1 files changed, 88 insertions, 0 deletions
diff --git a/src/support/istring.cpp b/src/support/istring.cpp
new file mode 100644
index 000000000..8a3319b5e
--- /dev/null
+++ b/src/support/istring.cpp
@@ -0,0 +1,88 @@
+/*
+ * Copyright 2022 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 "istring.h"
+
+namespace wasm {
+
+std::string_view IString::interned(std::string_view s, bool reuse) {
+  // We need a set of string_views that can be modified in-place to minimize
+  // the number of lookups we do. Since set elements cannot normally be
+  // modified, wrap the string_views in a container that provides mutability
+  // even through a const reference.
+  struct MutStringView {
+    mutable std::string_view str;
+    MutStringView(std::string_view str) : str(str) {}
+  };
+  struct MutStringViewHash {
+    size_t operator()(const MutStringView& mut) const {
+      return std::hash<std::string_view>{}(mut.str);
+    }
+  };
+  struct MutStringViewEqual {
+    bool operator()(const MutStringView& a, const MutStringView& b) const {
+      return a.str == b.str;
+    }
+  };
+  using StringSet =
+    std::unordered_set<MutStringView, MutStringViewHash, MutStringViewEqual>;
+
+  // The authoritative global set of interned string views.
+  static StringSet globalStrings;
+
+  // The global backing store for interned strings that do not otherwise have
+  // stable addresses.
+  static std::vector<std::vector<char>> allocated;
+
+  // Guards access to `globalStrings` and `allocated`.
+  static std::mutex mutex;
+
+  // A thread-local cache of strings to reduce contention.
+  thread_local static StringSet localStrings;
+
+  auto [localIt, localInserted] = localStrings.insert(s);
+  if (!localInserted) {
+    // We already had a local copy of this string.
+    return localIt->str;
+  }
+
+  // No copy yet in the local cache. Check the global cache.
+  std::unique_lock<std::mutex> lock(mutex);
+  auto [globalIt, globalInserted] = globalStrings.insert(s);
+  if (!globalInserted) {
+    // We already had a global copy of this string. Cache it locally.
+    localIt->str = globalIt->str;
+    return localIt->str;
+  }
+
+  if (!reuse) {
+    // We have a new string, but it doesn't have a stable address. Create a copy
+    // of the data at a stable address we can use. Make sure it is null
+    // terminated so legacy uses that get a C string still work.
+    allocated.emplace_back();
+    auto& data = allocated.back();
+    data.reserve(s.size() + 1);
+    data.insert(data.end(), s.begin(), s.end());
+    data.push_back('\0');
+    s = std::string_view(allocated.back().data(), s.size());
+  }
+
+  // Intern our new string.
+  localIt->str = globalIt->str = s;
+  return s;
+}
+
+} // namespace wasm