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116 lines
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116 lines
6.0 KiB
HTML
4 years ago
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<title>Predicates - GNU Compiler Collection (GCC) Internals</title>
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<link rel="up" href="Machine-Desc.html#Machine-Desc" title="Machine Desc">
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<link rel="prev" href="Output-Statement.html#Output-Statement" title="Output Statement">
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<div class="node">
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<a name="Predicates"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Constraints.html#Constraints">Constraints</a>,
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Previous: <a rel="previous" accesskey="p" href="Output-Statement.html#Output-Statement">Output Statement</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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<h3 class="section">16.7 Predicates</h3>
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<p><a name="index-predicates-3277"></a><a name="index-operand-predicates-3278"></a><a name="index-operator-predicates-3279"></a>
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A predicate determines whether a <code>match_operand</code> or
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<code>match_operator</code> expression matches, and therefore whether the
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surrounding instruction pattern will be used for that combination of
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operands. GCC has a number of machine-independent predicates, and you
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can define machine-specific predicates as needed. By convention,
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predicates used with <code>match_operand</code> have names that end in
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‘<samp><span class="samp">_operand</span></samp>’, and those used with <code>match_operator</code> have names
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that end in ‘<samp><span class="samp">_operator</span></samp>’.
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<p>All predicates are Boolean functions (in the mathematical sense) of
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two arguments: the RTL expression that is being considered at that
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position in the instruction pattern, and the machine mode that the
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<code>match_operand</code> or <code>match_operator</code> specifies. In this
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section, the first argument is called <var>op</var> and the second argument
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<var>mode</var>. Predicates can be called from C as ordinary two-argument
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functions; this can be useful in output templates or other
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machine-specific code.
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<p>Operand predicates can allow operands that are not actually acceptable
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to the hardware, as long as the constraints give reload the ability to
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fix them up (see <a href="Constraints.html#Constraints">Constraints</a>). However, GCC will usually generate
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better code if the predicates specify the requirements of the machine
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instructions as closely as possible. Reload cannot fix up operands
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that must be constants (“immediate operands”); you must use a
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predicate that allows only constants, or else enforce the requirement
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in the extra condition.
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<p><a name="index-predicates-and-machine-modes-3280"></a><a name="index-normal-predicates-3281"></a><a name="index-special-predicates-3282"></a>Most predicates handle their <var>mode</var> argument in a uniform manner.
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If <var>mode</var> is <code>VOIDmode</code> (unspecified), then <var>op</var> can have
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any mode. If <var>mode</var> is anything else, then <var>op</var> must have the
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same mode, unless <var>op</var> is a <code>CONST_INT</code> or integer
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<code>CONST_DOUBLE</code>. These RTL expressions always have
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<code>VOIDmode</code>, so it would be counterproductive to check that their
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mode matches. Instead, predicates that accept <code>CONST_INT</code> and/or
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integer <code>CONST_DOUBLE</code> check that the value stored in the
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constant will fit in the requested mode.
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<p>Predicates with this behavior are called <dfn>normal</dfn>.
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<samp><span class="command">genrecog</span></samp> can optimize the instruction recognizer based on
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knowledge of how normal predicates treat modes. It can also diagnose
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certain kinds of common errors in the use of normal predicates; for
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instance, it is almost always an error to use a normal predicate
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without specifying a mode.
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<p>Predicates that do something different with their <var>mode</var> argument
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are called <dfn>special</dfn>. The generic predicates
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<code>address_operand</code> and <code>pmode_register_operand</code> are special
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predicates. <samp><span class="command">genrecog</span></samp> does not do any optimizations or
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diagnosis when special predicates are used.
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<ul class="menu">
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<li><a accesskey="1" href="Machine_002dIndependent-Predicates.html#Machine_002dIndependent-Predicates">Machine-Independent Predicates</a>: Predicates available to all back ends.
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<li><a accesskey="2" href="Defining-Predicates.html#Defining-Predicates">Defining Predicates</a>: How to write machine-specific predicate
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functions.
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</ul>
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</body></html>
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