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.
219 lines
10 KiB
HTML
219 lines
10 KiB
HTML
4 years ago
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2018 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. -->
|
||
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<title>Comparisons (GNU Compiler Collection (GCC) Internals)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Comparisons (GNU Compiler Collection (GCC) Internals)">
|
||
|
|
<meta name="keywords" content="Comparisons (GNU Compiler Collection (GCC) Internals)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content="makeinfo">
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<link href="index.html#Top" rel="start" title="Top">
|
||
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="RTL.html#RTL" rel="up" title="RTL">
|
||
|
|
<link href="Bit_002dFields.html#Bit_002dFields" rel="next" title="Bit-Fields">
|
||
|
|
<link href="Arithmetic.html#Arithmetic" rel="prev" title="Arithmetic">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.indentedblock {margin-right: 0em}
|
||
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
||
|
|
blockquote.smallquotation {font-size: smaller}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
div.smalldisplay {margin-left: 3.2em}
|
||
|
|
div.smallexample {margin-left: 3.2em}
|
||
|
|
div.smalllisp {margin-left: 3.2em}
|
||
|
|
kbd {font-style: oblique}
|
||
|
|
pre.display {font-family: inherit}
|
||
|
|
pre.format {font-family: inherit}
|
||
|
|
pre.menu-comment {font-family: serif}
|
||
|
|
pre.menu-preformatted {font-family: serif}
|
||
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smallexample {font-size: smaller}
|
||
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smalllisp {font-size: smaller}
|
||
|
|
span.nolinebreak {white-space: nowrap}
|
||
|
|
span.roman {font-family: initial; font-weight: normal}
|
||
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<a name="Comparisons"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Bit_002dFields.html#Bit_002dFields" accesskey="n" rel="next">Bit-Fields</a>, Previous: <a href="Arithmetic.html#Arithmetic" accesskey="p" rel="prev">Arithmetic</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>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<a name="Comparison-Operations"></a>
|
||
|
|
<h3 class="section">14.10 Comparison Operations</h3>
|
||
|
|
<a name="index-RTL-comparison-operations"></a>
|
||
|
|
|
||
|
|
<p>Comparison operators test a relation on two operands and are considered
|
||
|
|
to represent a machine-dependent nonzero value described by, but not
|
||
|
|
necessarily equal to, <code>STORE_FLAG_VALUE</code> (see <a href="Misc.html#Misc">Misc</a>)
|
||
|
|
if the relation holds, or zero if it does not, for comparison operators
|
||
|
|
whose results have a ‘MODE_INT’ mode,
|
||
|
|
<code>FLOAT_STORE_FLAG_VALUE</code> (see <a href="Misc.html#Misc">Misc</a>) if the relation holds, or
|
||
|
|
zero if it does not, for comparison operators that return floating-point
|
||
|
|
values, and a vector of either <code>VECTOR_STORE_FLAG_VALUE</code> (see <a href="Misc.html#Misc">Misc</a>)
|
||
|
|
if the relation holds, or of zeros if it does not, for comparison operators
|
||
|
|
that return vector results.
|
||
|
|
The mode of the comparison operation is independent of the mode
|
||
|
|
of the data being compared. If the comparison operation is being tested
|
||
|
|
(e.g., the first operand of an <code>if_then_else</code>), the mode must be
|
||
|
|
<code>VOIDmode</code>.
|
||
|
|
</p>
|
||
|
|
<a name="index-condition-codes"></a>
|
||
|
|
<p>There are two ways that comparison operations may be used. The
|
||
|
|
comparison operators may be used to compare the condition codes
|
||
|
|
<code>(cc0)</code> against zero, as in <code>(eq (cc0) (const_int 0))</code>. Such
|
||
|
|
a construct actually refers to the result of the preceding instruction
|
||
|
|
in which the condition codes were set. The instruction setting the
|
||
|
|
condition code must be adjacent to the instruction using the condition
|
||
|
|
code; only <code>note</code> insns may separate them.
|
||
|
|
</p>
|
||
|
|
<p>Alternatively, a comparison operation may directly compare two data
|
||
|
|
objects. The mode of the comparison is determined by the operands; they
|
||
|
|
must both be valid for a common machine mode. A comparison with both
|
||
|
|
operands constant would be invalid as the machine mode could not be
|
||
|
|
deduced from it, but such a comparison should never exist in RTL due to
|
||
|
|
constant folding.
|
||
|
|
</p>
|
||
|
|
<p>In the example above, if <code>(cc0)</code> were last set to
|
||
|
|
<code>(compare <var>x</var> <var>y</var>)</code>, the comparison operation is
|
||
|
|
identical to <code>(eq <var>x</var> <var>y</var>)</code>. Usually only one style
|
||
|
|
of comparisons is supported on a particular machine, but the combine
|
||
|
|
pass will try to merge the operations to produce the <code>eq</code> shown
|
||
|
|
in case it exists in the context of the particular insn involved.
