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 <a name="Overview"></a>
 <div class="header">
 <p>
 Next: <a href="Patterns.html#Patterns" accesskey="n" rel="next">Patterns</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>
 </div>
 <hr>
 <a name="Overview-of-How-the-Machine-Description-is-Used"></a>
 <h3 class="section">16.1 Overview of How the Machine Description is Used</h3>

 <p>There are three main conversions that happen in the compiler:
 </p>
 <ol>
 <li> The front end reads the source code and builds a parse tree.

 </li><li> The parse tree is used to generate an RTL insn list based on named
 instruction patterns.

 </li><li> The insn list is matched against the RTL templates to produce assembler
 code.

 </li></ol>

 <p>For the generate pass, only the names of the insns matter, from either a
 named <code>define_insn</code> or a <code>define_expand</code>.  The compiler will
 choose the pattern with the right name and apply the operands according
 to the documentation later in this chapter, without regard for the RTL
 template or operand constraints.  Note that the names the compiler looks
 for are hard-coded in the compiler&mdash;it will ignore unnamed patterns and
 patterns with names it doesn&rsquo;t know about, but if you don&rsquo;t provide a
 named pattern it needs, it will abort.
 </p>
 <p>If a <code>define_insn</code> is used, the template given is inserted into the
 insn list.  If a <code>define_expand</code> is used, one of three things
 happens, based on the condition logic.  The condition logic may manually
 create new insns for the insn list, say via <code>emit_insn()</code>, and
 invoke <code>DONE</code>.  For certain named patterns, it may invoke <code>FAIL</code> to tell the
 compiler to use an alternate way of performing that task.  If it invokes
 neither <code>DONE</code> nor <code>FAIL</code>, the template given in the pattern
 is inserted, as if the <code>define_expand</code> were a <code>define_insn</code>.
 </p>
 <p>Once the insn list is generated, various optimization passes convert,
 replace, and rearrange the insns in the insn list.  This is where the
 <code>define_split</code> and <code>define_peephole</code> patterns get used, for
 example.
 </p>
 <p>Finally, the insn list&rsquo;s RTL is matched up with the RTL templates in the
 <code>define_insn</code> patterns, and those patterns are used to emit the
 final assembly code.  For this purpose, each named <code>define_insn</code>
 acts like it&rsquo;s unnamed, since the names are ignored.
 </p>
 <hr>
 <div class="header">
 <p>
 Next: <a href="Patterns.html#Patterns" accesskey="n" rel="next">Patterns</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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	Invariant Sections being "Funding Free Software", the Front-Cover
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	<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
	<a name="Overview"></a>
	<div class="header">
	<p>
	Next: <a href="Patterns.html#Patterns" accesskey="n" rel="next">Patterns</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="Overview-of-How-the-Machine-Description-is-Used"></a>
	<h3 class="section">16.1 Overview of How the Machine Description is Used</h3>

	<p>There are three main conversions that happen in the compiler:
	</p>
	<ol>
	<li> The front end reads the source code and builds a parse tree.

	</li><li> The parse tree is used to generate an RTL insn list based on named
	instruction patterns.

	</li><li> The insn list is matched against the RTL templates to produce assembler
	code.

	</li></ol>

	<p>For the generate pass, only the names of the insns matter, from either a
	named <code>define_insn</code> or a <code>define_expand</code>. The compiler will
	choose the pattern with the right name and apply the operands according
	to the documentation later in this chapter, without regard for the RTL
	template or operand constraints. Note that the names the compiler looks
	for are hard-coded in the compiler—it will ignore unnamed patterns and
	patterns with names it doesn’t know about, but if you don’t provide a
	named pattern it needs, it will abort.
	</p>
	<p>If a <code>define_insn</code> is used, the template given is inserted into the
	insn list. If a <code>define_expand</code> is used, one of three things
	happens, based on the condition logic. The condition logic may manually
	create new insns for the insn list, say via <code>emit_insn()</code>, and
	invoke <code>DONE</code>. For certain named patterns, it may invoke <code>FAIL</code> to tell the
	compiler to use an alternate way of performing that task. If it invokes
	neither <code>DONE</code> nor <code>FAIL</code>, the template given in the pattern
	is inserted, as if the <code>define_expand</code> were a <code>define_insn</code>.
	</p>
	<p>Once the insn list is generated, various optimization passes convert,
	replace, and rearrange the insns in the insn list. This is where the
	<code>define_split</code> and <code>define_peephole</code> patterns get used, for
	example.
	</p>
	<p>Finally, the insn list’s RTL is matched up with the RTL templates in the
	<code>define_insn</code> patterns, and those patterns are used to emit the
	final assembly code. For this purpose, each named <code>define_insn</code>
	acts like it’s unnamed, since the names are ignored.
	</p>
	<hr>
	<div class="header">
	<p>
	Next: <a href="Patterns.html#Patterns" accesskey="n" rel="next">Patterns</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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