158 lines
6.9 KiB
HTML
158 lines
6.9 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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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 no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Cheaper Pairs (Guile Reference Manual)</title>
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<meta name="description" content="Cheaper Pairs (Guile Reference Manual)">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Data-Representation.html" rel="up" title="Data Representation">
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<link href="Conservative-GC.html" rel="next" title="Conservative GC">
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<link href="Faster-Integers.html" rel="prev" title="Faster Integers">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Cheaper-Pairs">
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<div class="nav-panel">
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<p>
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Next: <a href="Conservative-GC.html" accesskey="n" rel="next">Conservative Garbage Collection</a>, Previous: <a href="Faster-Integers.html" accesskey="p" rel="prev">Faster Integers</a>, Up: <a href="Data-Representation.html" accesskey="u" rel="up">Data Representation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsection" id="Cheaper-Pairs-1"><span>9.2.3 Cheaper Pairs<a class="copiable-link" href="#Cheaper-Pairs-1"> ¶</a></span></h4>
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<p>However, there is yet another issue to confront. Most Scheme heaps
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contain more pairs than any other type of object; Jonathan Rees said at
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one point that pairs occupy 45% of the heap in his Scheme
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implementation, Scheme 48. However, our representation above spends
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three <code class="code">SCM</code>-sized words per pair — one for the type, and two for
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the <small class="sc">CAR</small> and <small class="sc">CDR</small>. Is there any way to represent pairs using
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only two words?
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</p>
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<p>Let us refine the convention we established earlier. Let us assert
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that:
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</p><ul class="itemize mark-bullet">
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<li>If the bottom three bits of an <code class="code">SCM</code> value are <code class="code">#b000</code>, then
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it is a pointer, as before.
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</li><li>If the bottom three bits are <code class="code">#b001</code>, then the upper bits are an
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integer. This is a bit more restrictive than before.
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</li><li>If the bottom two bits are <code class="code">#b010</code>, then the value, with the bottom
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three bits masked out, is the address of a pair.
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</li></ul>
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<p>Here is the new C code:
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</p><div class="example">
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<pre class="example-preformatted">enum type { string, vector, ... };
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typedef struct value *SCM;
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struct value {
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enum type type;
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union {
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struct { int length; char *elts; } string;
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struct { int length; SCM *elts; } vector;
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...
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} value;
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};
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struct pair {
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SCM car, cdr;
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};
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#define POINTER_P(x) (((int) (x) & 7) == 0)
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#define INTEGER_P(x) (((int) (x) & 7) == 1)
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#define GET_INTEGER(x) ((int) (x) >> 3)
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#define MAKE_INTEGER(x) ((SCM) (((x) << 3) | 1))
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#define PAIR_P(x) (((int) (x) & 7) == 2)
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#define GET_PAIR(x) ((struct pair *) ((int) (x) & ~7))
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</pre></div>
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<p>Notice that <code class="code">enum type</code> and <code class="code">struct value</code> now only contain
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provisions for vectors and strings; both integers and pairs have become
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special cases. The code above also assumes that an <code class="code">int</code> is large
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enough to hold a pointer, which isn’t generally true.
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</p>
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<p>Our list of examples is now as follows:
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</p><ul class="itemize mark-bullet">
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<li>To test if <var class="var">x</var> is an integer, we can write <code class="code">INTEGER_P
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(<var class="var">x</var>)</code>; this is as before.
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</li><li>To find its value, we can write <code class="code">GET_INTEGER (<var class="var">x</var>)</code>, as
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before.
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</li><li>To test if <var class="var">x</var> is a vector, we can write:
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<div class="example">
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<pre class="example-preformatted"> <code class="code">POINTER_P (<var class="var">x</var>) && <var class="var">x</var>->type == vector</code>
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</pre></div>
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<p>We must still make sure that <var class="var">x</var> is a pointer to a <code class="code">struct
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value</code> before dereferencing it to find its type.
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</p></li><li>If we know <var class="var">x</var> is a vector, we can write
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<code class="code"><var class="var">x</var>->value.vector.elts[0]</code> to refer to its first element, as
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before.
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</li><li>We can write <code class="code">PAIR_P (<var class="var">x</var>)</code> to determine if <var class="var">x</var> is a
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pair, and then write <code class="code">GET_PAIR (<var class="var">x</var>)->car</code> to refer to its
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car.
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</li></ul>
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<p>This change in representation reduces our heap size by 15%. It also
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makes it cheaper to decide if a value is a pair, because no memory
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references are necessary; it suffices to check the bottom two bits of
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the <code class="code">SCM</code> value. This may be significant when traversing lists, a
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common activity in a Scheme system.
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</p>
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<p>Again, most real Scheme systems use a slightly different implementation;
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for example, if GET_PAIR subtracts off the low bits of <code class="code">x</code>, instead
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of masking them off, the optimizer will often be able to combine that
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subtraction with the addition of the offset of the structure member we
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are referencing, making a modified pointer as fast to use as an
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unmodified pointer.
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Conservative-GC.html">Conservative Garbage Collection</a>, Previous: <a href="Faster-Integers.html">Faster Integers</a>, Up: <a href="Data-Representation.html">Data Representation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" 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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