[intro.multithread] - C++17 → C++20

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@@ -1,6 +1,6 @@
-## Multi-threaded executions and data races <a id="intro.multithread">[[intro.multithread]]</a>
 A *thread of execution* (also known as a *thread*) is a single flow of
 control within a program, including the initial invocation of a specific
 top-level function, and recursively including every function invocation
 subsequently executed by the thread.
@@ -8,15 +8,15 @@ subsequently executed by the thread.
 [*Note 1*: When one thread creates another, the initial call to the
 top-level function of the new thread is executed by the new thread, not
 by the creating thread. — *end note*]
 Every thread in a program can potentially access every object and
-function in a program.[^11] Under a hosted implementation, a C++program
 can have more than one thread running concurrently. The execution of
-each thread proceeds as defined by the remainder of this International
-Standard. The execution of the entire program consists of an execution
-of all of its threads.
 [*Note 2*: Usually the execution can be viewed as an interleaving of
 all its threads. However, some kinds of atomic operations, for example,
 allow executions inconsistent with a simple interleaving, as described
 below. — *end note*]
@@ -26,71 +26,67 @@ whether a program can have more than one thread of execution.
 For a signal handler that is not executed as a result of a call to the
 `std::raise` function, it is unspecified which thread of execution
 contains the signal handler invocation.
-### Data races <a id="intro.races">[[intro.races]]</a>
-The value of an object visible to a thread *T* at a particular point is
-the initial value of the object, a value assigned to the object by *T*,
-or a value assigned to the object by another thread, according to the
-rules below.
 [*Note 1*: In some cases, there may instead be undefined behavior. Much
-of this section is motivated by the desire to support atomic operations
-with explicit and detailed visibility constraints. However, it also
-implicitly supports a simpler view for more restricted
 programs. — *end note*]
 Two expression evaluations *conflict* if one of them modifies a memory
-location ([[intro.memory]]) and the other one reads or modifies the
-same memory location.
-The library defines a number of atomic operations (Clause  [[atomics]])
-and operations on mutexes (Clause  [[thread]]) that are specially
-identified as synchronization operations. These operations play a
-special role in making assignments in one thread visible to another. A
-synchronization operation on one or more memory locations is either a
-consume operation, an acquire operation, a release operation, or both an
-acquire and release operation. A synchronization operation without an
-associated memory location is a fence and can be either an acquire
-fence, a release fence, or both an acquire and release fence. In
-addition, there are relaxed atomic operations, which are not
-synchronization operations, and atomic read-modify-write operations,
-which have special characteristics.
 [*Note 2*: For example, a call that acquires a mutex will perform an
 acquire operation on the locations comprising the mutex.
 Correspondingly, a call that releases the same mutex will perform a
 release operation on those same locations. Informally, performing a
-release operation on *A* forces prior side effects on other memory
 locations to become visible to other threads that later perform a
-consume or an acquire operation on *A*. “Relaxed” atomic operations are
 not synchronization operations even though, like synchronization
 operations, they cannot contribute to data races. — *end note*]
-All modifications to a particular atomic object *M* occur in some
-particular total order, called the *modification order* of *M*.
 [*Note 3*: There is a separate order for each atomic object. There is
 no requirement that these can be combined into a single total order for
 all objects. In general this will be impossible since different threads
 may observe modifications to different objects in inconsistent
 orders. — *end note*]
-A *release sequence* headed by a release operation *A* on an atomic
-object *M* is a maximal contiguous sub-sequence of side effects in the
-modification order of *M*, where the first operation is `A`, and every
-subsequent operation
-- is performed by the same thread that performed `A`, or
-- is an atomic read-modify-write operation.
 Certain library calls *synchronize with* other library calls performed
 by another thread. For example, an atomic store-release synchronizes
-with a load-acquire that takes its value from the store (
-[[atomics.order]]).
