You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
115 lines
5.7 KiB
HTML
115 lines
5.7 KiB
HTML
4 years ago
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Accessors - GNU Compiler Collection (GCC) Internals</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="GNU Compiler Collection (GCC) Internals">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="up" href="RTL.html#RTL" title="RTL">
|
||
|
|
<link rel="prev" href="RTL-Classes.html#RTL-Classes" title="RTL Classes">
|
||
|
|
<link rel="next" href="Special-Accessors.html#Special-Accessors" title="Special Accessors">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<!--
|
||
|
|
Copyright (C) 1988-2015 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with the
|
||
|
|
Invariant Sections being ``Funding Free Software'', the Front-Cover
|
||
|
|
Texts being (a) (see below), and with the Back-Cover Texts being (b)
|
||
|
|
(see below). A copy of the license is included in the section entitled
|
||
|
|
``GNU Free Documentation License''.
|
||
|
|
|
||
|
|
(a) The FSF's Front-Cover Text is:
|
||
|
|
|
||
|
|
A GNU Manual
|
||
|
|
|
||
|
|
(b) The FSF's Back-Cover Text is:
|
||
|
|
|
||
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
||
|
|
software. Copies published by the Free Software Foundation raise
|
||
|
|
funds for GNU development.-->
|
||
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
||
|
|
<style type="text/css"><!--
|
||
|
|
pre.display { font-family:inherit }
|
||
|
|
pre.format { font-family:inherit }
|
||
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallexample { font-size:smaller }
|
||
|
|
pre.smalllisp { font-size:smaller }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="Accessors"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="Special-Accessors.html#Special-Accessors">Special Accessors</a>,
|
||
|
|
Previous: <a rel="previous" accesskey="p" href="RTL-Classes.html#RTL-Classes">RTL Classes</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="RTL.html#RTL">RTL</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h3 class="section">13.3 Access to Operands</h3>
|
||
|
|
|
||
|
|
<p><a name="index-accessors-2557"></a><a name="index-access-to-operands-2558"></a><a name="index-operand-access-2559"></a>
|
||
|
|
<a name="index-XEXP-2560"></a><a name="index-XINT-2561"></a><a name="index-XWINT-2562"></a><a name="index-XSTR-2563"></a>Operands of expressions are accessed using the macros <code>XEXP</code>,
|
||
|
|
<code>XINT</code>, <code>XWINT</code> and <code>XSTR</code>. Each of these macros takes
|
||
|
|
two arguments: an expression-pointer (RTX) and an operand number
|
||
|
|
(counting from zero). Thus,
|
||
|
|
|
||
|
|
<pre class="smallexample"> XEXP (<var>x</var>, 2)
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">accesses operand 2 of expression <var>x</var>, as an expression.
|
||
|
|
|
||
|
|
<pre class="smallexample"> XINT (<var>x</var>, 2)
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">accesses the same operand as an integer. <code>XSTR</code>, used in the same
|
||
|
|
fashion, would access it as a string.
|
||
|
|
|
||
|
|
<p>Any operand can be accessed as an integer, as an expression or as a string.
|
||
|
|
You must choose the correct method of access for the kind of value actually
|
||
|
|
stored in the operand. You would do this based on the expression code of
|
||
|
|
the containing expression. That is also how you would know how many
|
||
|
|
operands there are.
|
||
|
|
|
||
|
|
<p>For example, if <var>x</var> is a <code>subreg</code> expression, you know that it has
|
||
|
|
two operands which can be correctly accessed as <code>XEXP (</code><var>x</var><code>, 0)</code>
|
||
|
|
and <code>XINT (</code><var>x</var><code>, 1)</code>. If you did <code>XINT (</code><var>x</var><code>, 0)</code>, you
|
||
|
|
would get the address of the expression operand but cast as an integer;
|
||
|
|
that might occasionally be useful, but it would be cleaner to write
|
||
|
|
<code>(int) XEXP (</code><var>x</var><code>, 0)</code>. <code>XEXP (</code><var>x</var><code>, 1)</code> would also
|
||
|
|
compile without error, and would return the second, integer operand cast as
|
||
|
|
an expression pointer, which would probably result in a crash when
|
||
|
|
accessed. Nothing stops you from writing <code>XEXP (</code><var>x</var><code>, 28)</code> either,
|
||
|
|
but this will access memory past the end of the expression with
|
||
|
|
unpredictable results.
|
||
|
|
|
||
|
|
<p>Access to operands which are vectors is more complicated. You can use the
|
||
|
|
macro <code>XVEC</code> to get the vector-pointer itself, or the macros
|
||
|
|
<code>XVECEXP</code> and <code>XVECLEN</code> to access the elements and length of a
|
||
|
|
vector.
|
||
|
|
|
||
|
|
|
||
|
|
<a name="index-XVEC-2564"></a>
|
||
|
|
<dl><dt><code>XVEC (</code><var>exp</var><code>, </code><var>idx</var><code>)</code><dd>Access the vector-pointer which is operand number <var>idx</var> in <var>exp</var>.
|
||
|
|
|
||
|
|
<p><a name="index-XVECLEN-2565"></a><br><dt><code>XVECLEN (</code><var>exp</var><code>, </code><var>idx</var><code>)</code><dd>Access the length (number of elements) in the vector which is
|
||
|
|
in operand number <var>idx</var> in <var>exp</var>. This value is an <code>int</code>.
|
||
|
|
|
||
|
|
<p><a name="index-XVECEXP-2566"></a><br><dt><code>XVECEXP (</code><var>exp</var><code>, </code><var>idx</var><code>, </code><var>eltnum</var><code>)</code><dd>Access element number <var>eltnum</var> in the vector which is
|
||
|
|
in operand number <var>idx</var> in <var>exp</var>. This value is an RTX.
|
||
|
|
|
||
|
|
<p>It is up to you to make sure that <var>eltnum</var> is not negative
|
||
|
|
and is less than <code>XVECLEN (</code><var>exp</var><code>, </code><var>idx</var><code>)</code>.
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<p>All the macros defined in this section expand into lvalues and therefore
|
||
|
|
can be used to assign the operands, lengths and vector elements as well as
|
||
|
|
to access them.
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|