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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>Each instruction pattern 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 pattern or C code to generate the assembler
output, all wrapped up in a <code>define_insn</code> expression.
</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 entirely equivalent
to having a nameless pattern for all other purposes.
</p>
</li><li> The <em>RTL template</em> (see <a href="RTL-Template.html#RTL-Template">RTL Template</a>) is a vector of incomplete
RTL expressions which show what the instruction should look like.  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 only one element, that element is the template for the
instruction pattern.  If the vector has multiple elements, then the
instruction pattern is a <code>parallel</code> expression containing the
elements described.
</p>
</li><li> <a name="index-pattern-conditions"></a>
<a name="index-conditions_002c-in-patterns"></a>
A condition.  This is a string which contains a C expression that is
the final test to decide whether an insn body matches this pattern.

<a name="index-named-patterns-and-conditions"></a>
<p>For a named pattern, the condition (if present) 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>.  For an insn where the condition has once matched, it
can&rsquo;t be used to control register allocation, for example by excluding
certain hard registers or hard register combinations.
</p>
</li><li> The <em>output template</em>: a string that says how to output matching
insns as assembler code.  &lsquo;<samp>%</samp>&rsquo; in this string specifies where
to substitute the value of an operand.  See <a href="Output-Template.html#Output-Template">Output Template</a>.

<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> Optionally, a 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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