 [*Note 4*: Except in the specified cases, reading a later value does
 not necessarily ensure visibility as described below. Such a requirement
 would sometimes interfere with efficient implementation. — *end note*]
@@ -98,55 +94,52 @@ would sometimes interfere with efficient implementation. — *end note*]
 when one reads the value written by another. For atomic objects, the
 definition is clear. All operations on a given mutex occur in a single
 total order. Each mutex acquisition “reads the value written” by the
 last mutex release. — *end note*]
-An evaluation *A* *carries a dependency* to an evaluation *B* if
-- the value of *A* is used as an operand of *B*, unless:
-  - *B* is an invocation of any specialization of
- `std::kill_dependency` ([[atomics.order]]), or
-  - *A* is the left operand of a built-in logical AND (`&&`, see
-    [[expr.log.and]]) or logical OR (`||`, see  [[expr.log.or]])
- operator, or
-  - *A* is the left operand of a conditional (`?:`, see  [[expr.cond]])
     operator, or
-  - *A* is the left operand of the built-in comma (`,`) operator (
-    [[expr.comma]]);
   or
-- *A* writes a scalar object or bit-field *M*, *B* reads the value
- written by *A* from *M*, and *A* is sequenced before *B*, or
-- for some evaluation *X*, *A* carries a dependency to *X*, and *X*
- carries a dependency to *B*.
 [*Note 6*: “Carries a dependency to” is a subset of “is sequenced
 before”, and is similarly strictly intra-thread. — *end note*]
-An evaluation *A* is *dependency-ordered before* an evaluation *B* if
-- *A* performs a release operation on an atomic object *M*, and, in
- another thread, *B* performs a consume operation on *M* and reads a
- value written by any side effect in the release sequence headed by
-  *A*, or
-- for some evaluation *X*, *A* is dependency-ordered before *X* and *X*
-  carries a dependency to *B*.
 [*Note 7*: The relation “is dependency-ordered before” is analogous to
 “synchronizes with”, but uses release/consume in place of
 release/acquire. — *end note*]
-An evaluation *A* *inter-thread happens before* an evaluation *B* if
-- *A* synchronizes with *B*, or
-- *A* is dependency-ordered before *B*, or
-- for some evaluation *X*
-  - *A* synchronizes with *X* and *X* is sequenced before *B*, or
-  - *A* is sequenced before *X* and *X* inter-thread happens before *B*,
-    or
-  - *A* inter-thread happens before *X* and *X* inter-thread happens
-    before *B*.
 [*Note 8*: The “inter-thread happens before” relation describes
 arbitrary concatenations of “sequenced before”, “synchronizes with” and
 “dependency-ordered before” relationships, with two exceptions. The
 first exception is that a concatenation is not permitted to end with
@@ -161,101 +154,111 @@ exception is that a concatenation is not permitted to consist entirely
 of “sequenced before”. The reasons for this limitation are (1) to permit
 “inter-thread happens before” to be transitively closed and (2) the
 “happens before” relation, defined below, provides for relationships
 consisting entirely of “sequenced before”. — *end note*]
-An evaluation *A* *happens before* an evaluation *B* (or, equivalently,
-*B* *happens after* *A*) if:
-- *A* is sequenced before *B*, or
-- *A* inter-thread happens before *B*.
 The implementation shall ensure that no program execution demonstrates a
 cycle in the “happens before” relation.
 [*Note 9*: This cycle would otherwise be possible only through the use
 of consume operations. — *end note*]
-An evaluation *A* *strongly happens before* an evaluation *B* if either
-- *A* is sequenced before *B*, or
-- *A* synchronizes with *B*, or
-- *A* strongly happens before *X* and *X* strongly happens before *B*.
 [*Note 10*: In the absence of consume operations, the happens before
-and strongly happens before relations are identical. Strongly happens
-before essentially excludes consume operations. — *end note*]
-A *visible side effect* *A* on a scalar object or bit-field *M* with
-respect to a value computation *B* of *M* satisfies the conditions:
-- *A* happens before *B* and
-- there is no other side effect *X* to *M* such that *A* happens before
- *X* and *X* happens before *B*.
-The value of a non-atomic scalar object or bit-field *M*, as determined
-by evaluation *B*, shall be the value stored by the visible side effect
-*A*.
