. /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   3325995 ready_is_continuation(std::uintptr_t e) noexcept 34   3563893 ready_is_continuation(std::uintptr_t e) noexcept
35   { 35   {
HITCBC 36   3325995 return (e & ready_cont_bit) != 0; 36   3563893 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   1949626 ready_as_op(std::uintptr_t e) noexcept 41   2185404 ready_as_op(std::uintptr_t e) noexcept
42   { 42   {
HITCBC 43   1949626 return std::bit_cast<scheduler_op*>(e & ~ready_cont_bit); 43   2185404 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   1010785 ready_as_cont(std::uintptr_t e) noexcept 48   1009363 ready_as_cont(std::uintptr_t e) noexcept
49   { 49   {
HITCBC 50   1010785 return std::bit_cast<capy::continuation*>(e & ~ready_cont_bit); 50   1009363 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")
HITCBC 79   744956 static std::uintptr_t next_of(std::uintptr_t e) noexcept 79   819875 static std::uintptr_t next_of(std::uintptr_t e) noexcept
80   { 80   {
HITCBC 81   744956 if (ready_is_continuation(e)) 81   819875 if (ready_is_continuation(e))
HITCBC 82   253195 return std::bit_cast<std::uintptr_t>(ready_as_cont(e)->reserved); 82   252862 return std::bit_cast<std::uintptr_t>(ready_as_cont(e)->reserved);
HITCBC 83   491761 return ready_as_op(e)->q_next_; 83   567013 return ready_as_op(e)->q_next_;
84   } 84   }
85   85  
HITCBC 86   1432866 static void set_next(std::uintptr_t e, std::uintptr_t nxt) noexcept 86   1516017 static void set_next(std::uintptr_t e, std::uintptr_t nxt) noexcept
87   { 87   {
HITCBC 88   1432866 if (ready_is_continuation(e)) 88   1516017 if (ready_is_continuation(e))
HITCBC 89   504395 ready_as_cont(e)->reserved = std::bit_cast<void*>(nxt); 89   503639 ready_as_cont(e)->reserved = std::bit_cast<void*>(nxt);
90   else 90   else
HITCBC 91   928471 ready_as_op(e)->q_next_ = nxt; 91   1012378 ready_as_op(e)->q_next_ = nxt;
HITCBC 92   1432866 } 92   1516017 }
93   BOOST_COROSIO_GCC_WARNING_POP 93   BOOST_COROSIO_GCC_WARNING_POP
94   94  
HITCBC 95   744956 void push_entry(std::uintptr_t e) noexcept 95   819875 void push_entry(std::uintptr_t e) noexcept
96   { 96   {
HITCBC 97   744956 set_next(e, 0); 97   819875 set_next(e, 0);
HITCBC 98   744956 if (tail_) 98   819875 if (tail_)
HITCBC 99   347557 set_next(tail_, e); 99   350204 set_next(tail_, e);
100   else 100   else
HITCBC 101   397399 head_ = e; 101   469671 head_ = e;
HITCBC 102   744956 tail_ = e; 102   819875 tail_ = e;
HITCBC 103   744956 } 103   819875 }
104   104  
105   public: 105   public:
HITCBC 106   17944 ready_queue() = default; 106   17580 ready_queue() = default;
107   107  
108   ready_queue(ready_queue&& o) noexcept : head_(o.head_), tail_(o.tail_) 108   ready_queue(ready_queue&& o) noexcept : head_(o.head_), tail_(o.tail_)
109   { 109   {
110   o.head_ = 0; 110   o.head_ = 0;
111   o.tail_ = 0; 111   o.tail_ = 0;
112   } 112   }
113   113  
114   ready_queue(ready_queue const&) = delete; 114   ready_queue(ready_queue const&) = delete;
115   ready_queue& operator=(ready_queue const&) = delete; 115   ready_queue& operator=(ready_queue const&) = delete;
116   ready_queue& operator=(ready_queue&&) = delete; 116   ready_queue& operator=(ready_queue&&) = delete;
117   117  
118   /// Return true if the queue holds no entries. 118   /// Return true if the queue holds no entries.
HITCBC 119   2065440 bool empty() const noexcept 119   2290970 bool empty() const noexcept
120   { 120   {
HITCBC 121   2065440 return head_ == 0; 121   2290970 return head_ == 0;
122   } 122   }
123   123  
124   /// Append a scheduler_op to the back of the queue. 124   /// Append a scheduler_op to the back of the queue.
HITCBC 125   491761 void push(scheduler_op* op) noexcept 125   567013 void push(scheduler_op* op) noexcept
126   { 126   {
HITCBC 127   491761 push_entry(std::bit_cast<std::uintptr_t>(op)); 127   567013 push_entry(std::bit_cast<std::uintptr_t>(op));
HITCBC 128   491761 } 128   567013 }
129   129  
130   /// Append a continuation to the back of the queue. 130   /// Append a continuation to the back of the queue.
HITCBC 131   253195 void push(capy::continuation& c) noexcept 131   252862 void push(capy::continuation& c) noexcept
132   { 132   {
HITCBC 133   253195 push_entry(std::bit_cast<std::uintptr_t>(&c) | ready_cont_bit); 133   252862 push_entry(std::bit_cast<std::uintptr_t>(&c) | ready_cont_bit);
HITCBC 134   253195 } 134   252862 }
135   135  
136   /// Move all entries of @p other to the back in O(1); @p other is emptied. 136   /// Move all entries of @p other to the back in O(1); @p other is emptied.
HITCBC 137   620599 void splice(ready_queue& other) noexcept 137   695928 void splice(ready_queue& other) noexcept
138   { 138   {
HITCBC 139   620599 if (other.empty()) 139   695928 if (other.empty())
HITCBC 140   258604 return; 140   328155 return;
HITCBC 141   361995 if (tail_) 141   367773 if (tail_)
HITCBC 142   340353 set_next(tail_, other.head_); 142   345938 set_next(tail_, other.head_);
143   else 143   else
HITCBC 144   21642 head_ = other.head_; 144   21835 head_ = other.head_;
HITCBC 145   361995 tail_ = other.tail_; 145   367773 tail_ = other.tail_;
HITCBC 146   361995 other.head_ = 0; 146   367773 other.head_ = 0;
HITCBC 147   361995 other.tail_ = 0; 147   367773 other.tail_ = 0;
148   } 148   }
149   149  
150   /// Remove and return the front entry as a tagged value, or 0 when empty. 150   /// Remove and return the front entry as a tagged value, or 0 when empty.
HITCBC 151   791429 std::uintptr_t pop() noexcept 151   867715 std::uintptr_t pop() noexcept
152   { 152   {
HITCBC 153   791429 auto e = head_; 153   867715 auto e = head_;
HITCBC 154   791429 if (!e) 154   867715 if (!e)
HITCBC 155   46473 return 0; 155   47840 return 0;
HITCBC 156   744956 head_ = next_of(e); 156   819875 head_ = next_of(e);
HITCBC 157   744956 if (!head_) 157   819875 if (!head_)
HITCBC 158   57046 tail_ = 0; 158   123733 tail_ = 0;
HITCBC 159   744956 return e; 159   819875 return e;
160   } 160   }
161   }; 161   };
162   162  
163   } // namespace boost::corosio::detail 163   } // namespace boost::corosio::detail
164   164  
165   #endif 165   #endif