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/*
* 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
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