[libc++][ranges] Avoid using distance in ranges::contains_subrange

[frederick-vs-ja]

Either std::distance or ranges::distance are inefficient for non-sized ranges. Also, calculation the range in int type is seriously problematic.

This patch avoids using distance and calculation of the length of non-sized ranges.

Fixes #86833.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: A. Jiang (frederick-vs-ja)

Changes

Either std::distance or ranges::distance are inefficient for non-sized ranges. Also, calculation the range in int type is seriously problematic.

This patch avoids using distance and calculation of the length of non-sized ranges.

Fixes #86833.

---

Full diff: https://github.com/llvm/llvm-project/pull/87155.diff

2 Files Affected:

• (modified) libcxx/include/__algorithm/ranges_contains_subrange.h (+4-6)
• (modified) libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains_subrange.pass.cpp (+4)

diff --git a/libcxx/include/__algorithm/ranges_contains_subrange.h b/libcxx/include/__algorithm/ranges_contains_subrange.h
index 4cd03cbb537060..e4b1c78a4f282f 100644
--- a/libcxx/include/__algorithm/ranges_contains_subrange.h
+++ b/libcxx/include/__algorithm/ranges_contains_subrange.h
@@ -15,7 +15,6 @@
 #include <__functional/ranges_operations.h>
 #include <__functional/reference_wrapper.h>
 #include <__iterator/concepts.h>
-#include <__iterator/distance.h>
 #include <__iterator/indirectly_comparable.h>
 #include <__iterator/projected.h>
 #include <__ranges/access.h>
@@ -70,14 +69,13 @@ struct __fn {
     requires indirectly_comparable<iterator_t<_Range1>, iterator_t<_Range2>, _Pred, _Proj1, _Proj2>
   _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr bool static
   operator()(_Range1&& __range1, _Range2&& __range2, _Pred __pred = {}, _Proj1 __proj1 = {}, _Proj2 __proj2 = {}) {
-    auto __n2 = 0;
     if constexpr (sized_range<_Range2>) {
-      __n2 = ranges::size(__range2);
+      if (ranges::size(__range2) == 0)
+        return true;
     } else {
-      __n2 = std::distance(cbegin(__range2), cend(__range2));
+      if (ranges::begin(__range2) == ranges::end(__range2))
+        return true;
     }
-    if (__n2 == 0)
-      return true;
 
     auto __ret = ranges::search(__range1, __range2, __pred, std::ref(__proj1), std::ref(__proj2));
     return __ret.empty() == false;
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains_subrange.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains_subrange.pass.cpp
index d48ee9e4e7e02e..761691c2afdcb9 100644
--- a/libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains_subrange.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/alg.contains/ranges.contains_subrange.pass.cpp
@@ -309,6 +309,10 @@ constexpr bool test() {
     });
   });
 
+  assert(std::ranges::contains_subrange(
+      std::views::iota(0, 5), std::views::iota(0, 5) | std::views::filter([](int) { return true; })));
+  assert(!std::ranges::contains_subrange(std::views::iota(0ULL, 42ULL), std::views::iota(0ULL, 1ULL << 32)));
+
   return true;
 }
 

[var-const]

Thank you for the fix!