User Guide

Contents

Parallelize Data -
Intel® oneAPI Threading Building Blocks (oneTBB)
Loops with Complex Iteration Control

Sometimes the loop control is spread across complex control flow. Using
Intel® oneAPI Threading Building Blocks (oneTBB)
in this situation requires more features than the simple loops. Note that the task body must not access any of the auto variables defined within the annotation site, because they may have been destroyed before or while the task is running. Consider this serial code:
 extern char a[]; int previousEnd = -1; ANNOTATE_SITE_BEGIN(sitename); for (int i=0; i<=100; i++) { if (!a[i] || i==100) { ANNOTATE_TASK_BEGIN(do_something); DoSomething(previousEnd+1,i); ANNOTATE_TASK_END(); previousEnd=i; } } ANNOTATE_SITE_END();
In general, counted loops have better scalability than loops with complex iteration control, because the complex control is inherently sequential. Consider reformulating your code as a counted loop if possible.
The prior example is easily converted to parallelism by using the
task_group
feature of
oneTBB
:
  #include <tbb/tbb.h> ... extern char a[]; int previousEnd = -1; task_group g; for (int i=0; i<=100; i++) { if (!a[i] || i==100) { g.run([=]{DoSomething(previousEnd+1,i);} previousEnd=i; } } g.wait(); // Wait until all tasks in the group finish  
Here the lambda expression uses capture by value
[=]
because it is important for it to grab the values of
i
and
previousEnd
when the expression constructs its
functor
, because afterwards the value of
previousEnd
and
i
change.
For more information on
tbb::task_group
, see the
oneTBB
documentation.

Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.