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| <title>GNU Compiler Collection (GCC) Internals: Disable Insn Alternatives</title> |
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| <a name="Disable-Insn-Alternatives"></a> |
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| Next: <a href="Define-Constraints.html#Define-Constraints" accesskey="n" rel="next">Define Constraints</a>, Previous: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="p" rel="prev">Machine Constraints</a>, Up: <a href="Constraints.html#Constraints" accesskey="u" rel="up">Constraints</a> [<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="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>There are three insn attributes that may be used to selectively disable |
| instruction alternatives: |
| </p> |
| <dl compact="compact"> |
| <dt><code>enabled</code></dt> |
| <dd><p>Says whether an alternative is available on the current subtarget. |
| </p> |
| </dd> |
| <dt><code>preferred_for_size</code></dt> |
| <dd><p>Says whether an enabled alternative should be used in code that is |
| optimized for size. |
| </p> |
| </dd> |
| <dt><code>preferred_for_speed</code></dt> |
| <dd><p>Says whether an enabled alternative should be used in code that is |
| optimized for speed. |
| </p></dd> |
| </dl> |
| |
| <p>All these attributes should use <code>(const_int 1)</code> to allow an alternative |
| or <code>(const_int 0)</code> to disallow it. The attributes must be a static |
| property of the subtarget; they cannot for example depend on the |
| current operands, on the current optimization level, on the location |
| of the insn within the body of a loop, on whether register allocation |
| has finished, or on the current compiler pass. |
| </p> |
| <p>The <code>enabled</code> attribute is a correctness property. It tells GCC to act |
| as though the disabled alternatives were never defined in the first place. |
| This is useful when adding new instructions to an existing pattern in |
| cases where the new instructions are only available for certain cpu |
| architecture levels (typically mapped to the <code>-march=</code> command-line |
| option). |
| </p> |
| <p>In contrast, the <code>preferred_for_size</code> and <code>preferred_for_speed</code> |
| attributes are strong optimization hints rather than correctness properties. |
| <code>preferred_for_size</code> tells GCC which alternatives to consider when |
| adding or modifying an instruction that GCC wants to optimize for size. |
| <code>preferred_for_speed</code> does the same thing for speed. Note that things |
| like code motion can lead to cases where code optimized for size uses |
| alternatives that are not preferred for size, and similarly for speed. |
| </p> |
| <p>Although <code>define_insn</code>s can in principle specify the <code>enabled</code> |
| attribute directly, it is often clearer to have subsiduary attributes |
| for each architectural feature of interest. The <code>define_insn</code>s |
| can then use these subsiduary attributes to say which alternatives |
| require which features. The example below does this for <code>cpu_facility</code>. |
| </p> |
| <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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