to enqueue more work.
+inline bool FiniteIntPowersetLattice::Element::makeLeastUpperBound(
+  const FiniteIntPowersetLattice::Element& other) noexcept {
+  // Both must be from powerset lattice of the same set.
+  assert(other.bitvector.size() == bitvector.size());
+
+  bool modified = false;
+  for (size_t i = 0; i < bitvector.size(); ++i) {
+    // Bit is flipped on self only if self is false and other is true when self
+    // and other are OR'ed together.
+    modified |= (!bitvector[i] && other.bitvector[i]);
+    bitvector[i] = bitvector[i] || other.bitvector[i];
+  }
+
+  return modified;
+}
+
+inline void FiniteIntPowersetLattice::Element::print(std::ostream& os) {
+  // Element member 0 is on the left, element member N is on the right.
+  for (auto it : bitvector) {
+    os << it;
+  }
+}
+
+}; // namespace wasm::analysis
+
+#endif // wasm_analysis_lattices_powerset_impl_h
diff --git a/src/analysis/lattices/powerset.h b/src/analysis/lattices/powerset.h
new file mode 100644
index 000000000..92159da12
--- /dev/null
+++ b/src/analysis/lattices/powerset.h
@@ -0,0 +1,158 @@
+/*
+ * Copyright 2023 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <unordered_map>
+#include <vector>
+
+#include "wasm.h"
+
+#include "../lattice.h"
+
+#ifndef wasm_analysis_lattices_powerset_h
+#define wasm_analysis_lattices_powerset_h
+
+namespace wasm::analysis {
+
+// Represents a powerset lattice constructed from a finite set of consecutive
+// integers from 0 to n which can be represented by a bitvector. Set elements
+// are represented by FiniteIntPowersetLattice::Element, which represents
+// members present in each element by bits in the bitvector.
+class FiniteIntPowersetLattice {
+  // The size of the set that the powerset lattice was created from. This is
+  // equivalent to the size of the Top lattice element.
+  size_t setSize;
+
+public:
+  FiniteIntPowersetLattice(size_t setSize) : setSize(setSize) {}
+
+  // Returns the size of the set that the powerset lattices was created from.
+  size_t getSetSize() { return setSize; }
+
+  // This represents an element of a powerset lattice. The element is itself a
+  // set which has set members. The bitvector tracks which possible members of
+  // the element are actually present.
+  class Element {
+    // If bitvector[i] is true, then member i is present in the lattice element,
+    // otherwise it isn't.
+    std::vector<bool> bitvector;
+
+    // This constructs a bottom element, given the lattice set size. Used by the
+    // lattice's getBottom function.
+    Element(size_t latticeSetSize) : bitvector(latticeSetSize) {}
+
+  public:
+    Element(Element&& source) = default;
+    Element(const Element& source) = default;
+
+    Element& operator=(Element&& source) = default;
+    Element& operator=(const Element& source) = default;
+
+    // Counts the number of members present the element itself. For instance, if
+    // we had {true, false, true}, the count would be 2. O(N) operation which
+    // iterates through the bitvector.
+    size_t count() const;
+
+    bool get(size_t index) { return bitvector[index]; }
+    void set(size_t index, bool value) { bitvector[index] = value; }
+
+    bool isTop() const { return count() == bitvector.size(); }
+    bool isBottom() const { return count() == 0; }
+
+    // Calculates the LUB of this element with some other element and sets
+    // this element to the LUB in place. Returns true if this element before
+    // this method call was different than the LUB.
+    bool makeLeastUpperBound(const Element& other) noexcept;
+
+    // Prints out the bits in the bitvector for a lattice element.
+    void print(std::ostream& os);
+
+    friend FiniteIntPowersetLattice;
+  };
+
+  // Compares two lattice elements and returns a result indicating the
+  // left element's relation to the right element.
+  LatticeComparison compare(const Element& left,
+                            const Element& right) const noexcept;
+
+  // Returns an instance of the bottom lattice element.
+  Element getBottom() const noexcept;
+};
+
+// A layer of abstraction over FiniteIntPowersetLattice which maps
+// set members of some type T to indices in the bitvector. Allows
+// the finite powerset lattice to be generalized to arbitrary types.
+template<typename T> class FinitePowersetLattice {
+  FiniteIntPowersetLattice intLattice;
+
+  // Maps a bitvector index to some element member of type T.
+  // Used to produce initial ordering of element members.
+  std::vector<T> members;
+
+  // Maps an element member of type T to a bitvector index.
+  std::unordered_map<T, size_t> memberIndices;
+
+public:
+  using Element = FiniteIntPowersetLattice::Element;
+
+  // Takes in an ordered list of all elements of the set to create
+  // the powerset lattice from (i.e. the powerset lattice top element). This
+  // is used for mapping the elements to bitvector indices.
+  FinitePowersetLattice(std::vector<T>&& setMembers)
+    : intLattice(setMembers.size()), members(std::move(setMembers)) {
+    for (size_t i = 0; i < members.size(); ++i) {
+      memberIndices[members[i]] = i;
+    }
+  }
+
+  // Iterator to access the list of element members.
+  using membersIterator = typename std::vector<T>::const_iterator;
+  membersIterator membersBegin() { return members.cbegin(); }
+  membersIterator membersEnd() { return members.cend(); }
+  size_t getSetSize() { return intLattice.getSetSize(); }
+
+  T indexToMember(size_t index) { return members[index]; }
+
+  size_t memberToIndex(T member) { return memberIndices[member]; }
+
+  // Adds member to a powerset lattice element.
+  void add(Element* element, T member) {
+    element->set(memberIndices[member], true);
+  }
+
+  // Removes member from a powerset lattice element.
+  void remove(Element* element, T member) {
+    element->set(memberIndices[member], false);
+  }
+
+  // Checks if member is included in the element set.
+  bool exists(Element* element, T member) {
+    return element->get(memberIndices[member]);
+  }
+
+  // We use implementations from FiniteIntPowersetLattice here.
+  LatticeComparison compare(const Element& left,
+                            const Element& right) const noexcept {
+    return intLattice.compare(left, right);
+  }
+
+  Element getBottom() const noexcept { return intLattice.getBottom(); }
+};
+
+} // namespace wasm::analysis
+
+#include "powerset-impl.h"
+
+#endif // wasm_analysis_lattices_powerset_h