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| <a name="When-the-Order-of-Patterns-Matters"></a> |
| <h3 class="section">16.10 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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