. /include/boost/corosio /detail /ready_queue.hpp
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1   // 1   //
2   // Copyright (c) 2026 Steve Gerbino 2   // Copyright (c) 2026 Steve Gerbino
3   // 3   //
4   // Distributed under the Boost Software License, Version 1.0. (See accompanying 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) 5   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6   // 6   //
7   // Official repository: https://github.com/cppalliance/corosio 7   // Official repository: https://github.com/cppalliance/corosio
8   // 8   //
9   9  
10   #ifndef BOOST_COROSIO_DETAIL_READY_QUEUE_HPP 10   #ifndef BOOST_COROSIO_DETAIL_READY_QUEUE_HPP
11   #define BOOST_COROSIO_DETAIL_READY_QUEUE_HPP 11   #define BOOST_COROSIO_DETAIL_READY_QUEUE_HPP
12   12  
13   #include <boost/corosio/detail/scheduler_op.hpp> 13   #include <boost/corosio/detail/scheduler_op.hpp>
14   #include <boost/capy/continuation.hpp> 14   #include <boost/capy/continuation.hpp>
15   15  
16   #include <bit> 16   #include <bit>
17   #include <cstdint> 17   #include <cstdint>
18   18  
19   namespace boost::corosio::detail { 19   namespace boost::corosio::detail {
20   20  
21   // A queue entry is a tagged pointer: low bit selects the node kind, the rest 21   // A queue entry is a tagged pointer: low bit selects the node kind, the rest
22   // is the address. We steal a LOW bit (guaranteed zero by alignment), never a 22   // is the address. We steal a LOW bit (guaranteed zero by alignment), never a
23   // high bit (which would depend on a fragile platform canonical-address 23   // high bit (which would depend on a fragile platform canonical-address
24   // assumption). Both node types are >= 8-aligned, so the low 3 bits are free. 24   // assumption). Both node types are >= 8-aligned, so the low 3 bits are free.
25   static_assert(alignof(scheduler_op) >= 2); 25   static_assert(alignof(scheduler_op) >= 2);
26   static_assert(alignof(capy::continuation) >= 2); 26   static_assert(alignof(capy::continuation) >= 2);
27   static_assert(sizeof(void*) == sizeof(std::uintptr_t)); 27   static_assert(sizeof(void*) == sizeof(std::uintptr_t));
28   static_assert(sizeof(capy::continuation::reserved) >= sizeof(void*)); 28   static_assert(sizeof(capy::continuation::reserved) >= sizeof(void*));
29   29  
30   inline constexpr std::uintptr_t ready_cont_bit = 1; 30   inline constexpr std::uintptr_t ready_cont_bit = 1;
31   31  
32   /// Return true if a queue entry refers to a continuation (vs a scheduler_op). 32   /// Return true if a queue entry refers to a continuation (vs a scheduler_op).
33   inline bool 33   inline bool
HITCBC 34   3312224 ready_is_continuation(std::uintptr_t e) noexcept 34   3193452 ready_is_continuation(std::uintptr_t e) noexcept
35   { 35   {
HITCBC 36   3312224 return (e & ready_cont_bit) != 0; 36   3193452 return (e & ready_cont_bit) != 0;
37   } 37   }
38   38  
39   /// Recover the scheduler_op from an op-tagged entry. 39   /// Recover the scheduler_op from an op-tagged entry.
40   inline scheduler_op* 40   inline scheduler_op*
HITCBC 41   3082089 ready_as_op(std::uintptr_t e) noexcept 41   2966008 ready_as_op(std::uintptr_t e) noexcept
42   { 42   {
HITCBC 43   3082089 return std::bit_cast<scheduler_op*>(e & ~ready_cont_bit); 43   2966008 return std::bit_cast<scheduler_op*>(e & ~ready_cont_bit);
44   } 44   }
45   45  
46   /// Recover the continuation from a continuation-tagged entry. 46   /// Recover the continuation from a continuation-tagged entry.
47   inline capy::continuation* 47   inline capy::continuation*
HITCBC 48   95495 ready_as_cont(std::uintptr_t e) noexcept 48   94826 ready_as_cont(std::uintptr_t e) noexcept
49   { 49   {
HITCBC 50   95495 return std::bit_cast<capy::continuation*>(e & ~ready_cont_bit); 50   94826 return std::bit_cast<capy::continuation*>(e & ~ready_cont_bit);
51   } 51   }
52   52  
53   /** A unified intrusive FIFO of scheduler_ops and continuations. 53   /** A unified intrusive FIFO of scheduler_ops and continuations.
