[libcxx-commits] [libcxx] [libc++] Optimize ranges::{for_each, for_each_n} for segmented iterators (PR #132896)

[Louis Dionne via libcxx-commits]

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@@ -51,6 +57,47 @@ int main(int argc, char** argv) {
     bm.operator()<std::list<int>>("rng::for_each(list<int>)", std::ranges::for_each);
   }
 
+  // {std,ranges}::for_each for join_view
+  {
+    auto bm = []<class Container>(std::string name, auto for_each) {
+      using C1       = typename Container::value_type;
+      using ElemType = typename C1::value_type;
+
+      benchmark::RegisterBenchmark(
+          name,
+          [for_each](auto& st) {
+            std::size_t const size     = st.range(0);
+            std::size_t const seg_size = 256;
+            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), ElemType(1));
+            }
+
+            auto view  = c | std::views::join;
+            auto first = view.begin();
+            auto last  = view.end();
+
+            for ([[maybe_unused]] auto _ : st) {
+              benchmark::DoNotOptimize(c);
+              auto result = for_each(first, last, [](ElemType& x) { x = std::clamp<ElemType>(x, 10, 100); });
+              benchmark::DoNotOptimize(result);
+            }
+          })
+          ->Arg(8)
+          ->Arg(32)
+          ->Arg(50) // non power-of-two
+          ->Arg(1024)
+          ->Arg(4096)
+          ->Arg(8192)
+          ->Arg(1 << 14)
+          ->Arg(1 << 16)
+          ->Arg(1 << 18);
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ldionne wrote:

Same here for the benchmark sizes.

https://github.com/llvm/llvm-project/pull/132896