. /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   3426219 ready_is_continuation(std::uintptr_t e) noexcept 34   2753916 ready_is_continuation(std::uintptr_t e) noexcept
35   { 35   {
HITCBC 36   3426219 return (e & ready_cont_bit) != 0; 36   2753916 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   3226387 ready_as_op(std::uintptr_t e) noexcept 41   2586697 ready_as_op(std::uintptr_t e) noexcept
42   { 42   {
HITCBC 43   3226387 return std::bit_cast<scheduler_op*>(e & ~ready_cont_bit); 43   2586697 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   60741 ready_as_cont(std::uintptr_t e) noexcept 48   56856 ready_as_cont(std::uintptr_t e) noexcept
49   { 49   {
HITCBC 50   60741 return std::bit_cast<capy::continuation*>(e & ~ready_cont_bit); 50   56856 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")
79   static std::uintptr_t 79   static std::uintptr_t
HITCBC 80   816920 next_of(std::uintptr_t e) noexcept 80   658975 next_of(std::uintptr_t e) noexcept
81   { 81   {
HITCBC 82   816920 if (ready_is_continuation(e)) 82   658975 if (ready_is_continuation(e))
HITCBC 83   15187 return std::bit_cast<std::uintptr_t>(ready_as_cont(e)->reserved); 83   14216 return std::bit_cast<std::uintptr_t>(ready_as_cont(e)->reserved);
HITCBC 84   801733 return ready_as_op(e)->q_next_; 84   644759 return ready_as_op(e)->q_next_;
85   } 85   }
86   86  
87   static void 87   static void
HITCBC 88   1345497 set_next(std::uintptr_t e, std::uintptr_t nxt) noexcept 88   1076201 set_next(std::uintptr_t e, std::uintptr_t nxt) noexcept
89   { 89   {
HITCBC 90   1345497 if (ready_is_continuation(e)) 90   1076201 if (ready_is_continuation(e))
HITCBC 91   30367 ready_as_cont(e)->reserved = std::bit_cast<void*>(nxt); 91   28424 ready_as_cont(e)->reserved = std::bit_cast<void*>(nxt);
92   else 92   else
HITCBC 93   1315130 ready_as_op(e)->q_next_ = nxt; 93   1047777 ready_as_op(e)->q_next_ = nxt;
HITCBC 94   1345497 } 94   1076201 }
95   BOOST_COROSIO_GCC_WARNING_POP 95   BOOST_COROSIO_GCC_WARNING_POP
96   96  
97   void 97   void
HITCBC 98   816920 push_entry(std::uintptr_t e) noexcept 98   658975 push_entry(std::uintptr_t e) noexcept
99   { 99   {
HITCBC 100   816920 set_next(e, 0); 100   658975 set_next(e, 0);
HITCBC 101   816920 if (tail_) 101   658975 if (tail_)
HITCBC 102   363866 set_next(tail_, e); 102   294968 set_next(tail_, e);
103   else 103   else
HITCBC 104   453054 head_ = e; 104   364007 head_ = e;
HITCBC 105   816920 tail_ = e; 105   658975 tail_ = e;
HITCBC 106   816920 } 106   658975 }
107   107  
108   public: 108   public:
HITCBC 109   2149 ready_queue() = default; 109   2142 ready_queue() = default;
110   110  
111   ready_queue(ready_queue&& o) noexcept 111   ready_queue(ready_queue&& o) noexcept
112   : head_(o.head_) 112   : head_(o.head_)
113   , tail_(o.tail_) 113   , tail_(o.tail_)
114   { 114   {
115   o.head_ = 0; 115   o.head_ = 0;
116   o.tail_ = 0; 116   o.tail_ = 0;
117   } 117   }
118   118  
119   ready_queue(ready_queue const&) = delete; 119   ready_queue(ready_queue const&) = delete;
120   ready_queue& operator=(ready_queue const&) = delete; 120   ready_queue& operator=(ready_queue const&) = delete;
121   ready_queue& operator=(ready_queue&&) = delete; 121   ready_queue& operator=(ready_queue&&) = delete;
122   122  
123   /// Return true if the queue holds no entries. 123   /// Return true if the queue holds no entries.
HITCBC 124   2361002 bool empty() const noexcept { return head_ == 0; } 124   1906433 bool empty() const noexcept { return head_ == 0; }
125   125  
126   /// Append a scheduler_op to the back of the queue. 126   /// Append a scheduler_op to the back of the queue.
HITCBC 127   801733 void push(scheduler_op* op) noexcept 127   644759 void push(scheduler_op* op) noexcept
128   { 128   {
HITCBC 129   801733 push_entry(std::bit_cast<std::uintptr_t>(op)); 129   644759 push_entry(std::bit_cast<std::uintptr_t>(op));
HITCBC 130   801733 } 130   644759 }
131   131  
132   /// Append a continuation to the back of the queue. 132   /// Append a continuation to the back of the queue.
HITCBC 133   15187 void push(capy::continuation& c) noexcept 133   14216 void push(capy::continuation& c) noexcept
134   { 134   {
HITCBC 135   15187 push_entry(std::bit_cast<std::uintptr_t>(&c) | ready_cont_bit); 135   14216 push_entry(std::bit_cast<std::uintptr_t>(&c) | ready_cont_bit);
HITCBC 136   15187 } 136   14216 }
137   137  
138   /// 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 139   470071 void splice(ready_queue& other) noexcept 139   374272 void splice(ready_queue& other) noexcept
140   { 140   {
HITCBC 141   470071 if (other.empty()) 141   374272 if (other.empty())
HITCBC 142   36761 return; 142   28367 return;
HITCBC 143   433310 if (tail_) 143   345905 if (tail_)
HITCBC 144   164711 set_next(tail_, other.head_); 144   122258 set_next(tail_, other.head_);
145   else 145   else
HITCBC 146   268599 head_ = other.head_; 146   223647 head_ = other.head_;
HITCBC 147   433310 tail_ = other.tail_; 147   345905 tail_ = other.tail_;
HITCBC 148   433310 other.head_ = 0; 148   345905 other.head_ = 0;
HITCBC 149   433310 other.tail_ = 0; 149   345905 other.tail_ = 0;
150   } 150   }
151   151  
152   /// 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 153   1125930 std::uintptr_t pop() noexcept 153   909604 std::uintptr_t pop() noexcept
154   { 154   {
HITCBC 155   1125930 auto e = head_; 155   909604 auto e = head_;
HITCBC 156   1125930 if (!e) 156   909604 if (!e)
HITCBC 157   309010 return 0; 157   250629 return 0;
HITCBC 158   816920 head_ = next_of(e); 158   658975 head_ = next_of(e);
HITCBC 159   816920 if (!head_) 159   658975 if (!head_)
HITCBC 160   288343 tail_ = 0; 160   241749 tail_ = 0;
HITCBC 161   816920 return e; 161   658975 return e;
162   } 162   }
163   }; 163   };
164   164  
165   } // namespace boost::corosio::detail 165   } // namespace boost::corosio::detail
166   166  
167   #endif 167   #endif