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153 lines
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153 lines
7.7 KiB
HTML
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<title>Predicates (GNU Compiler Collection (GCC) Internals)</title>
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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="Machine-Desc.html#Machine-Desc" rel="up" title="Machine Desc">
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<link href="Machine_002dIndependent-Predicates.html#Machine_002dIndependent-Predicates" rel="next" title="Machine-Independent Predicates">
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<a name="Predicates"></a>
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<div class="header">
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<p>
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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>
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</div>
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<hr>
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<a name="Predicates-1"></a>
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<h3 class="section">17.7 Predicates</h3>
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<a name="index-predicates"></a>
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<a name="index-operand-predicates"></a>
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<a name="index-operator-predicates"></a>
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<p>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>_operand</samp>’, and those used with <code>match_operator</code> have names
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that end in ‘<samp>_operator</samp>’.
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</p>
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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>
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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>
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<a name="index-predicates-and-machine-modes"></a>
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<a name="index-normal-predicates"></a>
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<a name="index-special-predicates"></a>
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<p>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>
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<p>Predicates with this behavior are called <em>normal</em>.
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<code>genrecog</code> 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>
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<p>Predicates that do something different with their <var>mode</var> argument
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are called <em>special</em>. The generic predicates
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<code>address_operand</code> and <code>pmode_register_operand</code> are special
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predicates. <code>genrecog</code> does not do any optimizations or
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diagnosis when special predicates are used.
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</p>
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<table class="menu" border="0" cellspacing="0">
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<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.
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</td></tr>
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<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
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functions.
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</td></tr>
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</table>
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<hr>
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<div class="header">
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<p>
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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>
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</div>
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