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

[Peng Liu via libcxx-commits]

================
@@ -13,69 +13,113 @@
 //    constexpr InputIterator      // constexpr after C++17
 //    for_each_n(InputIterator first, Size n, Function f);
 
-
 #include <algorithm>
 #include <cassert>
+#include <deque>
 #include <functional>
+#include <iterator>
+#include <ranges>
+#include <vector>
 
 #include "test_macros.h"
 #include "test_iterators.h"
 
-#if TEST_STD_VER > 17
-TEST_CONSTEXPR bool test_constexpr() {
-    int ia[] = {1, 3, 6, 7};
-    int expected[] = {3, 5, 8, 9};
-    const std::size_t N = 4;
+struct for_each_test {
+  TEST_CONSTEXPR for_each_test(int c) : count(c) {}
+  int count;
+  TEST_CONSTEXPR_CXX14 void operator()(int& i) {
+    ++i;
+    ++count;
+  }
+};
 
-    auto it = std::for_each_n(std::begin(ia), N, [](int &a) { a += 2; });
-    return it == (std::begin(ia) + N)
-        && std::equal(std::begin(ia), std::end(ia), std::begin(expected))
-        ;
-    }
-#endif
+struct deque_test {
+  std::deque<int>* d_;
+  int* i_;
+
+  deque_test(std::deque<int>& d, int& i) : d_(&d), i_(&i) {}
 
-struct for_each_test
-{
-    for_each_test(int c) : count(c) {}
-    int count;
-    void operator()(int& i) {++i; ++count;}
+  void operator()(int& v) {
+    assert(&(*d_)[*i_] == &v);
+    ++*i_;
+  }
 };
 
-int main(int, char**)
-{
+/*TEST_CONSTEXPR_CXX26*/
+void test_segmented_deque_iterator() { // TODO: Mark as TEST_CONSTEXPR_CXX26 once std::deque is constexpr
+  // check that segmented deque iterators work properly
+  int sizes[] = {0, 1, 2, 1023, 1024, 1025, 2047, 2048, 2049};
+  for (const int size : sizes) {
+    std::deque<int> d(size);
+    int index = 0;
+
+    std::for_each_n(d.begin(), d.size(), deque_test(d, index));
+  }
+}
+
+TEST_CONSTEXPR_CXX20 bool test() {
+  {
     typedef cpp17_input_iterator<int*> Iter;
-    int ia[] = {0, 1, 2, 3, 4, 5};
-    const unsigned s = sizeof(ia)/sizeof(ia[0]);
+    int ia[]         = {0, 1, 2, 3, 4, 5};
+    const unsigned s = sizeof(ia) / sizeof(ia[0]);
 
     {
-    auto f = for_each_test(0);
-    Iter it = std::for_each_n(Iter(ia), 0, std::ref(f));
-    assert(it == Iter(ia));
-    assert(f.count == 0);
+      auto f  = for_each_test(0);
----------------
winner245 wrote:

I will apply the change by using a lambda with reference capture of a count variable, so that when `for_each` completes, the captured `count` is updated. This allows us to inline the test and also removes `std::ref` as suggested in your next comment.

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