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132 lines
5.8 KiB
HTML
132 lines
5.8 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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<!-- Copyright (C) 1988-2018 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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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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funds for GNU development. -->
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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>Consequences of using poly_int (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Consequences of using poly_int (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Consequences of using poly_int (GNU Compiler Collection (GCC) Internals)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="poly_005fint.html#poly_005fint" rel="up" title="poly_int">
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<link href="Comparisons-involving-poly_005fint.html#Comparisons-involving-poly_005fint" rel="next" title="Comparisons involving poly_int">
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<link href="Overview-of-poly_005fint.html#Overview-of-poly_005fint" rel="prev" title="Overview of poly_int">
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</head>
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<body lang="en">
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<a name="Consequences-of-using-poly_005fint"></a>
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<div class="header">
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<p>
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Next: <a href="Comparisons-involving-poly_005fint.html#Comparisons-involving-poly_005fint" accesskey="n" rel="next">Comparisons involving <code>poly_int</code></a>, Previous: <a href="Overview-of-poly_005fint.html#Overview-of-poly_005fint" accesskey="p" rel="prev">Overview of <code>poly_int</code></a>, Up: <a href="poly_005fint.html#poly_005fint" accesskey="u" rel="up">poly_int</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Consequences-of-using-poly_005fint-1"></a>
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<h3 class="section">10.2 Consequences of using <code>poly_int</code></h3>
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<p>The two main consequences of using polynomial sizes and offsets are that:
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</p>
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<ul>
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<li> there is no total ordering between the values at compile time, and
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</li><li> some operations might yield results that cannot be expressed as a
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<code>poly_int</code>.
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</li></ul>
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<p>For example, if <var>x</var> is a runtime invariant, we cannot tell at
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compile time whether:
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</p>
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<div class="smallexample">
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<pre class="smallexample">3 + 4<var>x</var> <= 1 + 5<var>x</var>
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</pre></div>
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<p>since the condition is false when <var>x</var> <= 1 and true when <var>x</var> >= 2.
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</p>
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<p>Similarly, <code>poly_int</code> cannot represent the result of:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(3 + 4<var>x</var>) * (1 + 5<var>x</var>)
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</pre></div>
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<p>since it cannot (and in practice does not need to) store powers greater
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than one. It also cannot represent the result of:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(3 + 4<var>x</var>) / (1 + 5<var>x</var>)
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</pre></div>
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<p>The following sections describe how we deal with these restrictions.
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</p>
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<a name="index-poly_005fint_002c-use-in-target_002dindependent-code"></a>
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<p>As described earlier, a <code>poly_int<1, <var>T</var>></code> has no indeterminates
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and so degenerates to a compile-time constant of type <var>T</var>. It would
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be possible in that case to do all normal arithmetic on the <var>T</var>,
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and to compare the <var>T</var> using the normal C++ operators. We deliberately
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prevent target-independent code from doing this, since the compiler needs
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to support other <code>poly_int<<var>n</var>, <var>T</var>></code> as well, regardless of
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the current target’s <code>NUM_POLY_INT_COEFFS</code>.
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</p>
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<a name="index-poly_005fint_002c-use-in-target_002dspecific-code"></a>
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<p>However, it would be very artificial to force target-specific code
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to follow these restrictions if the target has no runtime indeterminates.
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There is therefore an implicit conversion from <code>poly_int<1, <var>T</var>></code>
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to <var>T</var> when compiling target-specific translation units.
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</p>
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</body>
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</html>
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