[libcxx-commits] [libcxx] [libc++] Optimize {std, ranges}::distance for segmented iterators (PR #133612)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,73 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+#include <cstddef>
+#include <deque>
+#include <iterator>
+#include <ranges>
+#include <vector>
+
+#include <benchmark/benchmark.h>
+
+int main(int argc, char** argv) {
+  auto std_distance = [](auto first, auto last) { return std::distance(first, last); };
+
+  // {std,ranges}::distance
+  {
+    auto bm = []<class Container>(std::string name, auto distance, std::size_t seg_size) {
+      benchmark::RegisterBenchmark(
+          name,
+          [distance, seg_size](auto& st) {
+            std::size_t const size     = st.range(0);
+            std::size_t const segments = (size + seg_size - 1) / seg_size;
+            Container c(segments);
+            for (std::size_t i = 0, n = size; i < segments; ++i, n -= seg_size) {
+              c[i].resize(std::min(seg_size, n));
+            }
+
+            auto view  = c | std::views::join;
+            auto first = view.begin();
+            auto last  = view.end();
+
+            for ([[maybe_unused]] auto _ : st) {
+              benchmark::DoNotOptimize(c);
+              auto result = distance(first, last);
+              benchmark::DoNotOptimize(result);
+            }
+          })
+          ->Arg(50) // non power-of-two
+          ->Arg(1024)
+          ->Arg(4096)
+          ->Arg(8192);
+      // ->Arg(1 << 14)
+      // ->Arg(1 << 16)
+      // ->Arg(1 << 18)
+      // ->Arg(1 << 20);
----------------
ldionne wrote:

Did you mean to leave those commented? Let's either keep them uncommented or drop them.

https://github.com/llvm/llvm-project/pull/133612