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 <a name="Predicates"></a>
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 <p>
 Next: <a href="Constraints.html#Constraints" accesskey="n" rel="next">Constraints</a>, Previous: <a href="Output-Statement.html#Output-Statement" accesskey="p" rel="prev">Output Statement</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a> &nbsp; [<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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 <hr>
 <a name="Predicates-1"></a>
 <h3 class="section">16.7 Predicates</h3>
 <a name="index-predicates"></a>
 <a name="index-operand-predicates"></a>
 <a name="index-operator-predicates"></a>

 <p>A predicate determines whether a <code>match_operand</code> or
 <code>match_operator</code> expression matches, and therefore whether the
 surrounding instruction pattern will be used for that combination of
 operands.  GCC has a number of machine-independent predicates, and you
 can define machine-specific predicates as needed.  By convention,
 predicates used with <code>match_operand</code> have names that end in
 &lsquo;<samp>_operand</samp>&rsquo;, and those used with <code>match_operator</code> have names
 that end in &lsquo;<samp>_operator</samp>&rsquo;.
 </p>
 <p>All predicates are Boolean functions (in the mathematical sense) of
 two arguments: the RTL expression that is being considered at that
 position in the instruction pattern, and the machine mode that the
 <code>match_operand</code> or <code>match_operator</code> specifies.  In this
 section, the first argument is called <var>op</var> and the second argument
 <var>mode</var>.  Predicates can be called from C as ordinary two-argument
 functions; this can be useful in output templates or other
 machine-specific code.
 </p>
 <p>Operand predicates can allow operands that are not actually acceptable
 to the hardware, as long as the constraints give reload the ability to
 fix them up (see <a href="Constraints.html#Constraints">Constraints</a>).  However, GCC will usually generate
 better code if the predicates specify the requirements of the machine
 instructions as closely as possible.  Reload cannot fix up operands
 that must be constants (&ldquo;immediate operands&rdquo;); you must use a
 predicate that allows only constants, or else enforce the requirement
 in the extra condition.
 </p>
 <a name="index-predicates-and-machine-modes"></a>
 <a name="index-normal-predicates"></a>
 <a name="index-special-predicates"></a>
 <p>Most predicates handle their <var>mode</var> argument in a uniform manner.
 If <var>mode</var> is <code>VOIDmode</code> (unspecified), then <var>op</var> can have
 any mode.  If <var>mode</var> is anything else, then <var>op</var> must have the
 same mode, unless <var>op</var> is a <code>CONST_INT</code> or integer
 <code>CONST_DOUBLE</code>.  These RTL expressions always have
 <code>VOIDmode</code>, so it would be counterproductive to check that their
 mode matches.  Instead, predicates that accept <code>CONST_INT</code> and/or
 integer <code>CONST_DOUBLE</code> check that the value stored in the
 constant will fit in the requested mode.
 </p>
 <p>Predicates with this behavior are called <em>normal</em>.
 <code>genrecog</code> can optimize the instruction recognizer based on
 knowledge of how normal predicates treat modes.  It can also diagnose
 certain kinds of common errors in the use of normal predicates; for
 instance, it is almost always an error to use a normal predicate
 without specifying a mode.
 </p>
 <p>Predicates that do something different with their <var>mode</var> argument
 are called <em>special</em>.  The generic predicates
 <code>address_operand</code> and <code>pmode_register_operand</code> are special
 predicates.  <code>genrecog</code> does not do any optimizations or
 diagnosis when special predicates are used.
 </p>
 <table class="menu" border="0" cellspacing="0">
 <tr><td align="left" valign="top">&bull; <a href="Machine_002dIndependent-Predicates.html#Machine_002dIndependent-Predicates" accesskey="1">Machine-Independent Predicates</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Predicates available to all back ends.
 </td></tr>
 <tr><td align="left" valign="top">&bull; <a href="Defining-Predicates.html#Defining-Predicates" accesskey="2">Defining Predicates</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to write machine-specific predicate
                                     functions.
 </td></tr>
 </table>

