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122 lines
6.5 KiB
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122 lines
6.5 KiB
HTML
4 years ago
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<title>Constant expressions - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Expression-trees.html#Expression-trees" title="Expression trees">
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<link rel="next" href="Storage-References.html#Storage-References" title="Storage References">
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Copyright (C) 1988-2015 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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(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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</head>
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<body>
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<div class="node">
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<a name="Constant-expressions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Storage-References.html#Storage-References">Storage References</a>,
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Up: <a rel="up" accesskey="u" href="Expression-trees.html#Expression-trees">Expression trees</a>
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<hr>
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</div>
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<h4 class="subsection">10.6.1 Constant expressions</h4>
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<p><a name="index-INTEGER_005fCST-1824"></a><a name="index-tree_005fint_005fcst_005flt-1825"></a><a name="index-tree_005fint_005fcst_005fequal-1826"></a><a name="index-tree_005ffits_005fuhwi_005fp-1827"></a><a name="index-tree_005ffits_005fshwi_005fp-1828"></a><a name="index-tree_005fto_005fuhwi-1829"></a><a name="index-tree_005fto_005fshwi-1830"></a><a name="index-TREE_005fINT_005fCST_005fNUNITS-1831"></a><a name="index-TREE_005fINT_005fCST_005fELT-1832"></a><a name="index-TREE_005fINT_005fCST_005fLOW-1833"></a><a name="index-REAL_005fCST-1834"></a><a name="index-FIXED_005fCST-1835"></a><a name="index-COMPLEX_005fCST-1836"></a><a name="index-VECTOR_005fCST-1837"></a><a name="index-STRING_005fCST-1838"></a><a name="index-TREE_005fSTRING_005fLENGTH-1839"></a><a name="index-TREE_005fSTRING_005fPOINTER-1840"></a>
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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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<dl>
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<dt><code>INTEGER_CST</code><dd>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>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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<br><dt><code>REAL_CST</code><dd>
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FIXME: Talk about how to obtain representations of this constant, do
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comparisons, and so forth.
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<br><dt><code>FIXED_CST</code><dd>
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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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<br><dt><code>COMPLEX_CST</code><dd>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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<br><dt><code>VECTOR_CST</code><dd>These nodes are used to represent vector constants, whose parts are
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constant nodes. Each individual constant node is either an integer or a
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double constant node. The first operand is a <code>TREE_LIST</code> of the
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constant nodes and is accessed through <code>TREE_VECTOR_CST_ELTS</code>.
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<br><dt><code>STRING_CST</code><dd>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>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>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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</dl>
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</body></html>
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