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133 lines
6.1 KiB
HTML
133 lines
6.1 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 Writing Guide (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 Writing Guide (GNU MP 6.1.0)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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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="Internals.html#Internals" rel="next" title="Internals">
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<link href="Assembly-Loop-Unrolling.html#Assembly-Loop-Unrolling" rel="prev" title="Assembly Loop Unrolling">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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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-Writing-Guide"></a>
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<div class="header">
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<p>
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Previous: <a href="Assembly-Loop-Unrolling.html#Assembly-Loop-Unrolling" accesskey="p" rel="prev">Assembly Loop Unrolling</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="Writing-Guide"></a>
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<h4 class="subsection">15.8.10 Writing Guide</h4>
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<a name="index-Assembly-writing-guide"></a>
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<p>This is a guide to writing software pipelined loops for processing limb
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vectors in assembly.
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</p>
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<p>First determine the algorithm and which instructions are needed. Code it
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without unrolling or scheduling, to make sure it works. On a 3-operand CPU
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try to write each new value to a new register, this will greatly simplify later
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steps.
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</p>
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<p>Then note for each instruction the functional unit and/or issue port
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requirements. If an instruction can use either of two units, like U0 or U1
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then make a category “U0/U1”. Count the total using each unit (or combined
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unit), and count all instructions.
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</p>
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<p>Figure out from those counts the best possible loop time. The goal will be to
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find a perfect schedule where instruction latencies are completely hidden.
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The total instruction count might be the limiting factor, or perhaps a
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particular functional unit. It might be possible to tweak the instructions to
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help the limiting factor.
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</p>
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<p>Suppose the loop time is <em>N</em>, then make <em>N</em> issue buckets, with the
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final loop branch at the end of the last. Now fill the buckets with dummy
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instructions using the functional units desired. Run this to make sure the
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intended speed is reached.
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</p>
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<p>Now replace the dummy instructions with the real instructions from the slow
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but correct loop you started with. The first will typically be a load
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instruction. Then the instruction using that value is placed in a bucket an
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appropriate distance down. Run the loop again, to check it still runs at
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target speed.
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</p>
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<p>Keep placing instructions, frequently measuring the loop. After a few you
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will need to wrap around from the last bucket back to the top of the loop. If
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you used the new-register for new-value strategy above then there will be no
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register conflicts. If not then take care not to clobber something already in
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use. Changing registers at this time is very error prone.
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</p>
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<p>The loop will overlap two or more of the original loop iterations, and the
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computation of one vector element result will be started in one iteration of
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the new loop, and completed one or several iterations later.
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</p>
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<p>The final step is to create feed-in and wind-down code for the loop. A good
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way to do this is to make a copy (or copies) of the loop at the start and
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delete those instructions which don’t have valid antecedents, and at the end
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replicate and delete those whose results are unwanted (including any further
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loads).
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</p>
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<p>The loop will have a minimum number of limbs loaded and processed, so the
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feed-in code must test if the request size is smaller and skip either to a
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suitable part of the wind-down or to special code for small sizes.
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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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Previous: <a href="Assembly-Loop-Unrolling.html#Assembly-Loop-Unrolling" accesskey="p" rel="prev">Assembly Loop Unrolling</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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