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261 lines
12 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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<!-- 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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Constant expressions (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Constant expressions (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Constant expressions (GNU Compiler Collection (GCC) Internals)">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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="Expression-trees.html#Expression-trees" rel="up" title="Expression trees">
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<link href="Storage-References.html#Storage-References" rel="next" title="Storage References">
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<link href="Expression-trees.html#Expression-trees" rel="prev" title="Expression trees">
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</style>
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</head>
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<body lang="en">
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<a name="Constant-expressions"></a>
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<div class="header">
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<p>
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Next: <a href="Storage-References.html#Storage-References" accesskey="n" rel="next">Storage References</a>, Up: <a href="Expression-trees.html#Expression-trees" accesskey="u" rel="up">Expression trees</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="Constant-expressions-1"></a>
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<h4 class="subsection">11.6.1 Constant expressions</h4>
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<a name="index-INTEGER_005fCST"></a>
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<a name="index-tree_005fint_005fcst_005flt"></a>
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<a name="index-tree_005fint_005fcst_005fequal"></a>
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<a name="index-tree_005ffits_005fuhwi_005fp"></a>
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<a name="index-tree_005ffits_005fshwi_005fp"></a>
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<a name="index-tree_005fto_005fuhwi"></a>
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<a name="index-tree_005fto_005fshwi"></a>
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<a name="index-TREE_005fINT_005fCST_005fNUNITS"></a>
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<a name="index-TREE_005fINT_005fCST_005fELT"></a>
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<a name="index-TREE_005fINT_005fCST_005fLOW"></a>
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<a name="index-REAL_005fCST"></a>
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<a name="index-FIXED_005fCST"></a>
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<a name="index-COMPLEX_005fCST"></a>
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<a name="index-VECTOR_005fCST"></a>
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<a name="index-STRING_005fCST"></a>
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<a name="index-POLY_005fINT_005fCST"></a>
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<a name="index-TREE_005fSTRING_005fLENGTH"></a>
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<a name="index-TREE_005fSTRING_005fPOINTER"></a>
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<p>The table below begins with constants, moves on to unary expressions,
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then proceeds to binary expressions, and concludes with various other
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kinds of expressions:
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</p>
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<dl compact="compact">
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<dt><code>INTEGER_CST</code></dt>
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<dd><p>These nodes represent integer constants. Note that the type of these
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constants is obtained with <code>TREE_TYPE</code>; they are not always of type
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<code>int</code>. In particular, <code>char</code> constants are represented with
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<code>INTEGER_CST</code> nodes. The value of the integer constant <code>e</code> is
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represented in an array of HOST_WIDE_INT. There are enough elements
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in the array to represent the value without taking extra elements for
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redundant 0s or -1. The number of elements used to represent <code>e</code>
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is available via <code>TREE_INT_CST_NUNITS</code>. Element <code>i</code> can be
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extracted by using <code>TREE_INT_CST_ELT (e, i)</code>.
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<code>TREE_INT_CST_LOW</code> is a shorthand for <code>TREE_INT_CST_ELT (e, 0)</code>.
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</p>
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<p>The functions <code>tree_fits_shwi_p</code> and <code>tree_fits_uhwi_p</code>
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can be used to tell if the value is small enough to fit in a
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signed HOST_WIDE_INT or an unsigned HOST_WIDE_INT respectively.
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The value can then be extracted using <code>tree_to_shwi</code> and
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<code>tree_to_uhwi</code>.
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</p>
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</dd>
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<dt><code>REAL_CST</code></dt>
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<dd>
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<p>FIXME: Talk about how to obtain representations of this constant, do
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comparisons, and so forth.
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</p>
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</dd>
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<dt><code>FIXED_CST</code></dt>
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<dd>
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<p>These nodes represent fixed-point constants. The type of these constants
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is obtained with <code>TREE_TYPE</code>. <code>TREE_FIXED_CST_PTR</code> points to
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a <code>struct fixed_value</code>; <code>TREE_FIXED_CST</code> returns the structure
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itself. <code>struct fixed_value</code> contains <code>data</code> with the size of two
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<code>HOST_BITS_PER_WIDE_INT</code> and <code>mode</code> as the associated fixed-point
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machine mode for <code>data</code>.
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</p>
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</dd>
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<dt><code>COMPLEX_CST</code></dt>
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<dd><p>These nodes are used to represent complex number constants, that is a
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<code>__complex__</code> whose parts are constant nodes. The
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<code>TREE_REALPART</code> and <code>TREE_IMAGPART</code> return the real and the
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imaginary parts respectively.
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</p>
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</dd>
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<dt><code>VECTOR_CST</code></dt>
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<dd><p>These nodes are used to represent vector constants. Each vector
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constant <var>v</var> is treated as a specific instance of an arbitrary-length
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sequence that itself contains ‘<samp>VECTOR_CST_NPATTERNS (<var>v</var>)</samp>’
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interleaved patterns. Each pattern has the form:
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</p>
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<div class="smallexample">
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<pre class="smallexample">{ <var>base0</var>, <var>base1</var>, <var>base1</var> + <var>step</var>, <var>base1</var> + <var>step</var> * 2, … }
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</pre></div>
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<p>The first three elements in each pattern are enough to determine the
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values of the other elements. However, if all <var>step</var>s are zero,
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only the first two elements are needed. If in addition each <var>base1</var>
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is equal to the corresponding <var>base0</var>, only the first element in
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each pattern is needed. The number of encoded elements per pattern
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is given by ‘<samp>VECTOR_CST_NELTS_PER_PATTERN (<var>v</var>)</samp>’.
