To propagate CPU-specific settings for floating-point computations to tasks executed by the task scheduler, you can use one of the following two methods:
task_arenaor a task scheduler for a given application thread is initialized, they capture the current floating-point settings of the thread.
task_group_contextclass has a method to capture the current floating-point settings.
By default, worker threads use floating-point settings obtained during the initialization of a
task_arena or the implicit arena of the application thread. The settings are applied to all computations within that
task_arena or started by that application thread.
For better control over floating point behavior, a thread may capture the current settings in a task group context. Do it at context creation with a special flag passed to the constructor:
task_group_context ctx( task_group_context::isolated, task_group_context::default_traits | task_group_context::fp_settings );
Or call the
task_group_context ctx; ctx.capture_fp_settings();
You can then pass the task group context to most parallel algorithms, including
flow::graph, to ensure that all tasks related to this algorithm use the specified floating-point settings.
It is possible to execute the parallel algorithms with different floating-point settings captured to separate contexts, even at the same time.
Floating-point settings captured to a task group context prevail over the settings captured during task scheduler initialization. It means, if a context is passed to a parallel algorithm, the floating-point settings captured to the context are used. Otherwise, if floating-point settings are not captured to the context, or a context is not explicitly specified, the settings captured during the task arena initialization are used.
In a nested call to a parallel algorithm that does not use the context of a task group with explicitly captured floating-point settings, the outer-level settings are used. If none of the outer-level contexts capture floating-point settings, the settings captured during task arena initialization are used.
It guarantees that:
Floating-point settings are applied to all tasks executed within a task arena, if they are captured:
To a task group context.
During the arena initialization.
A call to a oneTBB parallel algorithm does not change the floating-point settings of the calling thread, even if the algorithm uses different settings.
The guarantees above apply only to the following conditions:
A user code inside a task should:
Not change the floating-point settings.
Revert any modifications.
Restore previous settings before the end of the task.
oneTBB task scheduler observers are not used to set or modify floating point settings.
Otherwise, the stated guarantees are not valid and the behavior related to floating-point settings is undefined.