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	<a name="Predicates"></a>
	<div class="header">
	<p>
	Next: <a href="Constraints.html#Constraints" accesskey="n" rel="next">Constraints</a>, Previous: <a href="Output-Statement.html#Output-Statement" accesskey="p" rel="prev">Output Statement</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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="Predicates-1"></a>
	<h3 class="section">16.7 Predicates</h3>
	<a name="index-predicates"></a>
	<a name="index-operand-predicates"></a>
	<a name="index-operator-predicates"></a>

	<p>A predicate determines whether a <code>match_operand</code> or
	<code>match_operator</code> expression matches, and therefore whether the
	surrounding instruction pattern will be used for that combination of
	operands. GCC has a number of machine-independent predicates, and you
	can define machine-specific predicates as needed. By convention,
	predicates used with <code>match_operand</code> have names that end in
	‘<samp>_operand</samp>’, and those used with <code>match_operator</code> have names
	that end in ‘<samp>_operator</samp>’.
	</p>
	<p>All predicates are Boolean functions (in the mathematical sense) of
	two arguments: the RTL expression that is being considered at that
	position in the instruction pattern, and the machine mode that the
	<code>match_operand</code> or <code>match_operator</code> specifies. In this
	section, the first argument is called <var>op</var> and the second argument
	<var>mode</var>. Predicates can be called from C as ordinary two-argument
	functions; this can be useful in output templates or other
	machine-specific code.
	</p>
	<p>Operand predicates can allow operands that are not actually acceptable
	to the hardware, as long as the constraints give reload the ability to
	fix them up (see <a href="Constraints.html#Constraints">Constraints</a>). However, GCC will usually generate
	better code if the predicates specify the requirements of the machine
	instructions as closely as possible. Reload cannot fix up operands
	that must be constants (“immediate operands”); you must use a
	predicate that allows only constants, or else enforce the requirement
	in the extra condition.
	</p>
	<a name="index-predicates-and-machine-modes"></a>
	<a name="index-normal-predicates"></a>
	<a name="index-special-predicates"></a>
	<p>Most predicates handle their <var>mode</var> argument in a uniform manner.
	If <var>mode</var> is <code>VOIDmode</code> (unspecified), then <var>op</var> can have
	any mode. If <var>mode</var> is anything else, then <var>op</var> must have the
	same mode, unless <var>op</var> is a <code>CONST_INT</code> or integer
	<code>CONST_DOUBLE</code>. These RTL expressions always have
	<code>VOIDmode</code>, so it would be counterproductive to check that their
	mode matches. Instead, predicates that accept <code>CONST_INT</code> and/or
	integer <code>CONST_DOUBLE</code> check that the value stored in the
	constant will fit in the requested mode.
	</p>
	<p>Predicates with this behavior are called <em>normal</em>.
	<code>genrecog</code> can optimize the instruction recognizer based on
	knowledge of how normal predicates treat modes. It can also diagnose
	certain kinds of common errors in the use of normal predicates; for
	instance, it is almost always an error to use a normal predicate
	without specifying a mode.
	</p>
	<p>Predicates that do something different with their <var>mode</var> argument
	are called <em>special</em>. The generic predicates
	<code>address_operand</code> and <code>pmode_register_operand</code> are special
	predicates. <code>genrecog</code> does not do any optimizations or
	diagnosis when special predicates are used.
	</p>
	<table class="menu" border="0" cellspacing="0">
	<tr><td align="left" valign="top">• <a href="Machine_002dIndependent-Predicates.html#Machine_002dIndependent-Predicates" accesskey="1">Machine-Independent Predicates</a>:</td><td>  </td><td align="left" valign="top">Predicates available to all back ends.
	</td></tr>
	<tr><td align="left" valign="top">• <a href="Defining-Predicates.html#Defining-Predicates" accesskey="2">Defining Predicates</a>:</td><td>  </td><td align="left" valign="top">How to write machine-specific predicate
	functions.
	</td></tr>
	</table>

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