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</p>
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<p>For example, the constant:
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</p>
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<div class="smallexample">
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<pre class="smallexample">{ 0, 1, 2, 6, 3, 8, 4, 10, 5, 12, 6, 14, 7, 16, 8, 18 }
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</pre></div>
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<p>is interpreted as an interleaving of the sequences:
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</p>
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<div class="smallexample">
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<pre class="smallexample">{ 0, 2, 3, 4, 5, 6, 7, 8 }
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{ 1, 6, 8, 10, 12, 14, 16, 18 }
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</pre></div>
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<p>where the sequences are represented by the following patterns:
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</p>
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<div class="smallexample">
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<pre class="smallexample"><var>base0</var> == 0, <var>base1</var> == 2, <var>step</var> == 1
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<var>base0</var> == 1, <var>base1</var> == 6, <var>step</var> == 2
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</pre></div>
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<p>In this case:
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</p>
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<div class="smallexample">
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<pre class="smallexample">VECTOR_CST_NPATTERNS (<var>v</var>) == 2
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VECTOR_CST_NELTS_PER_PATTERN (<var>v</var>) == 3
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</pre></div>
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<p>The vector is therefore encoded using the first 6 elements
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(‘<samp>{ 0, 1, 2, 6, 3, 8 }</samp>’), with the remaining 10 elements
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being implicit extensions of them.
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</p>
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<p>Sometimes this scheme can create two possible encodings of the same
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vector. For example { 0, 1 } could be seen as two patterns with
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one element each or one pattern with two elements (<var>base0</var> and
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<var>base1</var>). The canonical encoding is always the one with the
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fewest patterns or (if both encodings have the same number of
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petterns) the one with the fewest encoded elements.
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</p>
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<p>‘<samp>vector_cst_encoding_nelts (<var>v</var>)</samp>’ gives the total number of
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encoded elements in <var>v</var>, which is 6 in the example above.
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<code>VECTOR_CST_ENCODED_ELTS (<var>v</var>)</code> gives a pointer to the elements
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encoded in <var>v</var> and <code>VECTOR_CST_ENCODED_ELT (<var>v</var>, <var>i</var>)</code>
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accesses the value of encoded element <var>i</var>.
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</p>
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<p>‘<samp>VECTOR_CST_DUPLICATE_P (<var>v</var>)</samp>’ is true if <var>v</var> simply contains
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repeated instances of ‘<samp>VECTOR_CST_NPATTERNS (<var>v</var>)</samp>’ values. This is
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a shorthand for testing ‘<samp>VECTOR_CST_NELTS_PER_PATTERN (<var>v</var>) == 1</samp>’.
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</p>
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<p>‘<samp>VECTOR_CST_STEPPED_P (<var>v</var>)</samp>’ is true if at least one
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pattern in <var>v</var> has a nonzero step. This is a shorthand for
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testing ‘<samp>VECTOR_CST_NELTS_PER_PATTERN (<var>v</var>) == 3</samp>’.
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</p>
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<p>The utility function <code>vector_cst_elt</code> gives the value of an
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arbitrary index as a <code>tree</code>. <code>vector_cst_int_elt</code> gives
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the same value as a <code>wide_int</code>.
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</p>
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</dd>
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<dt><code>STRING_CST</code></dt>
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<dd><p>These nodes represent string-constants. The <code>TREE_STRING_LENGTH</code>
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returns the length of the string, as an <code>int</code>. The
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<code>TREE_STRING_POINTER</code> is a <code>char*</code> containing the string
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itself. The string may not be <code>NUL</code>-terminated, and it may contain
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embedded <code>NUL</code> characters. Therefore, the
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<code>TREE_STRING_LENGTH</code> includes the trailing <code>NUL</code> if it is
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present.
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</p>
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<p>For wide string constants, the <code>TREE_STRING_LENGTH</code> is the number
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of bytes in the string, and the <code>TREE_STRING_POINTER</code>
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points to an array of the bytes of the string, as represented on the
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target system (that is, as integers in the target endianness). Wide and
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non-wide string constants are distinguished only by the <code>TREE_TYPE</code>
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of the <code>STRING_CST</code>.
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</p>
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<p>FIXME: The formats of string constants are not well-defined when the
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target system bytes are not the same width as host system bytes.
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</p>
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</dd>
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<dt><code>POLY_INT_CST</code></dt>
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<dd><p>These nodes represent invariants that depend on some target-specific
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runtime parameters. They consist of <code>NUM_POLY_INT_COEFFS</code>
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coefficients, with the first coefficient being the constant term and
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the others being multipliers that are applied to the runtime parameters.
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</p>
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<p><code>POLY_INT_CST_ELT (<var>x</var>, <var>i</var>)</code> references coefficient number
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<var>i</var> of <code>POLY_INT_CST</code> node <var>x</var>. Each coefficient is an
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<code>INTEGER_CST</code>.
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Storage-References.html#Storage-References" accesskey="n" rel="next">Storage References</a>, Up: <a href="Expression-trees.html#Expression-trees" accesskey="u" rel="up">Expression trees</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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</body>
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</html>
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