296 lines
14 KiB
HTML
296 lines
14 KiB
HTML
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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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<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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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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<!-- 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: Constants</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Constants">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Constants">
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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="Regs-and-Memory.html#Regs-and-Memory" rel="next" title="Regs and Memory">
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<link href="Machine-Modes.html#Machine-Modes" rel="prev" title="Machine Modes">
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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="Constants"></a>
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<div class="header">
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<p>
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Next: <a href="Regs-and-Memory.html#Regs-and-Memory" accesskey="n" rel="next">Regs and Memory</a>, Previous: <a href="Machine-Modes.html#Machine-Modes" accesskey="p" rel="prev">Machine Modes</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="Constant-Expression-Types"></a>
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<h3 class="section">13.7 Constant Expression Types</h3>
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<a name="index-RTL-constants"></a>
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<a name="index-RTL-constant-expression-types"></a>
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<p>The simplest RTL expressions are those that represent constant values.
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</p>
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<dl compact="compact">
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<dd><a name="index-const_005fint"></a>
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</dd>
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<dt><code>(const_int <var>i</var>)</code></dt>
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<dd><p>This type of expression represents the integer value <var>i</var>. <var>i</var>
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is customarily accessed with the macro <code>INTVAL</code> as in
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<code>INTVAL (<var>exp</var>)</code>, which is equivalent to <code>XWINT (<var>exp</var>, 0)</code>.
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</p>
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<p>Constants generated for modes with fewer bits than in
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<code>HOST_WIDE_INT</code> must be sign extended to full width (e.g., with
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<code>gen_int_mode</code>). For constants for modes with more bits than in
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<code>HOST_WIDE_INT</code> the implied high order bits of that constant are
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copies of the top bit. Note however that values are neither
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inherently signed nor inherently unsigned; where necessary, signedness
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is determined by the rtl operation instead.
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</p>
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<a name="index-const0_005frtx"></a>
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<a name="index-const1_005frtx"></a>
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<a name="index-const2_005frtx"></a>
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<a name="index-constm1_005frtx"></a>
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<p>There is only one expression object for the integer value zero; it is
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the value of the variable <code>const0_rtx</code>. Likewise, the only
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expression for integer value one is found in <code>const1_rtx</code>, the only
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expression for integer value two is found in <code>const2_rtx</code>, and the
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only expression for integer value negative one is found in
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<code>constm1_rtx</code>. Any attempt to create an expression of code
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<code>const_int</code> and value zero, one, two or negative one will return
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<code>const0_rtx</code>, <code>const1_rtx</code>, <code>const2_rtx</code> or
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<code>constm1_rtx</code> as appropriate.
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</p>
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<a name="index-const_005ftrue_005frtx"></a>
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<p>Similarly, there is only one object for the integer whose value is
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<code>STORE_FLAG_VALUE</code>. It is found in <code>const_true_rtx</code>. If
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<code>STORE_FLAG_VALUE</code> is one, <code>const_true_rtx</code> and
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<code>const1_rtx</code> will point to the same object. If
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<code>STORE_FLAG_VALUE</code> is -1, <code>const_true_rtx</code> and
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<code>constm1_rtx</code> will point to the same object.
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</p>
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<a name="index-const_005fdouble"></a>
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</dd>
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<dt><code>(const_double:<var>m</var> <var>i0</var> <var>i1</var> …)</code></dt>
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<dd><p>This represents either a floating-point constant of mode <var>m</var> or
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(on older ports that do not define
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<code>TARGET_SUPPORTS_WIDE_INT</code>) an integer constant too large to fit
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into <code>HOST_BITS_PER_WIDE_INT</code> bits but small enough to fit within
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twice that number of bits. In the latter case, <var>m</var> will be
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<code>VOIDmode</code>. For integral values constants for modes with more
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bits than twice the number in <code>HOST_WIDE_INT</code> the implied high
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order bits of that constant are copies of the top bit of
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<code>CONST_DOUBLE_HIGH</code>. Note however that integral values are
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neither inherently signed nor inherently unsigned; where necessary,
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signedness is determined by the rtl operation instead.
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</p>
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<p>On more modern ports, <code>CONST_DOUBLE</code> only represents floating
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point values. New ports define <code>TARGET_SUPPORTS_WIDE_INT</code> to
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make this designation.