54   54  
55   Carries both completion handlers (`scheduler_op`, dispatched via 55   Carries both completion handlers (`scheduler_op`, dispatched via
56   `(*op)()`) and posted coroutine resumptions (`capy::continuation`, 56   `(*op)()`) and posted coroutine resumptions (`capy::continuation`,
57   dispatched via `h.resume()`) in one ordered queue, with no per-entry 57   dispatched via `h.resume()`) in one ordered queue, with no per-entry
58   allocation. The next-link lives in the node: `scheduler_op::q_next_` 58   allocation. The next-link lives in the node: `scheduler_op::q_next_`
59   for ops, `capy::continuation::reserved` for continuations. 59   for ops, `capy::continuation::reserved` for continuations.
60   60  
61   @par Thread Safety 61   @par Thread Safety
62   Not thread-safe; external synchronization required (the schedulers 62   Not thread-safe; external synchronization required (the schedulers
63   hold their dispatch mutex while touching it). 63   hold their dispatch mutex while touching it).
64   */ 64   */
65   class ready_queue 65   class ready_queue
66   { 66   {
67 - std::uintptr_t head_ = 0; // tagged first entry, 0 when empty 67 + std::uintptr_t head_ = 0; // tagged first entry, 0 when empty
68 - std::uintptr_t tail_ = 0; // tagged last entry, 0 when empty 68 + std::uintptr_t tail_ = 0; // tagged last entry, 0 when empty
69   69  
70   // Read a node's next-link by value. A continuation's link lives in its 70   // Read a node's next-link by value. A continuation's link lives in its
71   // void* `reserved` slot; bit_cast keeps us from forming a uintptr_t 71   // void* `reserved` slot; bit_cast keeps us from forming a uintptr_t
72   // lvalue over that void* object (which would violate strict aliasing). 72   // lvalue over that void* object (which would violate strict aliasing).
73   // 73   //
74   // GCC 12/13 false-positive: when inlining proves an entry refers to a 74   // GCC 12/13 false-positive: when inlining proves an entry refers to a
75   // continuation, -Warray-bounds still diagnoses the untaken scheduler_op 75   // continuation, -Warray-bounds still diagnoses the untaken scheduler_op
76   // branch against the smaller object. Fixed in GCC 14. 76   // branch against the smaller object. Fixed in GCC 14.
77   BOOST_COROSIO_GCC_WARNING_PUSH 77   BOOST_COROSIO_GCC_WARNING_PUSH
78   BOOST_COROSIO_GCC_WARNING_DISABLE("-Warray-bounds") 78   BOOST_COROSIO_GCC_WARNING_DISABLE("-Warray-bounds")
ECB 79 - 791036 static std::uintptr_t next_of(std::uintptr_t e) noexcept 79 + static std::uintptr_t
HITGNC   80 + 761703 next_of(std::uintptr_t e) noexcept
80   { 81   {
HITCBC 81   791036 if (ready_is_continuation(e)) 82   761703 if (ready_is_continuation(e))
HITCBC 82   23883 return std::bit_cast<std::uintptr_t>(ready_as_cont(e)->reserved); 83   23712 return std::bit_cast<std::uintptr_t>(ready_as_cont(e)->reserved);
HITCBC 83   767153 return ready_as_op(e)->q_next_; 84   737991 return ready_as_op(e)->q_next_;
84   } 85   }
85   86  
ECB 86 - 1291835 static void set_next(std::uintptr_t e, std::uintptr_t nxt) noexcept 87 + static void
HITGNC   88 + 1247239 set_next(std::uintptr_t e, std::uintptr_t nxt) noexcept
87   { 89   {
HITCBC 88   1291835 if (ready_is_continuation(e)) 90   1247239 if (ready_is_continuation(e))
HITCBC 89   47729 ready_as_cont(e)->reserved = std::bit_cast<void*>(nxt); 91   47402 ready_as_cont(e)->reserved = std::bit_cast<void*>(nxt);
90   else 92   else
HITCBC 91   1244106 ready_as_op(e)->q_next_ = nxt; 93   1199837 ready_as_op(e)->q_next_ = nxt;
HITCBC 92   1291835 } 94   1247239 }
93   BOOST_COROSIO_GCC_WARNING_POP 95   BOOST_COROSIO_GCC_WARNING_POP
94   96  
ECB 95 - 791036 void push_entry(std::uintptr_t e) noexcept 97 + void
HITGNC   98 + 761703 push_entry(std::uintptr_t e) noexcept
96   { 99   {
HITCBC 97   791036 set_next(e, 0); 100   761703 set_next(e, 0);
HITCBC 98   791036 if (tail_) 101   761703 if (tail_)
HITCBC 99   363648 set_next(tail_, e); 102   350068 set_next(tail_, e);
100   else 103   else
HITCBC 101   427388 head_ = e; 104   411635 head_ = e;
HITCBC 102   791036 tail_ = e; 105   761703 tail_ = e;
HITCBC 103   791036 } 106   761703 }
104   107  
105   public: 108   public:
HITCBC 106   4098 ready_queue() = default; 109   4110 ready_queue() = default;
107   110  
108 - ready_queue(ready_queue&& o) noexcept : head_(o.head_), tail_(o.tail_) 111 + ready_queue(ready_queue&& o) noexcept
  112 + : head_(o.head_)
  113 + , tail_(o.tail_)
109   { 114   {
110   o.head_ = 0; 115   o.head_ = 0;
111   o.tail_ = 0; 116   o.tail_ = 0;
112   } 117   }
113   118  
114   ready_queue(ready_queue const&) = delete; 119   ready_queue(ready_queue const&) = delete;
115   ready_queue& operator=(ready_queue const&) = delete; 120   ready_queue& operator=(ready_queue const&) = delete;
116   ready_queue& operator=(ready_queue&&) = delete; 121   ready_queue& operator=(ready_queue&&) = delete;
117   122  
118   /// Return true if the queue holds no entries. 123   /// Return true if the queue holds no entries.
