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121 lines
5.9 KiB
HTML
121 lines
5.9 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This manual describes how to install and use the GNU multiple precision
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arithmetic library, version 6.1.0.
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Copyright 1991, 1993-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document under
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the terms of the GNU Free Documentation License, Version 1.3 or any later
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version published by the Free Software Foundation; with no Invariant Sections,
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with the Front-Cover Texts being "A GNU Manual", and with the Back-Cover
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Texts being "You have freedom to copy and modify this GNU Manual, like GNU
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software". A copy of the license is included in
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GNU Free Documentation License. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Assembly Loop Unrolling (GNU MP 6.1.0)</title>
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<meta name="description" content="How to install and use the GNU multiple precision arithmetic library, version 6.1.0.">
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<meta name="keywords" content="Assembly Loop Unrolling (GNU MP 6.1.0)">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="Assembly-Coding.html#Assembly-Coding" rel="up" title="Assembly Coding">
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<link href="Assembly-Writing-Guide.html#Assembly-Writing-Guide" rel="next" title="Assembly Writing Guide">
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<link href="Assembly-Software-Pipelining.html#Assembly-Software-Pipelining" rel="prev" title="Assembly Software Pipelining">
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</style>
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</head>
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<body lang="en">
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<a name="Assembly-Loop-Unrolling"></a>
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<div class="header">
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<p>
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Next: <a href="Assembly-Writing-Guide.html#Assembly-Writing-Guide" accesskey="n" rel="next">Assembly Writing Guide</a>, Previous: <a href="Assembly-Software-Pipelining.html#Assembly-Software-Pipelining" accesskey="p" rel="prev">Assembly Software Pipelining</a>, Up: <a href="Assembly-Coding.html#Assembly-Coding" accesskey="u" rel="up">Assembly Coding</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Loop-Unrolling"></a>
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<h4 class="subsection">15.8.9 Loop Unrolling</h4>
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<a name="index-Assembly-loop-unrolling"></a>
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<p>Loop unrolling consists of replicating code so that several limbs are
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processed in each loop. At a minimum this reduces loop overheads by a
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corresponding factor, but it can also allow better register usage, for example
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alternately using one register combination and then another. Judicious use of
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<code>m4</code> macros can help avoid lots of duplication in the source code.
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</p>
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<p>Any amount of unrolling can be handled with a loop counter that’s decremented
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by <em>N</em> each time, stopping when the remaining count is less than the
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further <em>N</em> the loop will process. Or by subtracting <em>N</em> at the
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start, the termination condition becomes when the counter <em>C</em> is less
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than 0 (and the count of remaining limbs is <em>C+N</em>).
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</p>
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<p>Alternately for a power of 2 unroll the loop count and remainder can be
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established with a shift and mask. This is convenient if also making a
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computed jump into the middle of a large loop.
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</p>
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<p>The limbs not a multiple of the unrolling can be handled in various ways, for
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example
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</p>
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<ul>
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<li> A simple loop at the end (or the start) to process the excess. Care will be
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wanted that it isn’t too much slower than the unrolled part.
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</li><li> A set of binary tests, for example after an 8-limb unrolling, test for 4 more
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limbs to process, then a further 2 more or not, and finally 1 more or not.
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This will probably take more code space than a simple loop.
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</li><li> A <code>switch</code> statement, providing separate code for each possible excess,
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for example an 8-limb unrolling would have separate code for 0 remaining, 1
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remaining, etc, up to 7 remaining. This might take a lot of code, but may be
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the best way to optimize all cases in combination with a deep pipelined loop.
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</li><li> A computed jump into the middle of the loop, thus making the first iteration
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handle the excess. This should make times smoothly increase with size, which
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is attractive, but setups for the jump and adjustments for pointers can be
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tricky and could become quite difficult in combination with deep pipelining.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Assembly-Writing-Guide.html#Assembly-Writing-Guide" accesskey="n" rel="next">Assembly Writing Guide</a>, Previous: <a href="Assembly-Software-Pipelining.html#Assembly-Software-Pipelining" accesskey="p" rel="prev">Assembly Software Pipelining</a>, Up: <a href="Assembly-Coding.html#Assembly-Coding" accesskey="u" rel="up">Assembly Coding</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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