[libcxx-commits] [libcxx] [libc++][test] Refactor increasing_allocator (PR #115671)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef TEST_SUPPORT_INCREASING_ALLOCATOR_H
+#define TEST_SUPPORT_INCREASING_ALLOCATOR_H
+
+#include <cstddef>
+#include <memory>
+
+#include "test_macros.h"
+
+// The increasing_allocator is a custom allocator that enforces an increasing minimum allocation size,
+// ensuring that it allocates an increasing amount of memory, possibly exceeding the requested amount.
+// This unique behavior is particularly useful for testing the shrink_to_fit functionality in std::vector,
+// vector<bool>, and std::basic_string, ensuring that shrink_to_fit does not increase the capacity of
+// the allocated memory.
+
+template <typename T>
+struct increasing_allocator {
+  using value_type         = T;
+  std::size_t min_elements = 1000;
+  increasing_allocator()   = default;
+
+  template <typename U>
+  TEST_CONSTEXPR_CXX20 increasing_allocator(const increasing_allocator<U>& other) TEST_NOEXCEPT
+      : min_elements(other.min_elements) {}
+
+#if TEST_STD_VER >= 23
+  TEST_CONSTEXPR_CXX23 std::allocation_result<T*> allocate_at_least(std::size_t n) {
+    if (n < min_elements)
+      n = min_elements;
+    min_elements += 1000;
+    return std::allocator<T>{}.allocate_at_least(n);
+  }
+#endif // TEST_STD_VER >= 23
+
+  TEST_CONSTEXPR_CXX20 T* allocate(std::size_t n) {
+#if TEST_STD_VER >= 23
+    return allocate_at_least(n).ptr;
+#else
----------------
ldionne wrote:

Why don't we unconditionally call `std::allocator<T>().allocate(n)`?

Also, we still run into the issue that we had before, which is that we're calling `std::allocator<T>().allocate(n)` but calling `std::allocator<T>().deallocate(ptr, m)` with a smaller value than we allocated. That violates [allocator.members](http://eel.is/c++draft/allocator.members#10.1).

We can either do what @frederick-vs-ja suggested [here](https://github.com/llvm/llvm-project/pull/115671/files#r1843551032) (which TBH I don't fully understand why that makes the allocator ever-increasing), or we could try something like this: https://godbolt.org/z/P8PK6hvEj

```c++
template <typename T>
struct increasing_allocator {
  using value_type         = T;
  std::size_t min_elements = 1000;
  increasing_allocator()   = default;

private:
  tiny_map<void*, std::size_t> actual_sizes_;

public:
  template <typename U>
  constexpr increasing_allocator(const increasing_allocator<U>& other) noexcept
      : min_elements(other.min_elements), actual_sizes_(other.actual_sizes_) {}

  constexpr std::allocation_result<T*> allocate_at_least(std::size_t n) {
    if (n < min_elements)
      n = min_elements;
    min_elements += 1000;

    std::allocation_result<T*> result = std::allocator<T>().allocate_at_least(n);
    actual_sizes_.add(result.ptr, n);
    return result;
  }

  constexpr T* allocate(std::size_t n) {
    if (n < min_elements)
      n = min_elements;
    min_elements += 1000;

    T* result = std::allocator<T>().allocate(n);
    actual_sizes_.add(result, n);
    return result;
  }

  constexpr void deallocate(T* p, std::size_t n) noexcept {
    auto actual_size = actual_sizes_.find(p);
    std::allocator<T>().deallocate(p, actual_size->second);
    actual_sizes_.remove(p);
  }

  friend constexpr bool operator==(increasing_allocator const& a, increasing_allocator const& b) noexcept {
    return a.actual_sizes_ == b.actual_sizes_;
  }
};
```

This is a lot more complicated but I think it works. If you agree with that direction, we could land this PR as-is and then tackle this additional change as a follow-up, since it can be done independently from this refactoring.

https://github.com/llvm/llvm-project/pull/115671