2.19.8  flush Construct

Summary The flush construct executes the OpenMP flush operation. This operation makes a thread’s temporary view of memory consistent with memory and enforces an order on the memory operations of the variables explicitly specified or implied. See the memory model description in Section 1.4 for more details. The flush construct is a stand-alone directive.

Syntax

 
#pragma omp flush [memory-order-clause] [(list)] new-line
  

 
seq_cst 
acq_rel 
release 
acquire
       

 
!$omp flush [memory-order-clause] [(list)]
  

 
seq_cst 
acq_rel 
release 
acquire
       

Binding The binding thread set for a flush region is all threads in the device-set of its flush operation. Execution of a flush region affects the memory and it affects the temporary view of memory of the encountering thread. It does not affect the temporary view of other threads. Other threads on devices in the device-set must themselves execute a flush operation in order to be guaranteed to observe the effects of the flush operation of the encountering thread.

Description If neither memory-order-clause nor a list appears on the flush construct then the behavior is as if memory-order-clause is seq_cst.

A flush construct with the seq_cst clause, executed on a given thread, operates as if all data storage blocks that are accessible to the thread are flushed by a strong flush operation. A flush construct with a list applies a strong flush operation to the items in the list, and the flush operation does not complete until the operation is complete for all specified list items. An implementation may implement a flush construct with a list by ignoring the list and treating it the same as a flush construct with the seq_cst clause.

If no list items are specified, the flush operation has the release and/or acquire flush properties:

• If memory-order-clause is seq_cst or acq_rel, the flush operation is both a release flush and an acquire flush.
• If memory-order-clause is release, the flush operation is a release flush.
• If memory-order-clause is acquire, the flush operation is an acquire flush.

Note – Use of a flush construct with a list is extremely error prone and users are strongly discouraged from attempting it. The following examples illustrate the ordering properties of the flush operation. In the following incorrect pseudocode example, the programmer intends to prevent simultaneous execution of the protected section by the two threads, but the program does not work properly because it does not enforce the proper ordering of the operations on variables a and b. Any shared data accessed in the protected section is not guaranteed to be current or consistent during or after the protected section. The atomic notation in the pseudocode in the following two examples indicates that the accesses to a and b are atomic write and atomic read operations. Otherwise both examples would contain data races and automatically result in unspecified behavior. The flush operations are strong flushes that are applied to the specified flush lists

The problem with this example is that operations on variables a and b are not ordered with respect to each other. For instance, nothing prevents the compiler from moving the flush of b on thread 1 or the flush of a on thread 2 to a position completely after the protected section (assuming that the protected section on thread 1 does not reference b and the protected section on thread 2 does not reference a). If either re-ordering happens, both threads can simultaneously execute the protected section.

The following pseudocode example correctly ensures that the protected section is executed by only one thread at a time. Execution of the protected section by neither thread is considered correct in this example. This occurs if both flushes complete prior to either thread executing its if statement.

The compiler is prohibited from moving the flush at all for either thread, ensuring that the respective assignment is complete and the data is flushed before the if statement is executed.

Execution Model Events The flush event occurs in a thread that encounters the flush construct.

Tool Callbacks A thread dispatches a registered ompt_callback_flush callback for each occurrence of a flush event in that thread. This callback has the type signature ompt_callback_flush_t.

Restrictions Restrictions to the flush construct are as follows:

• If a memory-order-clause is specified, list items must not be specified on the flush directive.

Cross References

• ompt_callback_flush_t, see Section 4.5.2.17.

2.19.8.1 Implicit Flushes

Flush operations implied when executing an atomic region are described in Section 2.19.7.

A flush region that corresponds to a flush directive with the release clause present is implied at the following locations:

• During a barrier region;
• At entry to a parallel region;
• At entry to a teams region;
• At exit from a critical region;
• During an omp_unset_lock region;
• During an omp_unset_nest_lock region;
• Immediately before every task scheduling point;
• At exit from the task region of each implicit task;
• At exit from an ordered region, if a threads clause or a depend clause with a source dependence type is present, or if no clauses are present; and
• During a cancel region, if the cancel-var ICV is true.

For a target construct, the device-set of an implicit release flush that is performed in a target task during the generation of the target region and that is performed on exit from the initial task region that implicitly encloses the target region consists of the devices that execute the target task and the target region.

A flush region that corresponds to a flush directive with the acquire clause present is implied at the following locations:

• During a barrier region;
• At exit from a teams region;
• At entry to a critical region;
• If the region causes the lock to be set, during:
  • an omp_set_lock region;
  • an omp_test_lock region;
  • an omp_set_nest_lock region; and
  • an omp_test_nest_lock region;
• Immediately after every task scheduling point;
• At entry to the task region of each implicit task;
• At entry to an ordered region, if a threads clause or a depend clause with a sink dependence type is present, or if no clauses are present; and
• Immediately before a cancellation point, if the cancel-var ICV is true and cancellation has been activated.

For a target construct, the device-set of an implicit acquire flush that is performed in a target task following the generation of the target region or that is performed on entry to the initial task region that implicitly encloses the target region consists of the devices that execute the target task and the target region.

