A hot fix for a reduction parallel pattern of OpenMP backend

[MikeDvorskiy]

Summary

A hot fix for OpenMP backend; + "exit" condition for recursion of "__parallel_reduce_body"

[MikeDvorskiy]

clang format

[MikeDvorskiy]

A hot performance fix for parallel reduce pattern of OpenMP backend. The recursion should be limited by a grain size - __default_chunk_size.

[nadiasvertex]

Thank you! I fixed that bug once, but in the various changes I missed it the last time.

pstl/include/pstl/internal/omp/parallel_reduce.h

 namespace __omp_backend
 {
 
 template <class _RandomAccessIterator, class _Value, typename _RealBody, typename _Reduction>
 _Value
 __parallel_reduce_body(_RandomAccessIterator __first, _RandomAccessIterator __last, _Value __identity,
                        _RealBody __real_body, _Reduction __reduce)
 {
+    if (__should_run_serial(__first, __last))
+    {
+        return __real_body(__first, __last, __identity);
+    }
+
     auto __middle = __first + ((__last - __first) / 2);
     _Value __v1(__identity), __v2(__identity);
     __parallel_invoke_body(
         [&]() { __v1 = __parallel_reduce_body(__first, __middle, __identity, __real_body, __reduce); },
         [&]() { __v2 = __parallel_reduce_body(__middle, __last, __identity, __real_body, __reduce); });
 
     return __reduce(__v1, __v2);
 }