-[*Note 11*: If there is ambiguity about which side effect to a
 non-atomic object or bit-field is visible, then the behavior is either
 unspecified or undefined. — *end note*]
-[*Note 12*: This states that operations on ordinary objects are not
 visibly reordered. This is not actually detectable without data races,
 but it is necessary to ensure that data races, as defined below, and
 with suitable restrictions on the use of atomics, correspond to data
 races in a simple interleaved (sequentially consistent)
 execution. — *end note*]
-The value of an atomic object *M*, as determined by evaluation *B*,
-shall be the value stored by some side effect *A* that modifies *M*,
-where *B* does not happen before *A*.
-[*Note 13*: The set of such side effects is also restricted by the rest
 of the rules described here, and in particular, by the coherence
 requirements below. — *end note*]
-If an operation *A* that modifies an atomic object *M* happens before an
-operation *B* that modifies *M*, then *A* shall be earlier than *B* in
-the modification order of *M*.
-[*Note 14*: This requirement is known as write-write
 coherence. — *end note*]
-If a value computation *A* of an atomic object *M* happens before a
-value computation *B* of *M*, and *A* takes its value from a side effect
-*X* on *M*, then the value computed by *B* shall either be the value
-stored by *X* or the value stored by a side effect *Y* on *M*, where *Y*
-follows *X* in the modification order of *M*.
-[*Note 15*: This requirement is known as read-read
 coherence. — *end note*]
-If a value computation *A* of an atomic object *M* happens before an
-operation *B* that modifies *M*, then *A* shall take its value from a
-side effect *X* on *M*, where *X* precedes *B* in the modification order
-of *M*.
-[*Note 16*: This requirement is known as read-write
 coherence. — *end note*]
-If a side effect *X* on an atomic object *M* happens before a value
-computation *B* of *M*, then the evaluation *B* shall take its value
-from *X* or from a side effect *Y* that follows *X* in the modification
-order of *M*.
-[*Note 17*: This requirement is known as write-read
 coherence. — *end note*]
-[*Note 18*: The four preceding coherence requirements effectively
 disallow compiler reordering of atomic operations to a single object,
 even if both operations are relaxed loads. This effectively makes the
-cache coherence guarantee provided by most hardware available to
-C++atomic operations. — *end note*]
-[*Note 19*: The value observed by a load of an atomic depends on the
 “happens before” relation, which depends on the values observed by loads
 of atomics. The intended reading is that there must exist an association
 of atomic loads with modifications they observe that, together with
 suitably chosen modification orders and the “happens before” relation
 derived as described above, satisfy the resulting constraints as imposed
@@ -271,12 +274,12 @@ The execution of a program contains a *data race* if it contains two
 potentially concurrent conflicting actions, at least one of which is not
 atomic, and neither happens before the other, except for the special
 case for signal handlers described below. Any such data race results in
 undefined behavior.
-[*Note 20*: It can be shown that programs that correctly use mutexes
-and `memory_order_seq_cst` operations to prevent all data races and use
 no other synchronization operations behave as if the operations executed
 by their constituent threads were simply interleaved, with each value
 computation of an object being taken from the last side effect on that
 object in that interleaving. This is normally referred to as “sequential
 consistency”. However, this applies only to data-race-free programs, and
@@ -288,38 +291,38 @@ undefined operation. — *end note*]
 Two accesses to the same object of type `volatile std::sig_atomic_t` do
 not result in a data race if both occur in the same thread, even if one
 or more occurs in a signal handler. For each signal handler invocation,
 evaluations performed by the thread invoking a signal handler can be
-divided into two groups *A* and *B*, such that no evaluations in *B*
-happen before evaluations in *A*, and the evaluations of such
 `volatile std::sig_atomic_t` objects take values as though all
-evaluations in *A* happened before the execution of the signal handler
-and the execution of the signal handler happened before all evaluations
-in *B*.
