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 <a name="Patterns"></a>
 <div class="header">
 <p>
 Next: <a href="Example.html#Example" accesskey="n" rel="next">Example</a>, Previous: <a href="Overview.html#Overview" accesskey="p" rel="prev">Overview</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="Everything-about-Instruction-Patterns"></a>
 <h3 class="section">16.2 Everything about Instruction Patterns</h3>
 <a name="index-patterns"></a>
 <a name="index-instruction-patterns"></a>

 <a name="index-define_005finsn"></a>
 <p>A <code>define_insn</code> expression is used to define instruction patterns
 to which insns may be matched.  A <code>define_insn</code> expression contains
 an incomplete RTL expression, with pieces to be filled in later, operand
 constraints that restrict how the pieces can be filled in, and an output
 template or C code to generate the assembler output.
 </p>
 <p>A <code>define_insn</code> is an RTL expression containing four or five operands:
 </p>
 <ol>
 <li> An optional name.  The presence of a name indicate that this instruction
 pattern can perform a certain standard job for the RTL-generation
 pass of the compiler.  This pass knows certain names and will use
 the instruction patterns with those names, if the names are defined
 in the machine description.

 <p>The absence of a name is indicated by writing an empty string
 where the name should go.  Nameless instruction patterns are never
 used for generating RTL code, but they may permit several simpler insns
 to be combined later on.
 </p>
 <p>Names that are not thus known and used in RTL-generation have no
 effect; they are equivalent to no name at all.
 </p>
 <p>For the purpose of debugging the compiler, you may also specify a
 name beginning with the &lsquo;<samp>*</samp>&rsquo; character.  Such a name is used only
 for identifying the instruction in RTL dumps; it is equivalent to having
 a nameless pattern for all other purposes.  Names beginning with the
 &lsquo;<samp>*</samp>&rsquo; character are not required to be unique.
 </p>
 </li><li> The <em>RTL template</em>: This is a vector of incomplete RTL expressions
 which describe the semantics of the instruction (see <a href="RTL-Template.html#RTL-Template">RTL Template</a>).
 It is incomplete because it may contain <code>match_operand</code>,
 <code>match_operator</code>, and <code>match_dup</code> expressions that stand for
 operands of the instruction.

 <p>If the vector has multiple elements, the RTL template is treated as a
 <code>parallel</code> expression.
 </p>
 </li><li> <a name="index-pattern-conditions"></a>
 <a name="index-conditions_002c-in-patterns"></a>
 The condition: This is a string which contains a C expression.  When the
 compiler attempts to match RTL against a pattern, the condition is
 evaluated.  If the condition evaluates to <code>true</code>, the match is
 permitted.  The condition may be an empty string, which is treated
 as always <code>true</code>.

 <a name="index-named-patterns-and-conditions"></a>
 <p>For a named pattern, the condition may not depend on the data in the
 insn being matched, but only the target-machine-type flags.  The compiler
 needs to test these conditions during initialization in order to learn
 exactly which named instructions are available in a particular run.
 </p>
 <a name="index-operands-1"></a>
 <p>For nameless patterns, the condition is applied only when matching an
 individual insn, and only after the insn has matched the pattern&rsquo;s
 recognition template.  The insn&rsquo;s operands may be found in the vector
 <code>operands</code>.
 </p>
 <p>For an insn where the condition has once matched, it
 cannot later be used to control register allocation by excluding
 certain register or value combinations.
 </p>
 </li><li> The <em>output template</em> or <em>output statement</em>: This is either
 a string, or a fragment of C code which returns a string.

