[libcxx-commits] [libcxx] [libc++] Optimize {std, ranges}::for_each for iterating over __trees (PR #164405)

[Louis Dionne via libcxx-commits]

================
@@ -52,6 +55,66 @@ int main(int argc, char** argv) {
     bm.operator()<std::list<int>>("rng::for_each(list<int>)", std::ranges::for_each);
   }
 
+  // std::{,range::}for_each for associative containers
+  {
+    auto iterator_bm = []<class Container, bool IsMapLike>(
+                           std::type_identity<Container>, std::bool_constant<IsMapLike>, std::string name) {
+      benchmark::RegisterBenchmark(
+          name,
+          [](auto& st) {
+            Container c;
+            for (std::int64_t i = 0; i != st.range(0); ++i) {
+              if constexpr (IsMapLike)
+                c.emplace(i, i);
+              else
+                c.emplace(i);
+            }
+
+            for (auto _ : st) {
+              benchmark::DoNotOptimize(c);
+              std::for_each(c.begin(), c.end(), [](auto v) { benchmark::DoNotOptimize(&v); });
+            }
+          })
+          ->Arg(8)
+          ->Arg(32)
+          ->Arg(50) // non power-of-two
+          ->Arg(8192);
+    };
+    iterator_bm(std::type_identity<std::set<int>>{}, std::false_type{}, "rng::for_each(set<int>::iterator)");
+    iterator_bm(std::type_identity<std::multiset<int>>{}, std::false_type{}, "rng::for_each(multiset<int>::iterator)");
+    iterator_bm(std::type_identity<std::map<int, int>>{}, std::true_type{}, "rng::for_each(map<int>::iterator)");
+    iterator_bm(
+        std::type_identity<std::multimap<int, int>>{}, std::true_type{}, "rng::for_each(multimap<int>::iterator)");
+
+    auto container_bm = []<class Container, bool IsMapLike>(
+                            std::type_identity<Container>, std::bool_constant<IsMapLike>, std::string name) {
+      benchmark::RegisterBenchmark(
+          name,
+          [](auto& st) {
+            Container c;
+            for (std::int64_t i = 0; i != st.range(0); ++i) {
----------------
ldionne wrote:

If we use `std::size_t const size = st.range(0);`, Clang won't complain anymore and we can be consistent again.

https://github.com/llvm/llvm-project/pull/164405