-[*Note 21*: Compiler transformations that introduce assignments to a
 potentially shared memory location that would not be modified by the
-abstract machine are generally precluded by this International Standard,
-since such an assignment might overwrite another assignment by a
-different thread in cases in which an abstract machine execution would
-not have encountered a data race. This includes implementations of data
-member assignment that overwrite adjacent members in separate memory
-locations. Reordering of atomic loads in cases in which the atomics in
-question may alias is also generally precluded, since this may violate
-the coherence rules. — *end note*]
-[*Note 22*: Transformations that introduce a speculative read of a
 potentially shared memory location may not preserve the semantics of the
-C++program as defined in this International Standard, since they
-potentially introduce a data race. However, they are typically valid in
-the context of an optimizing compiler that targets a specific machine
-with well-defined semantics for data races. They would be invalid for a
 hypothetical machine that is not tolerant of races or provides hardware
 race detection. — *end note*]
-### Forward progress <a id="intro.progress">[[intro.progress]]</a>
 The implementation may assume that any thread will eventually do one of
 the following:
 - terminate,
@@ -329,17 +332,17 @@ the following:
 [*Note 1*: This is intended to allow compiler transformations such as
 removal of empty loops, even when termination cannot be
 proven. — *end note*]
-Executions of atomic functions that are either defined to be lock-free (
-[[atomics.flag]]) or indicated as lock-free ([[atomics.lockfree]]) are
 *lock-free executions*.
-- If there is only one thread that is not blocked ([[defns.block]]) in
- a standard library function, a lock-free execution in that thread
- shall complete. \[*Note 2*: Concurrently executing threads may prevent
   progress of a lock-free execution. For example, this situation can
   occur with load-locked store-conditional implementations. This
   property is sometimes termed obstruction-free. — *end note*]
 - When one or more lock-free executions run concurrently, at least one
   should complete. \[*Note 3*: It is difficult for some implementations
@@ -359,13 +362,13 @@ termed an *execution step*:
 - termination of the thread of execution,
 - performing an access through a volatile glvalue, or
 - completion of a call to a library I/O function, a synchronization
   operation, or an atomic operation.
-An invocation of a standard library function that blocks (
-[[defns.block]]) is considered to continuously execute execution steps
-while waiting for the condition that it blocks on to be satisfied.
 [*Example 1*: A library I/O function that blocks until the I/O
 operation is complete can be considered to continuously check whether
 the operation is complete. Each such check might consist of one or more
 execution steps, for example using observable behavior of the abstract
@@ -389,16 +392,17 @@ make progress for as long as it has not terminated.
 of executions (if any) have been or are making progress. To eventually
 fulfill this requirement means that this will happen in an unspecified
 but finite amount of time. — *end note*]
 It is *implementation-defined* whether the implementation-created thread
-of execution that executes `main` ([[basic.start.main]]) and the
-threads of execution created by `std::thread` ([[thread.thread.class]])
-provide concurrent forward progress guarantees.
-[*Note 6*: General-purpose implementations are encouraged to provide
-these guarantees. — *end note*]
 For a thread of execution providing *parallel forward progress
 guarantees*, the implementation is not required to ensure that the
 thread will eventually make progress if it has not yet executed any
 execution step; once this thread has executed a step, it provides
@@ -430,38 +434,38 @@ parallel forward progress guarantees.
 [*Note 9*: For example, some kinds of synchronization between threads
 of execution may only make progress if the respective threads of
 execution provide parallel forward progress guarantees, but will fail to
 make progress under weakly parallel guarantees. — *end note*]
-When a thread of execution *P* is specified to *block with forward
-progress guarantee delegation* on the completion of a set *S* of threads
-of execution, then throughout the whole time of *P* being blocked on
-*S*, the implementation shall ensure that the forward progress
-guarantees provided by at least one thread of execution in *S* is at
-least as strong as *P*’s forward progress guarantees.
-[*Note 10*: It is unspecified which thread or threads of execution in
-*S* are chosen and for which number of execution steps. The
-strengthening is not permanent and not necessarily in place for the rest
-of the lifetime of the affected thread of execution. As long as *P* is
-blocked, the implementation has to eventually select and potentially
-strengthen a thread of execution in *S*. — *end note*]
-Once a thread of execution in *S* terminates, it is removed from *S*.
-Once *S* is empty, *P* is unblocked.
