[analysis] Add a lattice for value types (#6064)

Add a lattice that is a thin wrapper around `wasm::Type` giving it the interface
of a lattice. As usual, `Type::unreachable` is the bottom element, but unlike in
the underlying API, we uniformly treat `Type::none` as the top type so that we
have a proper lattice.

1 files changed, 82 insertions, 0 deletions

diff --git a/src/analysis/lattices/valtype.h b/src/analysis/lattices/valtype.h
new file mode 100644
index 000000000..d63432ac6
--- /dev/null
+++ b/src/analysis/lattices/valtype.h
@@ -0,0 +1,82 @@
+/*
+ * 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_valtype_h
+#define wasm_analysis_lattices_valtype_h
+
+#include "../lattice.h"
+#include "wasm-type.h"
+
+namespace wasm::analysis {
+
+// Thin wrapper around `wasm::Type` giving it the interface of a lattice. As
+// usual, `Type::unreachable` is the bottom element, but unlike in the
+// underlying API, we uniformly treat `Type::none` as the top type so that we
+// have a proper lattice.
+struct ValType {
+  using Element = Type;
+
+  Element getBottom() const noexcept { return Type::unreachable; }
+
+  Element getTop() const noexcept { return Type::none; }
+
+  LatticeComparison compare(Element a, Element b) const noexcept {
+    if (a == b) {
+      return EQUAL;
+    }
+    if (b == Type::none || Type::isSubType(a, b)) {
+      return LESS;
+    }
+    if (a == Type::none || Type::isSubType(b, a)) {
+      return GREATER;
+    }
+    return NO_RELATION;
+  }
+
+  bool join(Element& joinee, Element joiner) const noexcept {
+    // `getLeastUpperBound` already treats `Type::none` as top.
+    auto lub = Type::getLeastUpperBound(joinee, joiner);
+    if (lub != joinee) {
+      joinee = lub;
+      return true;
+    }
+    return false;
+  }
+
+  bool meet(Element& meetee, Element meeter) const noexcept {
+    if (meetee == meeter || meeter == Type::none) {
+      return false;
+    }
+    if (meetee == Type::none) {
+      meetee = meeter;
+      return true;
+    }
+    auto glb = Type::getGreatestLowerBound(meetee, meeter);
+    if (glb != meetee) {
+      meetee = glb;
+      return true;
+    }
+    return false;
+  }
+};
+
+#if __cplusplus >= 202002L
+static_assert(FullLattice<ValType>);
+#endif
+
+} // namespace wasm::analysis
+
+#endif // wasm_analysis_lattices_valtype_h