[libcxx-commits] [libcxx] Optimize input iterator overload of `std::vector::assign(first, last)` (PR #113852)

[Peng Liu via libcxx-commits]

================
@@ -48,6 +48,76 @@ void BM_Assignment(benchmark::State& st, Container) {
   }
 }
 
+// Wrap any Iterator into an input iterator
+template <typename Iterator>
+class InputIterator {
+  using iter_traits = std::iterator_traits<Iterator>;
+
+public:
+  using iterator_category = std::input_iterator_tag;
+  using value_type        = typename iter_traits::value_type;
+  using difference_type   = typename iter_traits::difference_type;
+  using pointer           = typename iter_traits::pointer;
+  using reference         = typename iter_traits::reference;
+
+  InputIterator(Iterator it) : current_(it) {}
+
+  reference operator*() { return *current_; }
+  InputIterator& operator++() {
+    ++current_;
+    return *this;
+  }
+  InputIterator operator++(int) {
+    InputIterator tmp = *this;
+    ++(*this);
+    return tmp;
+  }
+
+  friend bool operator==(const InputIterator& lhs, const InputIterator& rhs) { return lhs.current_ == rhs.current_; }
+  friend bool operator!=(const InputIterator& lhs, const InputIterator& rhs) { return !(lhs == rhs); }
+
+private:
+  Iterator current_;
+};
+
+template <typename Iterator>
+InputIterator<Iterator> make_input_iterator(Iterator it) {
+  return InputIterator<Iterator>(it);
+}
+
+template <class Container,
+          class GenInputs,
+          typename std::enable_if<std::is_trivial<typename Container::value_type>::value>::type* = nullptr>
+void BM_AssignInputIterIter(benchmark::State& st, Container c, GenInputs gen) {
+  auto in = gen(st.range(1));
+  benchmark::DoNotOptimize(&in);
+  for (auto _ : st) {
+    st.PauseTiming();
+    c.resize(st.range(0));
+    benchmark::DoNotOptimize(&c);
+    st.ResumeTiming();
----------------
winner245 wrote:

Thank you for your clarification. I agree that we can add some extra but inexpensive operations within the benchmark loop, provided we do so consistently. Regarding my tests, since my goal is to measure the performance of `assign`, I prefer to avoid calling `resize` inside the loop to maintain a clean benchmark loop that only contains the operations I am measuring. I appreciate your insights on the general benchmarking practices.

https://github.com/llvm/llvm-project/pull/113852