-[*Note 11*: A thread of execution *B* thus can temporarily provide an
 effectively stronger forward progress guarantee for a certain amount of
-time, due to a second thread of execution *A* being blocked on it with
-forward progress guarantee delegation. In turn, if *B* then blocks with
-forward progress guarantee delegation on *C*, this may also temporarily
-provide a stronger forward progress guarantee to *C*. — *end note*]
-[*Note 12*: If all threads of execution in *S* finish executing (e.g.,
 they terminate and do not use blocking synchronization incorrectly),
-then *P*’s execution of the operation that blocks with forward progress
-guarantee delegation will not result in *P*’s progress guarantee being
 effectively weakened. — *end note*]
 [*Note 13*: This does not remove any constraints regarding blocking
 synchronization for threads of execution providing parallel or weakly
 parallel forward progress guarantees because the implementation is not

+### Multi-threaded executions and data races <a id="intro.multithread">[[intro.multithread]]</a>
 A *thread of execution* (also known as a *thread*) is a single flow of
 control within a program, including the initial invocation of a specific
 top-level function, and recursively including every function invocation
 subsequently executed by the thread.
 [*Note 1*: When one thread creates another, the initial call to the
 top-level function of the new thread is executed by the new thread, not
 by the creating thread. — *end note*]
 Every thread in a program can potentially access every object and
+function in a program.[^28] Under a hosted implementation, a C++ program
 can have more than one thread running concurrently. The execution of
+each thread proceeds as defined by the remainder of this document. The
+execution of the entire program consists of an execution of all of its
+threads.
 [*Note 2*: Usually the execution can be viewed as an interleaving of
 all its threads. However, some kinds of atomic operations, for example,
 allow executions inconsistent with a simple interleaving, as described
 below. — *end note*]
 For a signal handler that is not executed as a result of a call to the
 `std::raise` function, it is unspecified which thread of execution
 contains the signal handler invocation.
+#### Data races <a id="intro.races">[[intro.races]]</a>
+The value of an object visible to a thread T at a particular point is
+the initial value of the object, a value assigned to the object by T, or
+a value assigned to the object by another thread, according to the rules
+below.
 [*Note 1*: In some cases, there may instead be undefined behavior. Much
+of this subclause is motivated by the desire to support atomic
+operations with explicit and detailed visibility constraints. However,
+it also implicitly supports a simpler view for more restricted
 programs. — *end note*]
 Two expression evaluations *conflict* if one of them modifies a memory
+location [[intro.memory]] and the other one reads or modifies the same
+memory location.
+The library defines a number of atomic operations [[atomics]] and
+operations on mutexes [[thread]] that are specially identified as
+synchronization operations. These operations play a special role in
+making assignments in one thread visible to another. A synchronization
+operation on one or more memory locations is either a consume operation,
+an acquire operation, a release operation, or both an acquire and
+release operation. A synchronization operation without an associated
+memory location is a fence and can be either an acquire fence, a release
+fence, or both an acquire and release fence. In addition, there are
+relaxed atomic operations, which are not synchronization operations, and
+atomic read-modify-write operations, which have special characteristics.
 [*Note 2*: For example, a call that acquires a mutex will perform an
 acquire operation on the locations comprising the mutex.
 Correspondingly, a call that releases the same mutex will perform a
 release operation on those same locations. Informally, performing a
+release operation on A forces prior side effects on other memory
 locations to become visible to other threads that later perform a
+consume or an acquire operation on A. “Relaxed” atomic operations are
 not synchronization operations even though, like synchronization
 operations, they cannot contribute to data races. — *end note*]
+All modifications to a particular atomic object M occur in some
+particular total order, called the *modification order* of M.
 [*Note 3*: There is a separate order for each atomic object. There is
 no requirement that these can be combined into a single total order for
 all objects. In general this will be impossible since different threads
 may observe modifications to different objects in inconsistent
 orders. — *end note*]
+A *release sequence* headed by a release operation A on an atomic object
+M is a maximal contiguous sub-sequence of side effects in the
+modification order of M, where the first operation is A, and every
+subsequent operation is an atomic read-modify-write operation.
 Certain library calls *synchronize with* other library calls performed
 by another thread. For example, an atomic store-release synchronizes
+with a load-acquire that takes its value from the store
+[[atomics.order]].
