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| <a name="Disable-Insn-Alternatives"></a> |
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| <a name="Disable-insn-alternatives-using-the-enabled-attribute"></a> |
| <h4 class="subsection">16.8.6 Disable insn alternatives using the <code>enabled</code> attribute</h4> |
| <a name="index-enabled"></a> |
| |
| <p>The <code>enabled</code> insn attribute may be used to disable certain insn |
| alternatives for machine-specific reasons. This is useful when adding |
| new instructions to an existing pattern which are only available for |
| certain cpu architecture levels as specified with the <code>-march=</code> |
| option. |
| </p> |
| <p>If an insn alternative is disabled, then it will never be used. The |
| compiler treats the constraints for the disabled alternative as |
| unsatisfiable. |
| </p> |
| <p>In order to make use of the <code>enabled</code> attribute a back end has to add |
| in the machine description files: |
| </p> |
| <ol> |
| <li> A definition of the <code>enabled</code> insn attribute. The attribute is |
| defined as usual using the <code>define_attr</code> command. This |
| definition should be based on other insn attributes and/or target flags. |
| The <code>enabled</code> attribute is a numeric attribute and should evaluate to |
| <code>(const_int 1)</code> for an enabled alternative and to |
| <code>(const_int 0)</code> otherwise. |
| </li><li> A definition of another insn attribute used to describe for what |
| reason an insn alternative might be available or |
| not. E.g. <code>cpu_facility</code> as in the example below. |
| </li><li> An assignment for the second attribute to each insn definition |
| combining instructions which are not all available under the same |
| circumstances. (Note: It obviously only makes sense for definitions |
| with more than one alternative. Otherwise the insn pattern should be |
| disabled or enabled using the insn condition.) |
| </li></ol> |
| |
| <p>E.g. the following two patterns could easily be merged using the <code>enabled</code> |
| attribute: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> |
| (define_insn "*movdi_old" |
| [(set (match_operand:DI 0 "register_operand" "=d") |
| (match_operand:DI 1 "register_operand" " d"))] |
| "!TARGET_NEW" |
| "lgr %0,%1") |
| |
| (define_insn "*movdi_new" |
| [(set (match_operand:DI 0 "register_operand" "=d,f,d") |
| (match_operand:DI 1 "register_operand" " d,d,f"))] |
| "TARGET_NEW" |
| "@ |
| lgr %0,%1 |
| ldgr %0,%1 |
| lgdr %0,%1") |
| |
| </pre></div> |
| |
| <p>to: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> |
| (define_insn "*movdi_combined" |
| [(set (match_operand:DI 0 "register_operand" "=d,f,d") |
| (match_operand:DI 1 "register_operand" " d,d,f"))] |
| "" |
| "@ |
| lgr %0,%1 |
| ldgr %0,%1 |
| lgdr %0,%1" |
| [(set_attr "cpu_facility" "*,new,new")]) |
| |
| </pre></div> |
| |
| <p>with the <code>enabled</code> attribute defined like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> |
| (define_attr "cpu_facility" "standard,new" (const_string "standard")) |
| |
| (define_attr "enabled" "" |
| (cond [(eq_attr "cpu_facility" "standard") (const_int 1) |
| (and (eq_attr "cpu_facility" "new") |
| (ne (symbol_ref "TARGET_NEW") (const_int 0))) |
| (const_int 1)] |
| (const_int 0))) |
| |
| </pre></div> |
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