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  25. <title>Attr Example (GNU Compiler Collection (GCC) Internals)</title>

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  35. <link href="Insn-Attributes.html#Insn-Attributes" rel="up" title="Insn Attributes">

  36. <link href="Insn-Lengths.html#Insn-Lengths" rel="next" title="Insn Lengths">

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  70. <a name="Attr-Example"></a>

  71. <div class="header">

  72. <p>

  73. Next: <a href="Insn-Lengths.html#Insn-Lengths" accesskey="n" rel="next">Insn Lengths</a>, Previous: <a href="Tagging-Insns.html#Tagging-Insns" accesskey="p" rel="prev">Tagging Insns</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</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="Example-of-Attribute-Specifications"></a>

  77. <h4 class="subsection">17.19.4 Example of Attribute Specifications</h4>

  78. <a name="index-attribute-specifications-example"></a>

  79. <a name="index-attribute-specifications"></a>

  80. 

  81. <p>The judicious use of defaulting is important in the efficient use of

  82. insn attributes.  Typically, insns are divided into <em>types</em> and an

  83. attribute, customarily called <code>type</code>, is used to represent this

  84. value.  This attribute is normally used only to define the default value

  85. for other attributes.  An example will clarify this usage.

  86. </p>

  87. <p>Assume we have a RISC machine with a condition code and in which only

  88. full-word operations are performed in registers.  Let us assume that we

  89. can divide all insns into loads, stores, (integer) arithmetic

  90. operations, floating point operations, and branches.

  91. </p>

  92. <p>Here we will concern ourselves with determining the effect of an insn on

  93. the condition code and will limit ourselves to the following possible

  94. effects:  The condition code can be set unpredictably (clobbered), not

  95. be changed, be set to agree with the results of the operation, or only

  96. changed if the item previously set into the condition code has been

  97. modified.

  98. </p>

  99. <p>Here is part of a sample <samp>md</samp> file for such a machine:

  100. </p>

  101. <div class="smallexample">

  102. <pre class="smallexample">(define_attr &quot;type&quot; &quot;load,store,arith,fp,branch&quot; (const_string &quot;arith&quot;))

  103. 

  104. (define_attr &quot;cc&quot; &quot;clobber,unchanged,set,change0&quot;

  105.              (cond [(eq_attr &quot;type&quot; &quot;load&quot;)

  106.                         (const_string &quot;change0&quot;)

  107.                     (eq_attr &quot;type&quot; &quot;store,branch&quot;)

  108.                         (const_string &quot;unchanged&quot;)

  109.                     (eq_attr &quot;type&quot; &quot;arith&quot;)

  110.                         (if_then_else (match_operand:SI 0 &quot;&quot; &quot;&quot;)

  111.                                       (const_string &quot;set&quot;)

  112.                                       (const_string &quot;clobber&quot;))]

  113.                    (const_string &quot;clobber&quot;)))

  114. 

  115. (define_insn &quot;&quot;

  116.   [(set (match_operand:SI 0 &quot;general_operand&quot; &quot;=r,r,m&quot;)

  117.         (match_operand:SI 1 &quot;general_operand&quot; &quot;r,m,r&quot;))]

  118.   &quot;&quot;

  119.   &quot;@

  120.    move %0,%1

  121.    load %0,%1

  122.    store %0,%1&quot;

  123.   [(set_attr &quot;type&quot; &quot;arith,load,store&quot;)])

  124. </pre></div>

  125. 

  126. <p>Note that we assume in the above example that arithmetic operations

  127. performed on quantities smaller than a machine word clobber the condition

  128. code since they will set the condition code to a value corresponding to the

  129. full-word result.

  130. </p>

  131. <hr>

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