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127 lines
6.7 KiB
HTML
127 lines
6.7 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>RTL Objects - GNU Compiler Collection (GCC) Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="RTL.html#RTL" title="RTL">
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<link rel="next" href="RTL-Classes.html#RTL-Classes" title="RTL Classes">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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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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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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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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<style type="text/css"><!--
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span.sansserif { font-family:sans-serif; font-weight:normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="RTL-Objects"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="RTL-Classes.html#RTL-Classes">RTL Classes</a>,
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Up: <a rel="up" accesskey="u" href="RTL.html#RTL">RTL</a>
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<hr>
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</div>
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<h3 class="section">13.1 RTL Object Types</h3>
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<p><a name="index-RTL-object-types-2537"></a>
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<a name="index-RTL-integers-2538"></a><a name="index-RTL-strings-2539"></a><a name="index-RTL-vectors-2540"></a><a name="index-RTL-expression-2541"></a><a name="index-RTX-_0028See-RTL_0029-2542"></a>RTL uses five kinds of objects: expressions, integers, wide integers,
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strings and vectors. Expressions are the most important ones. An RTL
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expression (“RTX”, for short) is a C structure, but it is usually
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referred to with a pointer; a type that is given the typedef name
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<code>rtx</code>.
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<p>An integer is simply an <code>int</code>; their written form uses decimal
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digits. A wide integer is an integral object whose type is
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<code>HOST_WIDE_INT</code>; their written form uses decimal digits.
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<p>A string is a sequence of characters. In core it is represented as a
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<code>char *</code> in usual C fashion, and it is written in C syntax as well.
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However, strings in RTL may never be null. If you write an empty string in
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a machine description, it is represented in core as a null pointer rather
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than as a pointer to a null character. In certain contexts, these null
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pointers instead of strings are valid. Within RTL code, strings are most
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commonly found inside <code>symbol_ref</code> expressions, but they appear in
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other contexts in the RTL expressions that make up machine descriptions.
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<p>In a machine description, strings are normally written with double
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quotes, as you would in C. However, strings in machine descriptions may
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extend over many lines, which is invalid C, and adjacent string
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constants are not concatenated as they are in C. Any string constant
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may be surrounded with a single set of parentheses. Sometimes this
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makes the machine description easier to read.
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<p>There is also a special syntax for strings, which can be useful when C
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code is embedded in a machine description. Wherever a string can
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appear, it is also valid to write a C-style brace block. The entire
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brace block, including the outermost pair of braces, is considered to be
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the string constant. Double quote characters inside the braces are not
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special. Therefore, if you write string constants in the C code, you
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need not escape each quote character with a backslash.
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<p>A vector contains an arbitrary number of pointers to expressions. The
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number of elements in the vector is explicitly present in the vector.
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The written form of a vector consists of square brackets
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(‘<samp><span class="samp">[...]</span></samp>’) surrounding the elements, in sequence and with
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whitespace separating them. Vectors of length zero are not created;
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null pointers are used instead.
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<p><a name="index-expression-codes-2543"></a><a name="index-codes_002c-RTL-expression-2544"></a><a name="index-GET_005fCODE-2545"></a><a name="index-PUT_005fCODE-2546"></a>Expressions are classified by <dfn>expression codes</dfn> (also called RTX
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codes). The expression code is a name defined in <samp><span class="file">rtl.def</span></samp>, which is
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also (in uppercase) a C enumeration constant. The possible expression
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codes and their meanings are machine-independent. The code of an RTX can
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be extracted with the macro <code>GET_CODE (</code><var>x</var><code>)</code> and altered with
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<code>PUT_CODE (</code><var>x</var><code>, </code><var>newcode</var><code>)</code>.
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<p>The expression code determines how many operands the expression contains,
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and what kinds of objects they are. In RTL, unlike Lisp, you cannot tell
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by looking at an operand what kind of object it is. Instead, you must know
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from its context—from the expression code of the containing expression.
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For example, in an expression of code <code>subreg</code>, the first operand is
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to be regarded as an expression and the second operand as an integer. In
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an expression of code <code>plus</code>, there are two operands, both of which
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are to be regarded as expressions. In a <code>symbol_ref</code> expression,
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there is one operand, which is to be regarded as a string.
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<p>Expressions are written as parentheses containing the name of the
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expression type, its flags and machine mode if any, and then the operands
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of the expression (separated by spaces).
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<p>Expression code names in the ‘<samp><span class="samp">md</span></samp>’ file are written in lowercase,
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but when they appear in C code they are written in uppercase. In this
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manual, they are shown as follows: <code>const_int</code>.
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<p><a name="index-g_t_0028nil_0029-2547"></a><a name="index-nil-2548"></a>In a few contexts a null pointer is valid where an expression is normally
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wanted. The written form of this is <code>(nil)</code>.
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</body></html>
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