Note – A flush region is not implied at the following locations:

• At entry to worksharing regions; and
• At entry to or exit from masked regions.

The synchronization behavior of implicit flushes is as follows:

• When a thread executes an atomic region for which the corresponding construct has the release, acq_rel, or seq_cst clause and specifies an atomic operation that starts a given release sequence, the release flush that is performed on entry to the atomic operation synchronizes with an acquire flush that is performed by a different thread and has an associated atomic operation that reads a value written by a modification in the release sequence.
• When a thread executes an atomic region for which the corresponding construct has the acquire, acq_rel, or seq_cst clause and specifies an atomic operation that reads a value written by a given modification, a release flush that is performed by a different thread and has an associated release sequence that contains that modification synchronizes with the acquire flush that is performed on exit from the atomic operation.
• When a thread executes a critical region that has a given name, the behavior is as if the release flush performed on exit from the region synchronizes with the acquire flush performed on entry to the next critical region with the same name that is performed by a different thread, if it exists.
• When a thread team executes a barrier region, the behavior is as if the release flush performed by each thread within the region synchronizes with the acquire flush performed by all other threads within the region.
• When a thread executes a taskwait region that does not result in the creation of a dependent task and the task that encounters the corresponding taskwait construct has at least one child task, the behavior is as if each thread that executes a child task that is generated before the taskwait region performs a release flush upon completion of the child task that synchronizes with an acquire flush performed in the taskwait region.
• When a thread executes a taskgroup region, the behavior is as if each thread that executes a remaining descendant task performs a release flush upon completion of the descendant task that synchronizes with an acquire flush performed on exit from the taskgroup region.
• When a thread executes an ordered region that does not arise from a stand-alone ordered directive, the behavior is as if the release flush performed on exit from the region synchronizes with the acquire flush performed on entry to an ordered region encountered in the next logical iteration to be executed by a different thread, if it exists.
• When a thread executes an ordered region that arises from a stand-alone ordered directive, the behavior is as if the release flush performed in the ordered region from a given source iteration synchronizes with the acquire flush performed in all ordered regions executed by a different thread that are waiting for dependences on that iteration to be satisfied.
• When a thread team begins execution of a parallel region, the behavior is as if the release flush performed by the primary thread on entry to the parallel region synchronizes with the acquire flush performed on entry to each implicit task that is assigned to a different thread.
• When an initial thread begins execution of a target region that is generated by a different thread from a target task, the behavior is as if the release flush performed by the generating thread in the target task synchronizes with the acquire flush performed by the initial thread on entry to its initial task region.
• When an initial thread completes execution of a target region that is generated by a different thread from a target task, the behavior is as if the release flush performed by the initial thread on exit from its initial task region synchronizes with the acquire flush performed by the generating thread in the target task.
• When a thread encounters a teams construct, the behavior is as if the release flush performed by the thread on entry to the teams region synchronizes with the acquire flush performed on entry to each initial task that is executed by a different initial thread that participates in the execution of the teams region.
• When a thread that encounters a teams construct reaches the end of the teams region, the behavior is as if the release flush performed by each different participating initial thread at exit from its initial task synchronizes with the acquire flush performed by the thread at exit from the teams region.
• When a task generates an explicit task that begins execution on a different thread, the behavior is as if the thread that is executing the generating task performs a release flush that synchronizes with the acquire flush performed by the thread that begins to execute the explicit task.
• When an undeferred task completes execution on a given thread that is different from the thread on which its generating task is suspended, the behavior is as if a release flush performed by the thread that completes execution of the undeferred task synchronizes with an acquire flush performed by the thread that resumes execution of the generating task.
• When a dependent task with one or more predecessor tasks begins execution on a given thread, the behavior is as if each release flush performed by a different thread on completion of a predecessor task synchronizes with the acquire flush performed by the thread that begins to execute the dependent task.
• When a task begins execution on a given thread and it is mutually exclusive with respect to another sibling task that is executed by a different thread, the behavior is as if each release flush performed on completion of the sibling task synchronizes with the acquire flush performed by the thread that begins to execute the task.
• When a thread executes a cancel region, the cancel-var ICV is true, and cancellation is not already activated for the specified region, the behavior is as if the release flush performed during the cancel region synchronizes with the acquire flush performed by a different thread immediately before a cancellation point in which that thread observes cancellation was activated for the region.
• When a thread executes an omp_unset_lock region that causes the specified lock to be unset, the behavior is as if a release flush is performed during the omp_unset_lock region that synchronizes with an acquire flush that is performed during the next omp_set_lock or omp_test_lock region to be executed by a different thread that causes the specified lock to be set.
• When a thread executes an omp_unset_nest_lock region that causes the specified nested lock to be unset, the behavior is as if a release flush is performed during the omp_unset_nest_lock region that synchronizes with an acquire flush that is performed during the next omp_set_nest_lock or omp_test_nest_lock region to be executed by a different thread that causes the specified nested lock to be set.