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| Next: <a href="Implementing-ORDERED-construct.html#Implementing-ORDERED-construct" accesskey="n" rel="next">Implementing ORDERED construct</a>, Previous: <a href="Implementing-PARALLEL-construct.html#Implementing-PARALLEL-construct" accesskey="p" rel="prev">Implementing PARALLEL construct</a>, Up: <a href="The-libgomp-ABI.html#The-libgomp-ABI" accesskey="u" rel="up">The libgomp ABI</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Library-Index.html#Library-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Implementing-FOR-construct-1"></a> |
| <h3 class="section">4.11 Implementing FOR construct</h3> |
| |
| <div class="smallexample"> |
| <pre class="smallexample"> #pragma omp parallel for |
| for (i = lb; i <= ub; i++) |
| body; |
| </pre></div> |
| |
| <p>becomes |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> void subfunction (void *data) |
| { |
| long _s0, _e0; |
| while (GOMP_loop_static_next (&_s0, &_e0)) |
| { |
| long _e1 = _e0, i; |
| for (i = _s0; i < _e1; i++) |
| body; |
| } |
| GOMP_loop_end_nowait (); |
| } |
| |
| GOMP_parallel_loop_static (subfunction, NULL, 0, lb, ub+1, 1, 0); |
| subfunction (NULL); |
| GOMP_parallel_end (); |
| </pre></div> |
| |
| <div class="smallexample"> |
| <pre class="smallexample"> #pragma omp for schedule(runtime) |
| for (i = 0; i < n; i++) |
| body; |
| </pre></div> |
| |
| <p>becomes |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> { |
| long i, _s0, _e0; |
| if (GOMP_loop_runtime_start (0, n, 1, &_s0, &_e0)) |
| do { |
| long _e1 = _e0; |
| for (i = _s0, i < _e0; i++) |
| body; |
| } while (GOMP_loop_runtime_next (&_s0, _&e0)); |
| GOMP_loop_end (); |
| } |
| </pre></div> |
| |
| <p>Note that while it looks like there is trickiness to propagating |
| a non-constant STEP, there isn’t really. We’re explicitly allowed |
| to evaluate it as many times as we want, and any variables involved |
| should automatically be handled as PRIVATE or SHARED like any other |
| variables. So the expression should remain evaluable in the |
| subfunction. We can also pull it into a local variable if we like, |
| but since its supposed to remain unchanged, we can also not if we like. |
| </p> |
| <p>If we have SCHEDULE(STATIC), and no ORDERED, then we ought to be |
| able to get away with no work-sharing context at all, since we can |
| simply perform the arithmetic directly in each thread to divide up |
| the iterations. Which would mean that we wouldn’t need to call any |
| of these routines. |
| </p> |
| <p>There are separate routines for handling loops with an ORDERED |
| clause. Bookkeeping for that is non-trivial... |
| </p> |
| |
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| Next: <a href="Implementing-ORDERED-construct.html#Implementing-ORDERED-construct" accesskey="n" rel="next">Implementing ORDERED construct</a>, Previous: <a href="Implementing-PARALLEL-construct.html#Implementing-PARALLEL-construct" accesskey="p" rel="prev">Implementing PARALLEL construct</a>, Up: <a href="The-libgomp-ABI.html#The-libgomp-ABI" accesskey="u" rel="up">The libgomp ABI</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Library-Index.html#Library-Index" title="Index" rel="index">Index</a>]</p> |
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