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</p>
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<a name="index-CONST_005fDOUBLE_005fLOW"></a>
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<p>If <var>m</var> is <code>VOIDmode</code>, the bits of the value are stored in
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<var>i0</var> and <var>i1</var>. <var>i0</var> is customarily accessed with the macro
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<code>CONST_DOUBLE_LOW</code> and <var>i1</var> with <code>CONST_DOUBLE_HIGH</code>.
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</p>
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<p>If the constant is floating point (regardless of its precision), then
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the number of integers used to store the value depends on the size of
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<code>REAL_VALUE_TYPE</code> (see <a href="Floating-Point.html#Floating-Point">Floating Point</a>). The integers
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represent a floating point number, but not precisely in the target
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machine’s or host machine’s floating point format. To convert them to
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the precise bit pattern used by the target machine, use the macro
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<code>REAL_VALUE_TO_TARGET_DOUBLE</code> and friends (see <a href="Data-Output.html#Data-Output">Data Output</a>).
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</p>
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<a name="index-CONST_005fWIDE_005fINT"></a>
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</dd>
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<dt><code>(const_wide_int:<var>m</var> <var>nunits</var> <var>elt0</var> …)</code></dt>
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<dd><p>This contains an array of <code>HOST_WIDE_INT</code>s that is large enough
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to hold any constant that can be represented on the target. This form
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of rtl is only used on targets that define
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<code>TARGET_SUPPORTS_WIDE_INT</code> to be nonzero and then
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<code>CONST_DOUBLE</code>s are only used to hold floating-point values. If
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the target leaves <code>TARGET_SUPPORTS_WIDE_INT</code> defined as 0,
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<code>CONST_WIDE_INT</code>s are not used and <code>CONST_DOUBLE</code>s are as
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they were before.
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</p>
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<p>The values are stored in a compressed format. The higher-order
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0s or -1s are not represented if they are just the logical sign
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extension of the number that is represented.
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</p>
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<a name="index-CONST_005fWIDE_005fINT_005fVEC"></a>
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</dd>
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<dt><code>CONST_WIDE_INT_VEC (<var>code</var>)</code></dt>
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<dd><p>Returns the entire array of <code>HOST_WIDE_INT</code>s that are used to
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store the value. This macro should be rarely used.
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</p>
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<a name="index-CONST_005fWIDE_005fINT_005fNUNITS"></a>
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</dd>
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<dt><code>CONST_WIDE_INT_NUNITS (<var>code</var>)</code></dt>
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<dd><p>The number of <code>HOST_WIDE_INT</code>s used to represent the number.
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Note that this generally is smaller than the number of
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<code>HOST_WIDE_INT</code>s implied by the mode size.
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</p>
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<a name="index-CONST_005fWIDE_005fINT_005fELT"></a>
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</dd>
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<dt><code>CONST_WIDE_INT_NUNITS (<var>code</var>,<var>i</var>)</code></dt>
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<dd><p>Returns the <code>i</code>th element of the array. Element 0 is contains
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the low order bits of the constant.
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</p>
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<a name="index-const_005ffixed"></a>
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</dd>
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<dt><code>(const_fixed:<var>m</var> …)</code></dt>
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<dd><p>Represents a fixed-point constant of mode <var>m</var>.
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The operand is a data structure of type <code>struct fixed_value</code> and
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is accessed with the macro <code>CONST_FIXED_VALUE</code>. The high part of
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data is accessed with <code>CONST_FIXED_VALUE_HIGH</code>; the low part is
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accessed with <code>CONST_FIXED_VALUE_LOW</code>.
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</p>
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<a name="index-const_005fvector"></a>
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</dd>
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<dt><code>(const_vector:<var>m</var> [<var>x0</var> <var>x1</var> …])</code></dt>
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<dd><p>Represents a vector constant. The square brackets stand for the vector
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containing the constant elements. <var>x0</var>, <var>x1</var> and so on are
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the <code>const_int</code>, <code>const_double</code> or <code>const_fixed</code> elements.
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</p>
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<p>The number of units in a <code>const_vector</code> is obtained with the macro
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<code>CONST_VECTOR_NUNITS</code> as in <code>CONST_VECTOR_NUNITS (<var>v</var>)</code>.
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</p>
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<p>Individual elements in a vector constant are accessed with the macro
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<code>CONST_VECTOR_ELT</code> as in <code>CONST_VECTOR_ELT (<var>v</var>, <var>n</var>)</code>
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where <var>v</var> is the vector constant and <var>n</var> is the element
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desired.
