Index/include / boost / capy / ex / frame_alloc_mixin.hpp

include/boost/capy/ex/frame_alloc_mixin.hpp

100.0% Lines (19/0/19) 100.0% List of functions (2/0/2)
[ ] prev/next file
frame_alloc_mixin.hpp
f(x) Functions (2)
Function Calls Lines Blocks
boost::capy::frame_alloc_mixin::operator new(unsigned long) :73 3162x 100.0% 100.0% boost::capy::frame_alloc_mixin::operator delete(void*, unsigned long) :104 3162x 100.0% 100.0%
Line TLA Hits Source Code
1 //
2 // Copyright (c) 2026 Michael Vandeberg
3 //
4 // Distributed under the Boost Software License, Version 1.0. (See accompanying
5 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6 //
7 // Official repository: https://github.com/cppalliance/capy
8 //
9
10 #ifndef BOOST_CAPY_EX_FRAME_ALLOC_MIXIN_HPP
11 #define BOOST_CAPY_EX_FRAME_ALLOC_MIXIN_HPP
12
13 #include <boost/capy/detail/config.hpp>
14 #include <boost/capy/ex/frame_allocator.hpp>
15 #include <boost/capy/ex/recycling_memory_resource.hpp>
16
17 #include <cstddef>
18 #include <cstring>
19 #include <memory_resource>
20
21 namespace boost {
22 namespace capy {
23
24 /** Mixin that adds frame-allocator-aware allocation to a promise type.
25
26 Inherit from this class in any coroutine promise type to opt into
27 TLS-based frame allocation with the recycling memory resource
28 fast path. The mixin provides `operator new` and `operator delete`
29 that:
30
31 1. Read the thread-local frame allocator set by `run_async` or `run`.
32 2. Bypass virtual dispatch when the allocator is the default
33 recycling memory resource.
34 3. Store the allocator pointer at the end of each frame for
35 correct deallocation even when TLS changes between allocation
36 and deallocation.
37
38 This is the same allocation strategy used by @ref
39 io_awaitable_promise_base. Use this mixin directly when your
40 promise type does not need the full environment and continuation
41 support that `io_awaitable_promise_base` provides.
42
43 @par Example
44 @par !example example
45
46
47 @par Thread Safety
48 The allocation fast path uses thread-local storage and requires
49 no synchronization. The global pool fallback is mutex-protected.
50
51 @see io_awaitable_promise_base, frame_allocator, recycling_memory_resource
52 */
53 struct frame_alloc_mixin
54 {
55 /** Allocate a coroutine frame.
56
57 Uses the thread-local frame allocator set by run_async.
58 Falls back to default memory resource if not set.
59 Stores the allocator pointer at the end of each frame for
60 correct deallocation even when TLS changes. Uses memcpy
61 to avoid alignment requirements on the trailing pointer.
62 Bypasses virtual dispatch for the recycling allocator.
63
64 @param size The size, in bytes, of the coroutine frame.
65
66 @return A pointer to storage for the frame.
67
68 @par Exception Safety
69 Propagates any exception thrown by the underlying memory
70 resource's `allocate`, for example `std::bad_alloc` from
71 `::operator new`.
72 */
73 3162x static void* operator new(std::size_t size)
74 {
75 3162x static auto* const rmr = get_recycling_memory_resource();
76
77 3162x auto* mr = get_current_frame_allocator();
78 3162x if(!mr)
79 1116x mr = std::pmr::get_default_resource();
80
81 3162x auto total = size + sizeof(std::pmr::memory_resource*);
82 void* raw;
83 3162x if(mr == rmr)
84 raw = static_cast<recycling_memory_resource*>(mr)
85 1162x ->allocate_fast(total, alignof(std::max_align_t));
86 else
87 2000x raw = mr->allocate(total, alignof(std::max_align_t));
88 3162x std::memcpy(static_cast<char*>(raw) + size, &mr, sizeof(mr));
89 3162x return raw;
90 }
91
92 /** Deallocate a coroutine frame.
93
94 Reads the allocator pointer stored at the end of the frame
95 to ensure correct deallocation regardless of current TLS.
96 Bypasses virtual dispatch for the recycling allocator.
97
98 @param ptr The frame storage returned by `operator new`.
99
100 @param size The size, in bytes, that was passed to `operator new`.
101 The allocator pointer is read from `ptr + size`, which is where
102 `operator new` wrote it, so this value must match.
103 */
104 3162x static void operator delete(void* ptr, std::size_t size) noexcept
105 {
106 3162x static auto* const rmr = get_recycling_memory_resource();
107
108 std::pmr::memory_resource* mr;
109 3162x std::memcpy(&mr, static_cast<char*>(ptr) + size, sizeof(mr));
110 3162x auto total = size + sizeof(std::pmr::memory_resource*);
111 3162x if(mr == rmr)
112 static_cast<recycling_memory_resource*>(mr)
113 1162x ->deallocate_fast(ptr, total, alignof(std::max_align_t));
114 else
115 2000x mr->deallocate(ptr, total, alignof(std::max_align_t));
116 3162x }
117 };
118
119 } // namespace capy
120 } // namespace boost
121
122 #endif
123