[libcxx-commits] [libcxx] [libc++] Fix shrink_to_fit to swap buffer only when capacity is strictly smaller (PR #127321)

[Peng Liu via libcxx-commits]

================
@@ -49,4 +50,89 @@ TEST_CONSTEXPR_CXX20 bool operator==(increasing_allocator<T>, increasing_allocat
   return true;
 }
 
+template <std::size_t MinAllocSize, typename T>
+class min_size_allocator {
+public:
+  using value_type     = T;
+  min_size_allocator() = default;
+
+  template <typename U>
+  TEST_CONSTEXPR_CXX20 min_size_allocator(const min_size_allocator<MinAllocSize, U>&) TEST_NOEXCEPT {}
+
+#if TEST_STD_VER >= 23
+  TEST_CONSTEXPR_CXX23 std::allocation_result<T*> allocate_at_least(std::size_t n) {
+    if (n < MinAllocSize)
+      n = MinAllocSize;
+    return std::allocator<T>{}.allocate_at_least(n);
+  }
+#endif // TEST_STD_VER >= 23
+
+  TEST_NODISCARD TEST_CONSTEXPR_CXX20 T* allocate(std::size_t n) {
+    if (n < MinAllocSize)
+      n = MinAllocSize;
+    return std::allocator<T>().allocate(n);
+  }
+
+  TEST_CONSTEXPR_CXX20 void deallocate(T* p, std::size_t n) TEST_NOEXCEPT { std::allocator<T>().deallocate(p, n); }
+
+  template <typename U>
+  struct rebind {
+    using other = min_size_allocator<MinAllocSize, U>;
+  };
+};
+
+template <std::size_t MinAllocSize, typename T, typename U>
+TEST_CONSTEXPR_CXX20 bool
+operator==(const min_size_allocator<MinAllocSize, T>&, const min_size_allocator<MinAllocSize, U>&) {
+  return true;
+}
+
+template <std::size_t MinAllocSize, typename T, typename U>
+TEST_CONSTEXPR_CXX20 bool
+operator!=(const min_size_allocator<MinAllocSize, T>&, const min_size_allocator<MinAllocSize, U>&) {
+  return false;
+}
+
+template <typename T>
+class pow2_allocator {
+public:
+  using value_type = T;
+  pow2_allocator() = default;
+
+  template <typename U>
+  TEST_CONSTEXPR_CXX20 pow2_allocator(const pow2_allocator<U>&) TEST_NOEXCEPT {}
+
+#if TEST_STD_VER >= 23
+  TEST_CONSTEXPR_CXX23 std::allocation_result<T*> allocate_at_least(std::size_t n) {
+    return std::allocator<T>{}.allocate_at_least(next_power_of_two(n));
+  }
+#endif // TEST_STD_VER >= 23
+
+  TEST_NODISCARD TEST_CONSTEXPR_CXX20 T* allocate(std::size_t n) {
+    return std::allocator<T>().allocate(next_power_of_two(n));
+  }
+
+  TEST_CONSTEXPR_CXX20 void deallocate(T* p, std::size_t n) TEST_NOEXCEPT { std::allocator<T>().deallocate(p, n); }
+
+private:
+  TEST_CONSTEXPR_CXX20 std::size_t next_power_of_two(std::size_t n) const {
+    if ((n & (n - 1)) == 0)
+      return n;
+    for (std::size_t shift = 1; shift < std::numeric_limits<std::size_t>::digits; shift <<= 1) {
+      n |= n >> shift;
+    }
+    return n + 1;
+  }
+};
+
+template <typename T, typename U>
+TEST_CONSTEXPR_CXX20 bool operator==(const pow2_allocator<T>&, const pow2_allocator<U>&) {
+  return true;
+}
+
+template <typename T, typename U>
+TEST_CONSTEXPR_CXX20 bool operator!=(const pow2_allocator<T>&, const pow2_allocator<U>&) {
+  return false;
+}
----------------
winner245 wrote:

This allocator and the `min_size_allocator` are two instances of forcing equal allocation sizes so that we can trigger the QoI matter as described in the PR. I just wanted to make sure we have a slightly more intensive test coverage for `shrink_to_fit` given that its implementation can be very tricky. 

https://github.com/llvm/llvm-project/pull/127321