 [*Note 4*: Except in the specified cases, reading a later value does
 not necessarily ensure visibility as described below. Such a requirement
 would sometimes interfere with efficient implementation. — *end note*]
 when one reads the value written by another. For atomic objects, the
 definition is clear. All operations on a given mutex occur in a single
 total order. Each mutex acquisition “reads the value written” by the
 last mutex release. — *end note*]
+An evaluation A *carries a dependency* to an evaluation B if
+- the value of A is used as an operand of B, unless:
+  - B is an invocation of any specialization of `std::kill_dependency`
+    [[atomics.order]], or
+  - A is the left operand of a built-in logical (`&&`, see
+    [[expr.log.and]]) or logical (`||`, see  [[expr.log.or]]) operator,
+    or
+  - A is the left operand of a conditional (`?:`, see  [[expr.cond]])
     operator, or
+  - A is the left operand of the built-in comma (`,`) operator
+    [[expr.comma]];
   or
+- A writes a scalar object or bit-field M, B reads the value written by
+  A from M, and A is sequenced before B, or
+- for some evaluation X, A carries a dependency to X, and X carries a
+  dependency to B.
 [*Note 6*: “Carries a dependency to” is a subset of “is sequenced
 before”, and is similarly strictly intra-thread. — *end note*]
+An evaluation A is *dependency-ordered before* an evaluation B if
+- A performs a release operation on an atomic object M, and, in another
+  thread, B performs a consume operation on M and reads the value
+  written by A, or
+- for some evaluation X, A is dependency-ordered before X and X carries
+  a dependency to B.
 [*Note 7*: The relation “is dependency-ordered before” is analogous to
 “synchronizes with”, but uses release/consume in place of
 release/acquire. — *end note*]
+An evaluation A *inter-thread happens before* an evaluation B if
+- A synchronizes with B, or
+- A is dependency-ordered before B, or
+- for some evaluation X
+  - A synchronizes with X and X is sequenced before B, or
+  - A is sequenced before X and X inter-thread happens before B, or
+  - A inter-thread happens before X and X inter-thread happens before B.
 [*Note 8*: The “inter-thread happens before” relation describes
 arbitrary concatenations of “sequenced before”, “synchronizes with” and
 “dependency-ordered before” relationships, with two exceptions. The
 first exception is that a concatenation is not permitted to end with
 of “sequenced before”. The reasons for this limitation are (1) to permit
 “inter-thread happens before” to be transitively closed and (2) the
 “happens before” relation, defined below, provides for relationships
 consisting entirely of “sequenced before”. — *end note*]
+An evaluation A *happens before* an evaluation B (or, equivalently, B
+*happens after* A) if:
+- A is sequenced before B, or
+- A inter-thread happens before B.
 The implementation shall ensure that no program execution demonstrates a
 cycle in the “happens before” relation.
 [*Note 9*: This cycle would otherwise be possible only through the use
 of consume operations. — *end note*]
+An evaluation A *simply happens before* an evaluation B if either
+- A is sequenced before B, or
+- A synchronizes with B, or
+- A simply happens before X and X simply happens before B.
 [*Note 10*: In the absence of consume operations, the happens before
+and simply happens before relations are identical. — *end note*]
+An evaluation A *strongly happens before* an evaluation D if, either
+- A is sequenced before D, or
+- A synchronizes with D, and both A and D are sequentially consistent
+ atomic operations [[atomics.order]], or
+- there are evaluations B and C such that A is sequenced before B, B
+  simply happens before C, and C is sequenced before D, or
+- there is an evaluation B such that A strongly happens before B, and B
+  strongly happens before D.
+[*Note 11*: Informally, if A strongly happens before B, then A appears
+to be evaluated before B in all contexts. Strongly happens before
+excludes consume operations. — *end note*]
+A *visible side effect* A on a scalar object or bit-field M with respect
+to a value computation B of M satisfies the conditions:
+- A happens before B and
+- there is no other side effect X to M such that A happens before X and
+  X happens before B.
+The value of a non-atomic scalar object or bit-field M, as determined by
+evaluation B, shall be the value stored by the visible side effect A.
