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| <a name="Interdependence-of-Patterns"></a> |
| <h3 class="section">16.11 Interdependence of Patterns</h3> |
| <a name="index-Dependent-Patterns"></a> |
| <a name="index-Interdependence-of-Patterns"></a> |
| |
| <p>In some cases machines support instructions identical except for the |
| machine mode of one or more operands. For example, there may be |
| “sign-extend halfword” and “sign-extend byte” instructions whose |
| patterns are |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(set (match_operand:SI 0 …) |
| (extend:SI (match_operand:HI 1 …))) |
| |
| (set (match_operand:SI 0 …) |
| (extend:SI (match_operand:QI 1 …))) |
| </pre></div> |
| |
| <p>Constant integers do not specify a machine mode, so an instruction to |
| extend a constant value could match either pattern. The pattern it |
| actually will match is the one that appears first in the file. For correct |
| results, this must be the one for the widest possible mode (<code>HImode</code>, |
| here). If the pattern matches the <code>QImode</code> instruction, the results |
| will be incorrect if the constant value does not actually fit that mode. |
| </p> |
| <p>Such instructions to extend constants are rarely generated because they are |
| optimized away, but they do occasionally happen in nonoptimized |
| compilations. |
| </p> |
| <p>If a constraint in a pattern allows a constant, the reload pass may |
| replace a register with a constant permitted by the constraint in some |
| cases. Similarly for memory references. Because of this substitution, |
| you should not provide separate patterns for increment and decrement |
| instructions. Instead, they should be generated from the same pattern |
| that supports register-register add insns by examining the operands and |
| generating the appropriate machine instruction. |
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