[intro.races] - C++23 → Trunk

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@@ -9,27 +9,37 @@ below.
 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
@@ -38,11 +48,11 @@ 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
 can observe modifications to different objects in inconsistent
 orders. — *end note*]
@@ -54,181 +64,107 @@ 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*]
-[*Note 5*: The specifications of the synchronization operations define
 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
-“dependency-ordered before” followed by “sequenced before”. The reason
-for this limitation is that a consume operation participating in a
-“dependency-ordered before” relationship provides ordering only with
-respect to operations to which this consume operation actually carries a
-dependency. The reason that this limitation applies only to the end of
-such a concatenation is that any subsequent release operation will
-provide the required ordering for a prior consume operation. The second
-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 *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
@@ -244,49 +180,49 @@ 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 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
 data-race-free programs cannot observe most program transformations that
 do not change single-threaded program semantics. In fact, most
-single-threaded program transformations continue to be allowed, since
-any program that behaves differently as a result has undefined
 behavior. — *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 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 might alias is
 also generally precluded, since this could violate the coherence
 rules. — *end note*]
-[*Note 23*: Transformations that introduce a speculative read of a
-potentially shared memory location might 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*]

 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 [[defns.access]] a memory location [[intro.memory]] or
+- starts or ends the lifetime of an object in a memory location
+and the other one
+- reads or modifies the same memory location or
+- starts or ends the lifetime of an object occupying storage that
+  overlaps with the memory location.
+[*Note 2*: A modification can still conflict even if it does not alter
+the value of any bits. — *end note*]
 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 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 3*: 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
 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 4*: 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
 can observe modifications to different objects in inconsistent
 orders. — *end note*]
 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 5*: 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*]
+[*Note 6*: The specifications of the synchronization operations define
 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 *happens before* an evaluation B (or, equivalently, B
+happens after A) if either
 - A is sequenced before B, or
 - A synchronizes with B, or
+- A happens before X and X happens before B.
+[*Note 7*: An evaluation does not happen before itself. — *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
+  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 8*: Informally, if A strongly happens before B, then A appears
+to be evaluated before B in all contexts. — *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, is the value stored by the visible side effect A.
+[*Note 9*: 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 10*: This states that operations on ordinary objects are not
 visibly reordered. This is not actually detectable without data races,
+but is needed 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, is the
+value stored by some unspecified side effect A that modifies M, where B
+does not happen before A.
+[*Note 11*: 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 is earlier than B in the
 modification order of M.
+[*Note 12*: 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 is either 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 13*: 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 takes its value from a side effect X
+on M, where X precedes B in the modification order of M.
+[*Note 14*: 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 takes its value from X or from
+a side effect Y that follows X in the modification order of M.
+[*Note 15*: This requirement is known as write-read
 coherence. — *end note*]
+[*Note 16*: 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 17*: 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 18*: 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
 data-race-free programs cannot observe most program transformations that
 do not change single-threaded program semantics. In fact, most
+single-threaded program transformations remain possible, since any
+program that behaves differently as a result has undefined
 behavior. — *end note*]
+Two accesses to the same non-bit-field 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 19*: 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 might alias is
 also generally precluded, since this could violate the coherence
 rules. — *end note*]
+[*Note 20*: It is possible that transformations that introduce a
+speculative read of a potentially shared memory location do 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*]

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