[analysis] Add a ConeType lattice (#8050)

ConeType is an important part of PossibleContents. Implement it as a
standalone lattice that can be combined into a larger lattice if we
rewrite PossibleContents in terms of the lattice framework.

Author: tlively

src/analysis/lattices/conetype.h

@@ -0,0 +1,184 @@
+/*
+ * Copyright 2025 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 "analysis/lattice.h"
+#include "wasm-type.h"
+
+#ifndef wasm_analysis_lattices_conetype_h
+#define wasm_analysis_lattices_conetype_h
+
+namespace wasm::analysis {
+
+// The value type lattice augmented with subtyping depths on reference types. An
+// element {(ref $foo), 1}, for example, represents the set of values that are
+// exactly $foo or exactly one of $foo's immediate subtypes, but not any deeper
+// type. Non-reference types and bottom references types always have a depth of
+// 0.
+struct ConeType {
+  struct Element {
+    Type type;
+    Index depth;
+    bool operator==(const Element& other) const {
+      return type == other.type && depth == other.depth;
+    }
+    bool operator!=(const Element& other) const { return !(*this == other); }
+    bool isBottom() const { return type == Type::unreachable; }
+    bool isTop() const { return type == Type::none; }
+  };
+
+  // Used only for initializing depths for new elements.
+  const std::unordered_map<HeapType, Index> typeDepths;
+
+  ConeType(std::unordered_map<HeapType, Index>&& typeDepths)
+    : typeDepths(std::move(typeDepths)) {}
+
+  Element get(Type type) const noexcept {
+    assert(!type.isTuple());
+    if (!type.isRef() || type.isExact() || type.isNull() ||
+        type.getHeapType().isMaybeShared(HeapType::i31)) {
+      return Element{type, 0};
+    }
+    auto it = typeDepths.find(type.getHeapType());
+    assert(it != typeDepths.end());
+    return Element{type, it->second};
+  }
+
+  Element getBottom() const noexcept { return Element{Type::unreachable, 0}; }
+
+  Element getTop() const noexcept { return Element{Type::none, 0}; }
+
+  bool join(Element& joinee, const Element& joiner) const noexcept {
+    auto lub = Type::getLeastUpperBound(joinee.type, joiner.type);
+    bool changed = lub != joinee.type;
+    if (!lub.isRef()) {
+      joinee.type = lub;
+      joinee.depth = 0;
+      return changed;
+    }
+    Index joineeToLub = 0, joinerToLub = 0;
+    if (!joinee.isBottom() && !joinee.type.isNull()) {
+      joineeToLub = depthToSuper(joinee, lub);
+    }
+    if (!joiner.isBottom() && !joiner.type.isNull()) {
+      joinerToLub = depthToSuper(joiner, lub);
+    }
+    Index newDepth =
+      std::max(joinee.depth + joineeToLub, joiner.depth + joinerToLub);
+    changed = changed || newDepth != joinee.depth;
+    joinee.type = lub;
+    joinee.depth = newDepth;
+    return changed;
+  }
+
+  bool meet(Element& meetee, const Element& meeter) const noexcept {
+    // Type::none does not behave like the top type in
+    // Type::getGreatestLowerBound, so handle it separately first. Also handle
+    // unreachables so we don't have to worry about them later.
+    if (meetee.isBottom() || meeter.isTop()) {
+      return false;
+    }
+    if (meetee.isTop() || meeter.isBottom()) {
+      meetee = meeter;
+      return true;
+    }
+    if (meetee.type == meeter.type) {
+      auto newDepth = std::min(meetee.depth, meeter.depth);
+      bool changed = newDepth != meetee.depth;
+      meetee.depth = newDepth;
+      return changed;
+    }
+    Index newDepth;
+    auto glb = Type::getGreatestLowerBound(meetee.type, meeter.type);
+    if (glb == Type::unreachable || glb.isNull()) {
+      newDepth = 0;
+    } else if (HeapType::isSubType(meetee.type.getHeapType(),
+                                   meeter.type.getHeapType())) {
+      auto diff = depthToSuper(meetee, meeter.type);
+      if (meeter.depth < diff) {
+        glb = glb.with(glb.getHeapType().getBottom());
+        newDepth = 0;
+      } else {
+        newDepth = std::min(meeter.depth - diff, meetee.depth);
+      }
+    } else if (HeapType::isSubType(meeter.type.getHeapType(),
+                                   meetee.type.getHeapType())) {
+      auto diff = depthToSuper(meeter, meetee.type);
+      if (meetee.depth < diff) {
+        glb = glb.with(glb.getHeapType().getBottom());
+        newDepth = 0;
+      } else {
+        newDepth = std::min(meetee.depth - diff, meeter.depth);
+      }
+    } else {
+      WASM_UNREACHABLE("unexpected case");
+    }
+    bool changed = glb != meetee.type || newDepth != meetee.depth;
+    meetee.type = glb;
+    meetee.depth = newDepth;
+    return changed;
+  }
+
+  analysis::LatticeComparison compare(const Element& a,
+                                      const Element& b) const noexcept {
+    if (a == b) {
+      return analysis::EQUAL;
+    }
+    if (a.isBottom() || b.isTop()) {
+      return analysis::LESS;
+    }
+    if (a.isTop() || b.isBottom()) {
+      return analysis::GREATER;
+    }
+    if (a.type == b.type) {
+      return a.depth < b.depth ? analysis::LESS : analysis::GREATER;
+    }
+    if (Type::isSubType(a.type, b.type)) {
+      if (a.type.isNull()) {
+        return analysis::LESS;
+      }
+      Index diff = depthToSuper(a, b.type);
+      return a.depth + diff <= b.depth ? analysis::LESS : analysis::NO_RELATION;
+    }
+    if (Type::isSubType(b.type, a.type)) {
+      if (b.type.isNull()) {
+        return analysis::GREATER;
+      }
+      Index diff = depthToSuper(b, a.type);
+      return b.depth + diff <= a.depth ? analysis::GREATER
+                                       : analysis::NO_RELATION;
+    }
+    return analysis::NO_RELATION;
+  }
+
+private:
+  Index depthToSuper(const Element& e, Type super) const noexcept {
+    Index depth = 0;
+    for (HeapType type = e.type.getHeapType(); type != super.getHeapType();
+         type = *type.getSuperType()) {
+      ++depth;
+    }
+    return depth;
+  }
+};
+
+#if __cplusplus >= 202002L
+static_assert(Lattice<ConeType>);
+static_assert(FullLattice<ConeType>);
+#endif
+
+} // namespace wasm::analysis
+
+#endif // wasm_analysis_lattices_conetype_h