|
||
|
|
</p>
|
||
|
|
<p>Inequality comparisons come in two flavors, signed and unsigned. Thus,
|
||
|
|
there are distinct expression codes <code>gt</code> and <code>gtu</code> for signed and
|
||
|
|
unsigned greater-than. These can produce different results for the same
|
||
|
|
pair of integer values: for example, 1 is signed greater-than -1 but not
|
||
|
|
unsigned greater-than, because -1 when regarded as unsigned is actually
|
||
|
|
<code>0xffffffff</code> which is greater than 1.
|
||
|
|
</p>
|
||
|
|
<p>The signed comparisons are also used for floating point values. Floating
|
||
|
|
point comparisons are distinguished by the machine modes of the operands.
|
||
|
|
</p>
|
||
|
|
<dl compact="compact">
|
||
|
|
<dd><a name="index-eq"></a>
|
||
|
|
<a name="index-equal"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>(eq:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dd><p><code>STORE_FLAG_VALUE</code> if the values represented by <var>x</var> and <var>y</var>
|
||
|
|
are equal, otherwise 0.
|
||
|
|
</p>
|
||
|
|
<a name="index-ne"></a>
|
||
|
|
<a name="index-not-equal"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>(ne:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dd><p><code>STORE_FLAG_VALUE</code> if the values represented by <var>x</var> and <var>y</var>
|
||
|
|
are not equal, otherwise 0.
|
||
|
|
</p>
|
||
|
|
<a name="index-gt"></a>
|
||
|
|
<a name="index-greater-than"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>(gt:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dd><p><code>STORE_FLAG_VALUE</code> if the <var>x</var> is greater than <var>y</var>. If they
|
||
|
|
are fixed-point, the comparison is done in a signed sense.
|
||
|
|
</p>
|
||
|
|
<a name="index-gtu"></a>
|
||
|
|
<a name="index-greater-than-1"></a>
|
||
|
|
<a name="index-unsigned-greater-than"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>(gtu:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dd><p>Like <code>gt</code> but does unsigned comparison, on fixed-point numbers only.
|
||
|
|
</p>
|
||
|
|
<a name="index-lt"></a>
|
||
|
|
<a name="index-less-than"></a>
|
||
|
|
<a name="index-ltu"></a>
|
||
|
|
<a name="index-unsigned-less-than"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>(lt:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dt><code>(ltu:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dd><p>Like <code>gt</code> and <code>gtu</code> but test for “less than”.
|
||
|
|
</p>
|
||
|
|
<a name="index-ge"></a>
|
||
|
|
<a name="index-greater-than-2"></a>
|
||
|
|
<a name="index-geu"></a>
|
||
|
|
<a name="index-unsigned-greater-than-1"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>(ge:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dt><code>(geu:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dd><p>Like <code>gt</code> and <code>gtu</code> but test for “greater than or equal”.
|
||
|
|
</p>
|
||
|
|
<a name="index-le"></a>
|
||
|
|
<a name="index-less-than-or-equal"></a>
|
||
|
|
<a name="index-leu"></a>
|
||
|
|
<a name="index-unsigned-less-than-1"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>(le:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dt><code>(leu:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
|
||
|
|
<dd><p>Like <code>gt</code> and <code>gtu</code> but test for “less than or equal”.
|
||
|
|
</p>
|
||
|
|
<a name="index-if_005fthen_005felse"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>(if_then_else <var>cond</var> <var>then</var> <var>else</var>)</code></dt>
|
||
|
|
<dd><p>This is not a comparison operation but is listed here because it is
|
||
|
|
always used in conjunction with a comparison operation. To be
|
||
|
|
precise, <var>cond</var> is a comparison expression. This expression
|
||
|
|
represents a choice, according to <var>cond</var>, between the value
|
||
|
|
represented by <var>then</var> and the one represented by <var>else</var>.
|
||
|
|
</p>
|
||
|
|
<p>On most machines, <code>if_then_else</code> expressions are valid only
|
||
|
|
to express conditional jumps.
|
||
|
|
</p>
|
||
|
|
<a name="index-cond"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>(cond [<var>test1</var> <var>value1</var> <var>test2</var> <var>value2</var> …] <var>default</var>)</code></dt>
|
||
|
|
<dd><p>Similar to <code>if_then_else</code>, but more general. Each of <var>test1</var>,
|
||
|
|
<var>test2</var>, … is performed in turn. The result of this expression is
|
||
|
|
the <var>value</var> corresponding to the first nonzero test, or <var>default</var> if
|
||
|
|
none of the tests are nonzero expressions.
|
||
|
|
</p>
|
||
|
|
<p>This is currently not valid for instruction patterns and is supported only
|
||
|
|
for insn attributes. See <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>.
|
||
|
|
</p></dd>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Bit_002dFields.html#Bit_002dFields" accesskey="n" rel="next">Bit-Fields</a>, Previous: <a href="Arithmetic.html#Arithmetic" accesskey="p" rel="prev">Arithmetic</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>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|