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107 lines
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107 lines
4.8 KiB
HTML
4 years ago
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<title>Loop manipulation - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" title="Loop Analysis and Representation">
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<link rel="prev" href="Loop-querying.html#Loop-querying" title="Loop querying">
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<link rel="next" href="LCSSA.html#LCSSA" title="LCSSA">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Loop-manipulation"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="LCSSA.html#LCSSA">LCSSA</a>,
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Previous: <a rel="previous" accesskey="p" href="Loop-querying.html#Loop-querying">Loop querying</a>,
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Up: <a rel="up" accesskey="u" href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation">Loop Analysis and Representation</a>
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<hr>
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</div>
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<h3 class="section">15.3 Loop manipulation</h3>
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<p><a name="index-Loop-manipulation-3232"></a>
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The loops tree can be manipulated using the following functions:
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<ul>
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<li><code>flow_loop_tree_node_add</code>: Adds a node to the tree.
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<li><code>flow_loop_tree_node_remove</code>: Removes a node from the tree.
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<li><code>add_bb_to_loop</code>: Adds a basic block to a loop.
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<li><code>remove_bb_from_loops</code>: Removes a basic block from loops.
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</ul>
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<p>Most low-level CFG functions update loops automatically. The following
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functions handle some more complicated cases of CFG manipulations:
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<ul>
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<li><code>remove_path</code>: Removes an edge and all blocks it dominates.
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<li><code>split_loop_exit_edge</code>: Splits exit edge of the loop,
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ensuring that PHI node arguments remain in the loop (this ensures that
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loop-closed SSA form is preserved). Only useful on GIMPLE.
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</ul>
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<p>Finally, there are some higher-level loop transformations implemented.
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While some of them are written so that they should work on non-innermost
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loops, they are mostly untested in that case, and at the moment, they
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are only reliable for the innermost loops:
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<ul>
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<li><code>create_iv</code>: Creates a new induction variable. Only works on
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GIMPLE. <code>standard_iv_increment_position</code> can be used to find a
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suitable place for the iv increment.
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<li><code>duplicate_loop_to_header_edge</code>,
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<code>tree_duplicate_loop_to_header_edge</code>: These functions (on RTL and
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on GIMPLE) duplicate the body of the loop prescribed number of times on
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one of the edges entering loop header, thus performing either loop
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unrolling or loop peeling. <code>can_duplicate_loop_p</code>
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(<code>can_unroll_loop_p</code> on GIMPLE) must be true for the duplicated
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loop.
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<li><code>loop_version</code>, <code>tree_ssa_loop_version</code>: These function
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create a copy of a loop, and a branch before them that selects one of
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them depending on the prescribed condition. This is useful for
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optimizations that need to verify some assumptions in runtime (one of
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the copies of the loop is usually left unchanged, while the other one is
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transformed in some way).
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<li><code>tree_unroll_loop</code>: Unrolls the loop, including peeling the
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extra iterations to make the number of iterations divisible by unroll
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factor, updating the exit condition, and removing the exits that now
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cannot be taken. Works only on GIMPLE.
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</ul>
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</body></html>
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