[libcxx] Avoid hash key in __hash_table::find() if it is empty.

libcxx/include/__hash_table

@@ -1771,9 +1771,9 @@ template <class _Tp, class _Hash, class _Equal, class _Alloc>
 template <class _Key>
 typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator
 __hash_table<_Tp, _Hash, _Equal, _Alloc>::find(const _Key& __k) {
-  size_t __hash  = hash_function()(__k);
   size_type __bc = bucket_count();
-  if (__bc != 0) {
+  if (__bc != 0 && size() != 0) {
+    size_t __hash       = hash_function()(__k);
     size_t __chash      = std::__constrain_hash(__hash, __bc);
     __next_pointer __nd = __bucket_list_[__chash];
     if (__nd != nullptr) {
@@ -1792,9 +1792,9 @@ template <class _Tp, class _Hash, class _Equal, class _Alloc>
 template <class _Key>
 typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::const_iterator
 __hash_table<_Tp, _Hash, _Equal, _Alloc>::find(const _Key& __k) const {
-  size_t __hash  = hash_function()(__k);
   size_type __bc = bucket_count();
-  if (__bc != 0) {
+  if (__bc != 0 && size() != 0) {
+    size_t __hash       = hash_function()(__k);
     size_t __chash      = std::__constrain_hash(__hash, __bc);
     __next_pointer __nd = __bucket_list_[__chash];
     if (__nd != nullptr) {

libcxx/test/benchmarks/containers/associative/associative_container_benchmarks.h

@@ -57,7 +57,7 @@ void associative_container_benchmarks(std::string container) {
   auto get_key = [](Value const& v) { return adapt_operations<Container>::key_from_value(v); };
 
   auto bench = [&](std::string operation, auto f) {
-    benchmark::RegisterBenchmark(container + "::" + operation, f)->Arg(32)->Arg(1024)->Arg(8192);
+    benchmark::RegisterBenchmark(container + "::" + operation, f)->Arg(0)->Arg(32)->Arg(1024)->Arg(8192);
   };
 
   static constexpr bool is_multi_key_container =
@@ -172,7 +172,7 @@ void associative_container_benchmarks(std::string container) {
   // Insertion
   /////////////////////////
   bench("insert(value) (already present)", [=](auto& st) {
-    const std::size_t size = st.range(0);
+    const std::size_t size = st.range(0) ? st.range(0) : 1;
     std::vector<Value> in  = make_value_types(generate_unique_keys(size));
     Value to_insert        = in[in.size() / 2]; // pick any existing value
     std::vector<Container> c(BatchSize, Container(in.begin(), in.end()));
@@ -325,7 +325,7 @@ void associative_container_benchmarks(std::string container) {
   // Erasure
   /////////////////////////
   bench("erase(key) (existent)", [=](auto& st) {
-    const std::size_t size = st.range(0);
+    const std::size_t size = st.range(0) ? st.range(0) : 1; // avoid empty container
     std::vector<Value> in  = make_value_types(generate_unique_keys(size));
     Value element          = in[in.size() / 2]; // pick any element
     std::vector<Container> c(BatchSize, Container(in.begin(), in.end()));
@@ -369,7 +369,7 @@ void associative_container_benchmarks(std::string container) {
   });
 
   bench("erase(iterator)", [=](auto& st) {
-    const std::size_t size = st.range(0);
+    const std::size_t size = st.range(0) ? st.range(0) : 1; // avoid empty container
     std::vector<Value> in  = make_value_types(generate_unique_keys(size));
     Value element          = in[in.size() / 2]; // pick any element
 
@@ -448,7 +448,7 @@ void associative_container_benchmarks(std::string container) {
       Container c(in.begin(), in.end());
 
       while (st.KeepRunningBatch(BatchSize)) {
-        for (std::size_t i = 0; i != BatchSize; ++i) {
+        for (std::size_t i = 0; i != keys.size(); ++i) { // possible empty keys when Arg(0)
           auto result = func(c, keys[i]);
           benchmark::DoNotOptimize(c);
           benchmark::DoNotOptimize(result);

libcxx/test/benchmarks/containers/associative/unordered_set.bench.cpp

@@ -8,6 +8,7 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
+#include <string>
 #include <unordered_set>
 #include <utility>
 
@@ -27,6 +28,7 @@ struct support::adapt_operations<std::unordered_set<K>> {
 
 int main(int argc, char** argv) {
   support::associative_container_benchmarks<std::unordered_set<int>>("std::unordered_set<int>");
+  support::associative_container_benchmarks<std::unordered_set<std::string>>("std::unordered_set<std::string>");
 
   benchmark::Initialize(&argc, argv);
   benchmark::RunSpecifiedBenchmarks();