171 lines
8.1 KiB
HTML
171 lines
8.1 KiB
HTML
<!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) 1988-2016 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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"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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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: RTL Objects</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: RTL Objects">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: RTL Objects">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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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="RTL.html#RTL" rel="up" title="RTL">
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<link href="RTL-Classes.html#RTL-Classes" rel="next" title="RTL Classes">
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<link href="RTL.html#RTL" rel="prev" title="RTL">
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="RTL-Objects"></a>
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<div class="header">
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<p>
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Next: <a href="RTL-Classes.html#RTL-Classes" accesskey="n" rel="next">RTL Classes</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</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="RTL-Object-Types"></a>
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<h3 class="section">13.1 RTL Object Types</h3>
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<a name="index-RTL-object-types"></a>
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<a name="index-RTL-integers"></a>
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<a name="index-RTL-strings"></a>
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<a name="index-RTL-vectors"></a>
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<a name="index-RTL-expression"></a>
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<a name="index-RTX-_0028See-RTL_0029"></a>
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<p>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>
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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>
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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>
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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>
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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>
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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>[…]</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>
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<a name="index-expression-codes"></a>
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<a name="index-codes_002c-RTL-expression"></a>
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<a name="index-GET_005fCODE"></a>
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<a name="index-PUT_005fCODE"></a>
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<p>Expressions are classified by <em>expression codes</em> (also called RTX
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codes). The expression code is a name defined in <samp>rtl.def</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 (<var>x</var>)</code> and altered with
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<code>PUT_CODE (<var>x</var>, <var>newcode</var>)</code>.
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</p>
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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>
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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>
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<p>Expression code names in the ‘<samp>md</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>
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<a name="index-_0028nil_0029"></a>
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<a name="index-nil"></a>
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<p>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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="RTL-Classes.html#RTL-Classes" accesskey="n" rel="next">RTL Classes</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</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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