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96 lines
4.5 KiB
HTML
96 lines
4.5 KiB
HTML
4 years ago
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<title>LCSSA - 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-manipulation.html#Loop-manipulation" title="Loop manipulation">
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<link rel="next" href="Scalar-evolutions.html#Scalar-evolutions" title="Scalar evolutions">
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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="LCSSA"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Scalar-evolutions.html#Scalar-evolutions">Scalar evolutions</a>,
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Previous: <a rel="previous" accesskey="p" href="Loop-manipulation.html#Loop-manipulation">Loop manipulation</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">15.4 Loop-closed SSA form</h3>
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<p><a name="index-LCSSA-3233"></a><a name="index-Loop_002dclosed-SSA-form-3234"></a>
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Throughout the loop optimizations on tree level, one extra condition is
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enforced on the SSA form: No SSA name is used outside of the loop in
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that it is defined. The SSA form satisfying this condition is called
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“loop-closed SSA form” – LCSSA. To enforce LCSSA, PHI nodes must be
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created at the exits of the loops for the SSA names that are used
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outside of them. Only the real operands (not virtual SSA names) are
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held in LCSSA, in order to save memory.
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<p>There are various benefits of LCSSA:
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<ul>
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<li>Many optimizations (value range analysis, final value
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replacement) are interested in the values that are defined in the loop
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and used outside of it, i.e., exactly those for that we create new PHI
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nodes.
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<li>In induction variable analysis, it is not necessary to specify the
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loop in that the analysis should be performed – the scalar evolution
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analysis always returns the results with respect to the loop in that the
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SSA name is defined.
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<li>It makes updating of SSA form during loop transformations simpler.
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Without LCSSA, operations like loop unrolling may force creation of PHI
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nodes arbitrarily far from the loop, while in LCSSA, the SSA form can be
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updated locally. However, since we only keep real operands in LCSSA, we
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cannot use this advantage (we could have local updating of real
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operands, but it is not much more efficient than to use generic SSA form
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updating for it as well; the amount of changes to SSA is the same).
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</ul>
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<p>However, it also means LCSSA must be updated. This is usually
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straightforward, unless you create a new value in loop and use it
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outside, or unless you manipulate loop exit edges (functions are
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provided to make these manipulations simple).
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<code>rewrite_into_loop_closed_ssa</code> is used to rewrite SSA form to
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LCSSA, and <code>verify_loop_closed_ssa</code> to check that the invariant of
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LCSSA is preserved.
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</body></html>
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