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104 lines
4.9 KiB
HTML
104 lines
4.9 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 Functional Units (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 Functional Units (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-Floating-Point.html#Assembly-Floating-Point" rel="next" title="Assembly Floating Point">
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<link href="Assembly-Cache-Handling.html#Assembly-Cache-Handling" rel="prev" title="Assembly Cache Handling">
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<style type="text/css">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Assembly-Functional-Units"></a>
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<div class="header">
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<p>
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Next: <a href="Assembly-Floating-Point.html#Assembly-Floating-Point" accesskey="n" rel="next">Assembly Floating Point</a>, Previous: <a href="Assembly-Cache-Handling.html#Assembly-Cache-Handling" accesskey="p" rel="prev">Assembly Cache Handling</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="Functional-Units"></a>
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<h4 class="subsection">15.8.5 Functional Units</h4>
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<p>When choosing an approach for an assembly loop, consideration is given to
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what operations can execute simultaneously and what throughput can thereby be
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achieved. In some cases an algorithm can be tweaked to accommodate available
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resources.
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</p>
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<p>Loop control will generally require a counter and pointer updates, costing as
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much as 5 instructions, plus any delays a branch introduces. CPU addressing
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modes might reduce pointer updates, perhaps by allowing just one updating
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pointer and others expressed as offsets from it, or on CISC chips with all
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addressing done with the loop counter as a scaled index.
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</p>
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<p>The final loop control cost can be amortised by processing several limbs in
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each iteration (see <a href="Assembly-Loop-Unrolling.html#Assembly-Loop-Unrolling">Assembly Loop Unrolling</a>). This at least ensures loop
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control isn’t a big fraction the work done.
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</p>
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<p>Memory throughput is always a limit. If perhaps only one load or one store
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can be done per cycle then 3 cycles/limb will the top speed for “binary”
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operations like <code>mpn_add_n</code>, and any code achieving that is optimal.
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</p>
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<p>Integer resources can be freed up by having the loop counter in a float
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register, or by pressing the float units into use for some multiplying,
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perhaps doing every second limb on the float side (see <a href="Assembly-Floating-Point.html#Assembly-Floating-Point">Assembly Floating Point</a>).
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</p>
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<p>Float resources can be freed up by doing carry propagation on the integer
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side, or even by doing integer to float conversions in integers using bit
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twiddling.
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</p>
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</body>
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</html>
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