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<a name="Patterns"></a>
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<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>

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