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  25. <title>Disable Insn Alternatives (GNU Compiler Collection (GCC) Internals)</title>

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  35. <link href="Constraints.html#Constraints" rel="up" title="Constraints">

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  70. <a name="Disable-Insn-Alternatives"></a>

  71. <div class="header">

  72. <p>

  73. 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> &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>

  74. </div>

  75. <hr>

  76. <a name="Disable-insn-alternatives-using-the-enabled-attribute"></a>

  77. <h4 class="subsection">17.8.6 Disable insn alternatives using the <code>enabled</code> attribute</h4>

  78. <a name="index-enabled"></a>

  79. 

  80. <p>There are three insn attributes that may be used to selectively disable

  81. instruction alternatives:

  82. </p>

  83. <dl compact="compact">

  84. <dt><code>enabled</code></dt>

  85. <dd><p>Says whether an alternative is available on the current subtarget.

  86. </p>

  87. </dd>

  88. <dt><code>preferred_for_size</code></dt>

  89. <dd><p>Says whether an enabled alternative should be used in code that is

  90. optimized for size.

  91. </p>

  92. </dd>

  93. <dt><code>preferred_for_speed</code></dt>

  94. <dd><p>Says whether an enabled alternative should be used in code that is

  95. optimized for speed.

  96. </p></dd>

  97. </dl>

  98. 

  99. <p>All these attributes should use <code>(const_int 1)</code> to allow an alternative

  100. or <code>(const_int 0)</code> to disallow it.  The attributes must be a static

  101. property of the subtarget; they cannot for example depend on the

  102. current operands, on the current optimization level, on the location

  103. of the insn within the body of a loop, on whether register allocation

  104. has finished, or on the current compiler pass.

  105. </p>

  106. <p>The <code>enabled</code> attribute is a correctness property.  It tells GCC to act

  107. as though the disabled alternatives were never defined in the first place.

  108. This is useful when adding new instructions to an existing pattern in

  109. cases where the new instructions are only available for certain cpu

  110. architecture levels (typically mapped to the <code>-march=</code> command-line

  111. option).

  112. </p>

  113. <p>In contrast, the <code>preferred_for_size</code> and <code>preferred_for_speed</code>

  114. attributes are strong optimization hints rather than correctness properties.

  115. <code>preferred_for_size</code> tells GCC which alternatives to consider when

  116. adding or modifying an instruction that GCC wants to optimize for size.

  117. <code>preferred_for_speed</code> does the same thing for speed.  Note that things

  118. like code motion can lead to cases where code optimized for size uses

  119. alternatives that are not preferred for size, and similarly for speed.

  120. </p>

  121. <p>Although <code>define_insn</code>s can in principle specify the <code>enabled</code>

  122. attribute directly, it is often clearer to have subsiduary attributes

  123. for each architectural feature of interest.  The <code>define_insn</code>s

  124. can then use these subsiduary attributes to say which alternatives

  125. require which features.  The example below does this for <code>cpu_facility</code>.

  126. </p>

  127. <p>E.g. the following two patterns could easily be merged using the <code>enabled</code>

  128. attribute:

  129. </p>

  130. <div class="smallexample">

  131. <pre class="smallexample">

  132. 

  133. (define_insn &quot;*movdi_old&quot;

  134.   [(set (match_operand:DI 0 &quot;register_operand&quot; &quot;=d&quot;)

  135.         (match_operand:DI 1 &quot;register_operand&quot; &quot; d&quot;))]

  136.   &quot;!TARGET_NEW&quot;

  137.   &quot;lgr %0,%1&quot;)

  138. 

  139. (define_insn &quot;*movdi_new&quot;

  140.   [(set (match_operand:DI 0 &quot;register_operand&quot; &quot;=d,f,d&quot;)

  141.         (match_operand:DI 1 &quot;register_operand&quot; &quot; d,d,f&quot;))]

  142.   &quot;TARGET_NEW&quot;

  143.   &quot;@

  144.    lgr  %0,%1

  145.    ldgr %0,%1

  146.    lgdr %0,%1&quot;)

  147. 

  148. </pre></div>

  149. 

  150. <p>to:

  151. </p>

  152. <div class="smallexample">

  153. <pre class="smallexample">

  154. 

  155. (define_insn &quot;*movdi_combined&quot;

  156.   [(set (match_operand:DI 0 &quot;register_operand&quot; &quot;=d,f,d&quot;)

  157.         (match_operand:DI 1 &quot;register_operand&quot; &quot; d,d,f&quot;))]

  158.   &quot;&quot;

  159.   &quot;@

  160.    lgr  %0,%1

  161.    ldgr %0,%1

  162.    lgdr %0,%1&quot;

  163.   [(set_attr &quot;cpu_facility&quot; &quot;*,new,new&quot;)])

  164. 

  165. </pre></div>

  166. 

  167. <p>with the <code>enabled</code> attribute defined like this:

  168. </p>

  169. <div class="smallexample">

  170. <pre class="smallexample">

  171. 

  172. (define_attr &quot;cpu_facility&quot; &quot;standard,new&quot; (const_string &quot;standard&quot;))

  173. 

  174. (define_attr &quot;enabled&quot; &quot;&quot;

  175.   (cond [(eq_attr &quot;cpu_facility&quot; &quot;standard&quot;) (const_int 1)

  176.          (and (eq_attr &quot;cpu_facility&quot; &quot;new&quot;)

  177.               (ne (symbol_ref &quot;TARGET_NEW&quot;) (const_int 0)))

  178.          (const_int 1)]

  179.         (const_int 0)))

  180. 

  181. </pre></div>

  182. 

  183. 

  184. <hr>

  185. <div class="header">

  186. <p>

  187. 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> &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>

  188. </div>

  189. 

  190. 

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