 <p>When simple substitution isn&rsquo;t general enough, you can specify a piece
 of C code to compute the output.  See <a href="Output-Statement.html#Output-Statement">Output Statement</a>.
 </p>
 </li><li> The <em>insn attributes</em>: This is an optional vector containing the values of
 attributes for insns matching this pattern (see <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>).
 </li></ol>

 <hr>
 <div class="header">
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 Next: <a href="Example.html#Example" accesskey="n" rel="next">Example</a>, Previous: <a href="Overview.html#Overview" accesskey="p" rel="prev">Overview</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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	<a name="Patterns"></a>
	<div class="header">
	<p>
	Next: <a href="Example.html#Example" accesskey="n" rel="next">Example</a>, Previous: <a href="Overview.html#Overview" accesskey="p" rel="prev">Overview</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="Everything-about-Instruction-Patterns"></a>
	<h3 class="section">16.2 Everything about Instruction Patterns</h3>
	<a name="index-patterns"></a>
	<a name="index-instruction-patterns"></a>

	<a name="index-define_005finsn"></a>
	<p>A <code>define_insn</code> expression is used to define instruction patterns
	to which insns may be matched. A <code>define_insn</code> expression contains
	an incomplete RTL expression, with pieces to be filled in later, operand
	constraints that restrict how the pieces can be filled in, and an output
	template or C code to generate the assembler output.
	</p>
	<p>A <code>define_insn</code> is an RTL expression containing four or five operands:
	</p>
	<ol>
	<li> An optional name. The presence of a name indicate that this instruction
	pattern can perform a certain standard job for the RTL-generation
	pass of the compiler. This pass knows certain names and will use
	the instruction patterns with those names, if the names are defined
	in the machine description.

	<p>The absence of a name is indicated by writing an empty string
	where the name should go. Nameless instruction patterns are never
	used for generating RTL code, but they may permit several simpler insns
	to be combined later on.
	</p>
	<p>Names that are not thus known and used in RTL-generation have no
	effect; they are equivalent to no name at all.
	</p>
	<p>For the purpose of debugging the compiler, you may also specify a
	name beginning with the ‘<samp>*</samp>’ character. Such a name is used only
	for identifying the instruction in RTL dumps; it is equivalent to having
	a nameless pattern for all other purposes. Names beginning with the
	‘<samp>*</samp>’ character are not required to be unique.
	</p>
	</li><li> The <em>RTL template</em>: This is a vector of incomplete RTL expressions
	which describe the semantics of the instruction (see <a href="RTL-Template.html#RTL-Template">RTL Template</a>).
	It is incomplete because it may contain <code>match_operand</code>,
	<code>match_operator</code>, and <code>match_dup</code> expressions that stand for
	operands of the instruction.

	<p>If the vector has multiple elements, the RTL template is treated as a
	<code>parallel</code> expression.
	</p>
	</li><li> <a name="index-pattern-conditions"></a>
	<a name="index-conditions_002c-in-patterns"></a>
	The condition: This is a string which contains a C expression. When the
	compiler attempts to match RTL against a pattern, the condition is
	evaluated. If the condition evaluates to <code>true</code>, the match is
	permitted. The condition may be an empty string, which is treated
	as always <code>true</code>.

	<a name="index-named-patterns-and-conditions"></a>
	<p>For a named pattern, the condition may not depend on the data in the
	insn being matched, but only the target-machine-type flags. The compiler
	needs to test these conditions during initialization in order to learn
	exactly which named instructions are available in a particular run.
	</p>
	<a name="index-operands-1"></a>
	<p>For nameless patterns, the condition is applied only when matching an
	individual insn, and only after the insn has matched the pattern’s
	recognition template. The insn’s operands may be found in the vector
	<code>operands</code>.
	</p>
	<p>For an insn where the condition has once matched, it
	cannot later be used to control register allocation by excluding
	certain register or value combinations.
	</p>
	</li><li> The <em>output template</em> or <em>output statement</em>: This is either
	a string, or a fragment of C code which returns a string.

	<p>When simple substitution isn’t general enough, you can specify a piece
	of C code to compute the output. See <a href="Output-Statement.html#Output-Statement">Output Statement</a>.
	</p>
	</li><li> The <em>insn attributes</em>: This is an optional vector containing the values of
	attributes for insns matching this pattern (see <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>).
	</li></ol>

	<hr>
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	Next: <a href="Example.html#Example" accesskey="n" rel="next">Example</a>, Previous: <a href="Overview.html#Overview" accesskey="p" rel="prev">Overview</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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