HITCBC 119 - 2012603 bool empty() const noexcept 124 + 1939443 bool empty() const noexcept { return head_ == 0; }
120 - {  
DCB 121 - 2012603 return head_ == 0;  
122 - }  
123   125  
124   /// Append a scheduler_op to the back of the queue. 126   /// Append a scheduler_op to the back of the queue.
HITCBC 125   767153 void push(scheduler_op* op) noexcept 127   737991 void push(scheduler_op* op) noexcept
126   { 128   {
HITCBC 127   767153 push_entry(std::bit_cast<std::uintptr_t>(op)); 129   737991 push_entry(std::bit_cast<std::uintptr_t>(op));
HITCBC 128   767153 } 130   737991 }
129   131  
130   /// Append a continuation to the back of the queue. 132   /// Append a continuation to the back of the queue.
HITCBC 131   23883 void push(capy::continuation& c) noexcept 133   23712 void push(capy::continuation& c) noexcept
132   { 134   {
HITCBC 133   23883 push_entry(std::bit_cast<std::uintptr_t>(&c) | ready_cont_bit); 135   23712 push_entry(std::bit_cast<std::uintptr_t>(&c) | ready_cont_bit);
HITCBC 134   23883 } 136   23712 }
135   137  
136   /// Move all entries of @p other to the back in O(1); @p other is emptied. 138   /// Move all entries of @p other to the back in O(1); @p other is emptied.
HITCBC 137   445347 void splice(ready_queue& other) noexcept 139   430361 void splice(ready_queue& other) noexcept
138   { 140   {
HITCBC 139   445347 if (other.empty()) 141   430361 if (other.empty())
HITCBC 140   33814 return; 142   34371 return;
HITCBC 141   411533 if (tail_) 143   395990 if (tail_)
HITCBC 142   137151 set_next(tail_, other.head_); 144   135468 set_next(tail_, other.head_);
143   else 145   else
HITCBC 144   274382 head_ = other.head_; 146   260522 head_ = other.head_;
HITCBC 145 - 411533 tail_ = other.tail_; 147 + 395990 tail_ = other.tail_;
HITCBC 146   411533 other.head_ = 0; 148   395990 other.head_ = 0;
HITCBC 147   411533 other.tail_ = 0; 149   395990 other.tail_ = 0;
148   } 150   }
149   151  
150   /// Remove and return the front entry as a tagged value, or 0 when empty. 152   /// Remove and return the front entry as a tagged value, or 0 when empty.
HITCBC 151   1096821 std::uintptr_t pop() noexcept 153   1054000 std::uintptr_t pop() noexcept
152   { 154   {
HITCBC 153   1096821 auto e = head_; 155   1054000 auto e = head_;
HITCBC 154   1096821 if (!e) 156   1054000 if (!e)
HITCBC 155   305785 return 0; 157   292297 return 0;
HITCBC 156   791036 head_ = next_of(e); 158   761703 head_ = next_of(e);
HITCBC 157   791036 if (!head_) 159   761703 if (!head_)
HITCBC 158   290237 tail_ = 0; 160   276167 tail_ = 0;
HITCBC 159   791036 return e; 161   761703 return e;
160   } 162   }
161   }; 163   };
162   164  
163   } // namespace boost::corosio::detail 165   } // namespace boost::corosio::detail
164   166  
165   #endif 167   #endif