Attr-Example.html 6.6 KB

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  68. <a name="Example-of-Attribute-Specifications"></a>
  69. <h4 class="subsection">16.19.4 Example of Attribute Specifications</h4>
  70. <a name="index-attribute-specifications-example"></a>
  71. <a name="index-attribute-specifications"></a>
  72. <p>The judicious use of defaulting is important in the efficient use of
  73. insn attributes. Typically, insns are divided into <em>types</em> and an
  74. attribute, customarily called <code>type</code>, is used to represent this
  75. value. This attribute is normally used only to define the default value
  76. for other attributes. An example will clarify this usage.
  77. </p>
  78. <p>Assume we have a RISC machine with a condition code and in which only
  79. full-word operations are performed in registers. Let us assume that we
  80. can divide all insns into loads, stores, (integer) arithmetic
  81. operations, floating point operations, and branches.
  82. </p>
  83. <p>Here we will concern ourselves with determining the effect of an insn on
  84. the condition code and will limit ourselves to the following possible
  85. effects: The condition code can be set unpredictably (clobbered), not
  86. be changed, be set to agree with the results of the operation, or only
  87. changed if the item previously set into the condition code has been
  88. modified.
  89. </p>
  90. <p>Here is part of a sample <samp>md</samp> file for such a machine:
  91. </p>
  92. <div class="smallexample">
  93. <pre class="smallexample">(define_attr &quot;type&quot; &quot;load,store,arith,fp,branch&quot; (const_string &quot;arith&quot;))
  94. (define_attr &quot;cc&quot; &quot;clobber,unchanged,set,change0&quot;
  95. (cond [(eq_attr &quot;type&quot; &quot;load&quot;)
  96. (const_string &quot;change0&quot;)
  97. (eq_attr &quot;type&quot; &quot;store,branch&quot;)
  98. (const_string &quot;unchanged&quot;)
  99. (eq_attr &quot;type&quot; &quot;arith&quot;)
  100. (if_then_else (match_operand:SI 0 &quot;&quot; &quot;&quot;)
  101. (const_string &quot;set&quot;)
  102. (const_string &quot;clobber&quot;))]
  103. (const_string &quot;clobber&quot;)))
  104. (define_insn &quot;&quot;
  105. [(set (match_operand:SI 0 &quot;general_operand&quot; &quot;=r,r,m&quot;)
  106. (match_operand:SI 1 &quot;general_operand&quot; &quot;r,m,r&quot;))]
  107. &quot;&quot;
  108. &quot;@
  109. move %0,%1
  110. load %0,%1
  111. store %0,%1&quot;
  112. [(set_attr &quot;type&quot; &quot;arith,load,store&quot;)])
  113. </pre></div>
  114. <p>Note that we assume in the above example that arithmetic operations
  115. performed on quantities smaller than a machine word clobber the condition
  116. code since they will set the condition code to a value corresponding to the
  117. full-word result.
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