156 lines
6.3 KiB
HTML
156 lines
6.3 KiB
HTML
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<head>
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<title>GNU libgomp: Implementing FOR construct</title>
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<link href="Library-Index.html#Library-Index" rel="index" title="Library Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="The-libgomp-ABI.html#The-libgomp-ABI" rel="up" title="The libgomp ABI">
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<link href="Implementing-ORDERED-construct.html#Implementing-ORDERED-construct" rel="next" title="Implementing ORDERED construct">
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<a name="Implementing-FOR-construct"></a>
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<div class="header">
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<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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</div>
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<hr>
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<a name="Implementing-FOR-construct-1"></a>
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<h3 class="section">9.11 Implementing FOR construct</h3>
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<div class="smallexample">
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<pre class="smallexample"> #pragma omp parallel for
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for (i = lb; i <= ub; i++)
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body;
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</pre></div>
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<p>becomes
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</p>
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<div class="smallexample">
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<pre class="smallexample"> void subfunction (void *data)
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{
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long _s0, _e0;
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while (GOMP_loop_static_next (&_s0, &_e0))
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{
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long _e1 = _e0, i;
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for (i = _s0; i < _e1; i++)
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body;
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}
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GOMP_loop_end_nowait ();
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}
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GOMP_parallel_loop_static (subfunction, NULL, 0, lb, ub+1, 1, 0);
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subfunction (NULL);
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GOMP_parallel_end ();
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</pre></div>
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<div class="smallexample">
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<pre class="smallexample"> #pragma omp for schedule(runtime)
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for (i = 0; i < n; i++)
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body;
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</pre></div>
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<p>becomes
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</p>
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<div class="smallexample">
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<pre class="smallexample"> {
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long i, _s0, _e0;
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if (GOMP_loop_runtime_start (0, n, 1, &_s0, &_e0))
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do {
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long _e1 = _e0;
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for (i = _s0, i < _e0; i++)
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body;
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} while (GOMP_loop_runtime_next (&_s0, _&e0));
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GOMP_loop_end ();
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}
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</pre></div>
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<p>Note that while it looks like there is trickiness to propagating
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a non-constant STEP, there isn’t really. We’re explicitly allowed
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to evaluate it as many times as we want, and any variables involved
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should automatically be handled as PRIVATE or SHARED like any other
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variables. So the expression should remain evaluable in the
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subfunction. We can also pull it into a local variable if we like,
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but since its supposed to remain unchanged, we can also not if we like.
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</p>
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<p>If we have SCHEDULE(STATIC), and no ORDERED, then we ought to be
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able to get away with no work-sharing context at all, since we can
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simply perform the arithmetic directly in each thread to divide up
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the iterations. Which would mean that we wouldn’t need to call any
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of these routines.
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</p>
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<p>There are separate routines for handling loops with an ORDERED
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clause. Bookkeeping for that is non-trivial...
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</p>
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<hr>
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<div class="header">
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<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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</div>
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