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  65. 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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  68. <a name="Overview-of-How-the-Machine-Description-is-Used"></a>
  69. <h3 class="section">16.1 Overview of How the Machine Description is Used</h3>
  70. <p>There are three main conversions that happen in the compiler:
  71. </p>
  72. <ol>
  73. <li> The front end reads the source code and builds a parse tree.
  74. </li><li> The parse tree is used to generate an RTL insn list based on named
  75. instruction patterns.
  76. </li><li> The insn list is matched against the RTL templates to produce assembler
  77. code.
  78. </li></ol>
  79. <p>For the generate pass, only the names of the insns matter, from either a
  80. named <code>define_insn</code> or a <code>define_expand</code>. The compiler will
  81. choose the pattern with the right name and apply the operands according
  82. to the documentation later in this chapter, without regard for the RTL
  83. template or operand constraints. Note that the names the compiler looks
  84. for are hard-coded in the compiler&mdash;it will ignore unnamed patterns and
  85. patterns with names it doesn&rsquo;t know about, but if you don&rsquo;t provide a
  86. named pattern it needs, it will abort.
  87. </p>
  88. <p>If a <code>define_insn</code> is used, the template given is inserted into the
  89. insn list. If a <code>define_expand</code> is used, one of three things
  90. happens, based on the condition logic. The condition logic may manually
  91. create new insns for the insn list, say via <code>emit_insn()</code>, and
  92. invoke <code>DONE</code>. For certain named patterns, it may invoke <code>FAIL</code> to tell the
  93. compiler to use an alternate way of performing that task. If it invokes
  94. neither <code>DONE</code> nor <code>FAIL</code>, the template given in the pattern
  95. is inserted, as if the <code>define_expand</code> were a <code>define_insn</code>.
  96. </p>
  97. <p>Once the insn list is generated, various optimization passes convert,
  98. replace, and rearrange the insns in the insn list. This is where the
  99. <code>define_split</code> and <code>define_peephole</code> patterns get used, for
  100. example.
  101. </p>
  102. <p>Finally, the insn list&rsquo;s RTL is matched up with the RTL templates in the
  103. <code>define_insn</code> patterns, and those patterns are used to emit the
  104. final assembly code. For this purpose, each named <code>define_insn</code>
  105. acts like it&rsquo;s unnamed, since the names are ignored.
  106. </p>
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  110. 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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