+[*Note 12*: If there is ambiguity about which side effect to a
 non-atomic object or bit-field is visible, then the behavior is either
 unspecified or undefined. — *end note*]
+[*Note 13*: This states that operations on ordinary objects are not
 visibly reordered. This is not actually detectable without data races,
 but it is necessary to ensure that data races, as defined below, and
 with suitable restrictions on the use of atomics, correspond to data
 races in a simple interleaved (sequentially consistent)
 execution. — *end note*]
+The value of an atomic object M, as determined by evaluation B, shall be
+the value stored by some side effect A that modifies M, where B does not
+happen before A.
+[*Note 14*: The set of such side effects is also restricted by the rest
 of the rules described here, and in particular, by the coherence
 requirements below. — *end note*]
+If an operation A that modifies an atomic object M happens before an
+operation B that modifies M, then A shall be earlier than B in the
+modification order of M.
+[*Note 15*: This requirement is known as write-write
 coherence. — *end note*]
+If a value computation A of an atomic object M happens before a value
+computation B of M, and A takes its value from a side effect X on M,
+then the value computed by B shall either be the value stored by X or
+the value stored by a side effect Y on M, where Y follows X in the
+modification order of M.
+[*Note 16*: This requirement is known as read-read
 coherence. — *end note*]
+If a value computation A of an atomic object M happens before an
+operation B that modifies M, then A shall take its value from a side
+effect X on M, where X precedes B in the modification order of M.
+[*Note 17*: This requirement is known as read-write
 coherence. — *end note*]
+If a side effect X on an atomic object M happens before a value
+computation B of M, then the evaluation B shall take its value from X or
+from a side effect Y that follows X in the modification order of M.
+[*Note 18*: This requirement is known as write-read
 coherence. — *end note*]
+[*Note 19*: The four preceding coherence requirements effectively
 disallow compiler reordering of atomic operations to a single object,
 even if both operations are relaxed loads. This effectively makes the
+cache coherence guarantee provided by most hardware available to C++
+atomic operations. — *end note*]
+[*Note 20*: The value observed by a load of an atomic depends on the
 “happens before” relation, which depends on the values observed by loads
 of atomics. The intended reading is that there must exist an association
 of atomic loads with modifications they observe that, together with
 suitably chosen modification orders and the “happens before” relation
 derived as described above, satisfy the resulting constraints as imposed
 potentially concurrent conflicting actions, at least one of which is not
 atomic, and neither happens before the other, except for the special
 case for signal handlers described below. Any such data race results in
 undefined behavior.
+[*Note 21*: It can be shown that programs that correctly use mutexes
+and `memory_order::seq_cst` operations to prevent all data races and use
 no other synchronization operations behave as if the operations executed
 by their constituent threads were simply interleaved, with each value
 computation of an object being taken from the last side effect on that
 object in that interleaving. This is normally referred to as “sequential
 consistency”. However, this applies only to data-race-free programs, and
 Two accesses to the same object of type `volatile std::sig_atomic_t` do
 not result in a data race if both occur in the same thread, even if one
 or more occurs in a signal handler. For each signal handler invocation,
 evaluations performed by the thread invoking a signal handler can be
+divided into two groups A and B, such that no evaluations in B happen
+before evaluations in A, and the evaluations of such
 `volatile std::sig_atomic_t` objects take values as though all
+evaluations in A happened before the execution of the signal handler and
+the execution of the signal handler happened before all evaluations in
+B.
+[*Note 22*: Compiler transformations that introduce assignments to a
 potentially shared memory location that would not be modified by the
+abstract machine are generally precluded by this document, since such an
+assignment might overwrite another assignment by a different thread in
+cases in which an abstract machine execution would not have encountered
+a data race. This includes implementations of data member assignment
+that overwrite adjacent members in separate memory locations. Reordering
+of atomic loads in cases in which the atomics in question may alias is
+also generally precluded, since this may violate the coherence
+rules. — *end note*]
+[*Note 23*: Transformations that introduce a speculative read of a
 potentially shared memory location may not preserve the semantics of the
+C++ program as defined in this document, since they potentially
+introduce a data race. However, they are typically valid in the context
+of an optimizing compiler that targets a specific machine with
+well-defined semantics for data races. They would be invalid for a
 hypothetical machine that is not tolerant of races or provides hardware
 race detection. — *end note*]
+#### Forward progress <a id="intro.progress">[[intro.progress]]</a>
 The implementation may assume that any thread will eventually do one of
 the following:
 - terminate,
 [*Note 1*: This is intended to allow compiler transformations such as
 removal of empty loops, even when termination cannot be
 proven. — *end note*]
+Executions of atomic functions that are either defined to be lock-free
+[[atomics.flag]] or indicated as lock-free [[atomics.lockfree]] are
 *lock-free executions*.
