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<a name="Predicates"></a>
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<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.
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