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</p>
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<a name="index-const_005fstring"></a>
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</dd>
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<dt><code>(const_string <var>str</var>)</code></dt>
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<dd><p>Represents a constant string with value <var>str</var>. Currently this is
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used only for insn attributes (see <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>) since constant
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strings in C are placed in memory.
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</p>
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<a name="index-symbol_005fref"></a>
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</dd>
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<dt><code>(symbol_ref:<var>mode</var> <var>symbol</var>)</code></dt>
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<dd><p>Represents the value of an assembler label for data. <var>symbol</var> is
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a string that describes the name of the assembler label. If it starts
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with a ‘<samp>*</samp>’, the label is the rest of <var>symbol</var> not including
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the ‘<samp>*</samp>’. Otherwise, the label is <var>symbol</var>, usually prefixed
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with ‘<samp>_</samp>’.
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</p>
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<p>The <code>symbol_ref</code> contains a mode, which is usually <code>Pmode</code>.
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Usually that is the only mode for which a symbol is directly valid.
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</p>
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<a name="index-label_005fref"></a>
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</dd>
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<dt><code>(label_ref:<var>mode</var> <var>label</var>)</code></dt>
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<dd><p>Represents the value of an assembler label for code. It contains one
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operand, an expression, which must be a <code>code_label</code> or a <code>note</code>
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of type <code>NOTE_INSN_DELETED_LABEL</code> that appears in the instruction
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sequence to identify the place where the label should go.
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</p>
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<p>The reason for using a distinct expression type for code label
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references is so that jump optimization can distinguish them.
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</p>
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<p>The <code>label_ref</code> contains a mode, which is usually <code>Pmode</code>.
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Usually that is the only mode for which a label is directly valid.
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</p>
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<a name="index-const"></a>
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</dd>
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<dt><code>(const:<var>m</var> <var>exp</var>)</code></dt>
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<dd><p>Represents a constant that is the result of an assembly-time
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arithmetic computation. The operand, <var>exp</var>, is an expression that
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contains only constants (<code>const_int</code>, <code>symbol_ref</code> and
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<code>label_ref</code> expressions) combined with <code>plus</code> and
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<code>minus</code>. However, not all combinations are valid, since the
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assembler cannot do arbitrary arithmetic on relocatable symbols.
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</p>
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<p><var>m</var> should be <code>Pmode</code>.
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</p>
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<a name="index-high"></a>
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</dd>
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<dt><code>(high:<var>m</var> <var>exp</var>)</code></dt>
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<dd><p>Represents the high-order bits of <var>exp</var>, usually a
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<code>symbol_ref</code>. The number of bits is machine-dependent and is
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normally the number of bits specified in an instruction that initializes
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the high order bits of a register. It is used with <code>lo_sum</code> to
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represent the typical two-instruction sequence used in RISC machines to
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reference a global memory location.
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</p>
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<p><var>m</var> should be <code>Pmode</code>.
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</p></dd>
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</dl>
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<a name="index-CONST0_005fRTX"></a>
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<a name="index-CONST1_005fRTX"></a>
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<a name="index-CONST2_005fRTX"></a>
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<p>The macro <code>CONST0_RTX (<var>mode</var>)</code> refers to an expression with
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value 0 in mode <var>mode</var>. If mode <var>mode</var> is of mode class
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<code>MODE_INT</code>, it returns <code>const0_rtx</code>. If mode <var>mode</var> is of
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mode class <code>MODE_FLOAT</code>, it returns a <code>CONST_DOUBLE</code>
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expression in mode <var>mode</var>. Otherwise, it returns a
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<code>CONST_VECTOR</code> expression in mode <var>mode</var>. Similarly, the macro
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<code>CONST1_RTX (<var>mode</var>)</code> refers to an expression with value 1 in
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mode <var>mode</var> and similarly for <code>CONST2_RTX</code>. The
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<code>CONST1_RTX</code> and <code>CONST2_RTX</code> macros are undefined
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for vector modes.
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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="Regs-and-Memory.html#Regs-and-Memory" accesskey="n" rel="next">Regs and Memory</a>, Previous: <a href="Machine-Modes.html#Machine-Modes" accesskey="p" rel="prev">Machine Modes</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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