Optimize vector::assign for InputIterator-only pair inputs

libcxx/docs/ReleaseNotes/20.rst

@@ -69,6 +69,10 @@ Improvements and New Features
 - The ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY`` ABI configuration was added, which allows storing valid bounds
   in ``std::array::iterator`` and detecting OOB accesses when the appropriate hardening mode is enabled.
 
+- The input iterator overload of `assign(_InputIterator, _InputIterator)` in `std::vector<_Tp, _Allocator>` has been
+  optimized, resulting in a performance improvement of up to 2x for trivial element types (e.g., `std::vector<int>`),
+  and up to 3.4x for non-trivial element types (e.g., `std::vector<std::vector<int>>`).
+
 Deprecations and Removals
 -------------------------
 

libcxx/include/__vector/vector.h

@@ -1003,9 +1003,15 @@ template <class _Tp, class _Allocator>
 template <class _Iterator, class _Sentinel>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
 vector<_Tp, _Allocator>::__assign_with_sentinel(_Iterator __first, _Sentinel __last) {
-  clear();
-  for (; __first != __last; ++__first)
-    emplace_back(*__first);
+  pointer __cur = __begin_;
+  for (; __first != __last && __cur != __end_; ++__first, (void)++__cur)
+    *__cur = *__first;
+  if (__cur != __end_) {
+    __destruct_at_end(__cur);
+  } else {
+    for (; __first != __last; ++__first)
+      emplace_back(*__first);
+  }
 }
 
 template <class _Tp, class _Allocator>

libcxx/test/benchmarks/ContainerBenchmarks.h

@@ -14,8 +14,9 @@
 #include <iterator>
 #include <utility>
 
-#include "Utilities.h"
 #include "benchmark/benchmark.h"
+#include "Utilities.h"
+#include "test_iterators.h"
 
 namespace ContainerBenchmarks {
 
@@ -50,6 +51,19 @@ void BM_Assignment(benchmark::State& st, Container) {
   }
 }
 
+template <std::size_t... sz, typename Container, typename GenInputs>
+void BM_AssignInputIterIter(benchmark::State& st, Container c, GenInputs gen) {
+  auto v = gen(1, sz...);
+  c.resize(st.range(0), v[0]);
+  auto in = gen(st.range(1), sz...);
+  benchmark::DoNotOptimize(&in);
+  benchmark::DoNotOptimize(&c);
+  for (auto _ : st) {
+    c.assign(cpp17_input_iterator(in.begin()), cpp17_input_iterator(in.end()));
+    benchmark::ClobberMemory();
+  }
+}
+
 template <class Container>
 void BM_ConstructSizeValue(benchmark::State& st, Container, typename Container::value_type const& val) {
   const auto size = st.range(0);

libcxx/test/benchmarks/GenerateInput.h

@@ -108,16 +108,29 @@ std::vector<IntT> getRandomIntegerInputs(std::size_t N) {
   return inputs;
 }
 
+inline std::vector<std::string> getRandomStringInputsWithLength(std::size_t N, std::size_t len) { // N-by-len
+  std::vector<std::string> inputs;
+  inputs.reserve(N);
+  for (std::size_t i = 0; i < N; ++i)
+    inputs.push_back(getRandomString(len));
+  return inputs;
+}
+
 inline std::vector<std::string> getDuplicateStringInputs(std::size_t N) {
   std::vector<std::string> inputs(N, getRandomString(1024));
   return inputs;
 }
 
 inline std::vector<std::string> getRandomStringInputs(std::size_t N) {
-  std::vector<std::string> inputs;
+  return getRandomStringInputsWithLength(N, 1024);
+}
+
+template <class IntT>
+std::vector<std::vector<IntT>> getRandomIntegerInputsWithLength(std::size_t N, std::size_t len) { // N-by-len
+  std::vector<std::vector<IntT>> inputs;
   inputs.reserve(N);
   for (std::size_t i = 0; i < N; ++i)
-    inputs.push_back(getRandomString(1024));
+    inputs.push_back(getRandomIntegerInputs<IntT>(len));
   return inputs;
 }
 

libcxx/test/benchmarks/vector_operations.bench.cpp

@@ -18,7 +18,6 @@
 #include <vector>
 
 #include "benchmark/benchmark.h"
-
 #include "ContainerBenchmarks.h"
 #include "GenerateInput.h"
 
@@ -79,4 +78,17 @@ BENCHMARK(bm_grow<std::string>);
 BENCHMARK(bm_grow<std::unique_ptr<int>>);
 BENCHMARK(bm_grow<std::deque<int>>);
 
+BENCHMARK_CAPTURE(BM_AssignInputIterIter, vector_int, std::vector<int>{}, getRandomIntegerInputs<int>)
+    ->Args({TestNumInputs, TestNumInputs});
+
+BENCHMARK_CAPTURE(
+    BM_AssignInputIterIter<32>, vector_string, std::vector<std::string>{}, getRandomStringInputsWithLength)
+    ->Args({TestNumInputs, TestNumInputs});
+
+BENCHMARK_CAPTURE(BM_AssignInputIterIter<100>,
+                  vector_vector_int,
+                  std::vector<std::vector<int>>{},
+                  getRandomIntegerInputsWithLength<int>)
+    ->Args({TestNumInputs, TestNumInputs});
+
 BENCHMARK_MAIN();