+- If there is only one thread that is not blocked [[defns.block]] in a
+  standard library function, a lock-free execution in that thread shall
+  complete. \[*Note 2*: Concurrently executing threads may prevent
   progress of a lock-free execution. For example, this situation can
   occur with load-locked store-conditional implementations. This
   property is sometimes termed obstruction-free. — *end note*]
 - When one or more lock-free executions run concurrently, at least one
   should complete. \[*Note 3*: It is difficult for some implementations
 - termination of the thread of execution,
 - performing an access through a volatile glvalue, or
 - completion of a call to a library I/O function, a synchronization
   operation, or an atomic operation.
+An invocation of a standard library function that blocks [[defns.block]]
+is considered to continuously execute execution steps while waiting for
+the condition that it blocks on to be satisfied.
 [*Example 1*: A library I/O function that blocks until the I/O
 operation is complete can be considered to continuously check whether
 the operation is complete. Each such check might consist of one or more
 execution steps, for example using observable behavior of the abstract
 of executions (if any) have been or are making progress. To eventually
 fulfill this requirement means that this will happen in an unspecified
 but finite amount of time. — *end note*]
 It is *implementation-defined* whether the implementation-created thread
+of execution that executes `main` [[basic.start.main]] and the threads
+of execution created by `std::thread` [[thread.thread.class]] or
+`std::jthread` [[thread.jthread.class]] provide concurrent forward
+progress guarantees.
+[*Note 6*: General-purpose implementations should provide these
+guarantees. — *end note*]
 For a thread of execution providing *parallel forward progress
 guarantees*, the implementation is not required to ensure that the
 thread will eventually make progress if it has not yet executed any
 execution step; once this thread has executed a step, it provides
 [*Note 9*: For example, some kinds of synchronization between threads
 of execution may only make progress if the respective threads of
 execution provide parallel forward progress guarantees, but will fail to
 make progress under weakly parallel guarantees. — *end note*]
+When a thread of execution P is specified to *block with forward
+progress guarantee delegation* on the completion of a set S of threads
+of execution, then throughout the whole time of P being blocked on S,
+the implementation shall ensure that the forward progress guarantees
+provided by at least one thread of execution in S is at least as strong
+as P’s forward progress guarantees.
+[*Note 10*: It is unspecified which thread or threads of execution in S
+are chosen and for which number of execution steps. The strengthening is
+not permanent and not necessarily in place for the rest of the lifetime
+of the affected thread of execution. As long as P is blocked, the
+implementation has to eventually select and potentially strengthen a
+thread of execution in S. — *end note*]
+Once a thread of execution in S terminates, it is removed from S. Once S
+is empty, P is unblocked.
+[*Note 11*: A thread of execution B thus can temporarily provide an
 effectively stronger forward progress guarantee for a certain amount of
+time, due to a second thread of execution A being blocked on it with
+forward progress guarantee delegation. In turn, if B then blocks with
+forward progress guarantee delegation on C, this may also temporarily
+provide a stronger forward progress guarantee to C. — *end note*]
+[*Note 12*: If all threads of execution in S finish executing (e.g.,
 they terminate and do not use blocking synchronization incorrectly),
+then P’s execution of the operation that blocks with forward progress
+guarantee delegation will not result in P’s progress guarantee being
 effectively weakened. — *end note*]
 [*Note 13*: This does not remove any constraints regarding blocking
 synchronization for threads of execution providing parallel or weakly
 parallel forward progress guarantees because the implementation is not

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