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

include/boost/capy/ex/frame_alloc_mixin.hpp

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frame_alloc_mixin.hpp
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boost::capy::frame_alloc_mixin::operator new(unsigned long) :84 3153x 100.0% 100.0% boost::capy::frame_alloc_mixin::operator delete(void*, unsigned long) :115 3153x 100.0% 100.0%
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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 @code
45 struct my_internal_coroutine
46 {
47 struct promise_type : frame_alloc_mixin
48 {
49 my_internal_coroutine get_return_object();
50 std::suspend_always initial_suspend() noexcept;
51 std::suspend_always final_suspend() noexcept;
52 void return_void();
53 void unhandled_exception() noexcept;
54 };
55 };
56 @endcode
57
58 @par Thread Safety
59 The allocation fast path uses thread-local storage and requires
60 no synchronization. The global pool fallback is mutex-protected.
61
62 @see io_awaitable_promise_base, frame_allocator, recycling_memory_resource
63 */
64 struct frame_alloc_mixin
65 {
66 /** Allocate a coroutine frame.
67
68 Uses the thread-local frame allocator set by run_async.
69 Falls back to default memory resource if not set.
70 Stores the allocator pointer at the end of each frame for
71 correct deallocation even when TLS changes. Uses memcpy
72 to avoid alignment requirements on the trailing pointer.
73 Bypasses virtual dispatch for the recycling allocator.
74
75 @param size The size, in bytes, of the coroutine frame.
76
77 @return A pointer to storage for the frame.
78
79 @par Exception Safety
80 Propagates any exception thrown by the underlying memory
81 resource's `allocate`, for example `std::bad_alloc` from
82 `::operator new`.
83 */
84 3153x static void* operator new(std::size_t size)
85 {
86 3153x static auto* const rmr = get_recycling_memory_resource();
87
88 3153x auto* mr = get_current_frame_allocator();
89 3153x if(!mr)
90 1116x mr = std::pmr::get_default_resource();
91
92 3153x auto total = size + sizeof(std::pmr::memory_resource*);
93 void* raw;
94 3153x if(mr == rmr)
95 raw = static_cast<recycling_memory_resource*>(mr)
96 1162x ->allocate_fast(total, alignof(std::max_align_t));
97 else
98 1991x raw = mr->allocate(total, alignof(std::max_align_t));
99 3153x std::memcpy(static_cast<char*>(raw) + size, &mr, sizeof(mr));
100 3153x return raw;
101 }
102
103 /** Deallocate a coroutine frame.
104
105 Reads the allocator pointer stored at the end of the frame
106 to ensure correct deallocation regardless of current TLS.
107 Bypasses virtual dispatch for the recycling allocator.
108
109 @param ptr The frame storage returned by `operator new`.
110
111 @param size The size, in bytes, that was passed to `operator new`.
112 The allocator pointer is read from `ptr + size`, which is where
113 `operator new` wrote it, so this value must match.
114 */
115 3153x static void operator delete(void* ptr, std::size_t size) noexcept
116 {
117 3153x static auto* const rmr = get_recycling_memory_resource();
118
119 std::pmr::memory_resource* mr;
120 3153x std::memcpy(&mr, static_cast<char*>(ptr) + size, sizeof(mr));
121 3153x auto total = size + sizeof(std::pmr::memory_resource*);
122 3153x if(mr == rmr)
123 static_cast<recycling_memory_resource*>(mr)
124 1162x ->deallocate_fast(ptr, total, alignof(std::max_align_t));
125 else
126 1991x mr->deallocate(ptr, total, alignof(std::max_align_t));
127 3153x }
128 };
129
130 } // namespace capy
131 } // namespace boost
132
133 #endif
134