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+Next: <a href="Dependent-Patterns.html#Dependent-Patterns" accesskey="n" rel="next">Dependent Patterns</a>, Previous: <a href="Standard-Names.html#Standard-Names" accesskey="p" rel="previous">Standard Names</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="When-the-Order-of-Patterns-Matters"></a>
+<h3 class="section">17.10 When the Order of Patterns Matters</h3>
+<a name="index-Pattern-Ordering"></a>
+<a name="index-Ordering-of-Patterns"></a>
+
+<p>Sometimes an insn can match more than one instruction pattern.  Then the
+pattern that appears first in the machine description is the one used.
+Therefore, more specific patterns (patterns that will match fewer things)
+and faster instructions (those that will produce better code when they
+do match) should usually go first in the description.
+</p>
+<p>In some cases the effect of ordering the patterns can be used to hide
+a pattern when it is not valid.  For example, the 68000 has an
+instruction for converting a fullword to floating point and another
+for converting a byte to floating point.  An instruction converting
+an integer to floating point could match either one.  We put the
+pattern to convert the fullword first to make sure that one will
+be used rather than the other.  (Otherwise a large integer might
+be generated as a single-byte immediate quantity, which would not work.)
+Instead of using this pattern ordering it would be possible to make the
+pattern for convert-a-byte smart enough to deal properly with any
+constant value.
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