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  15. A GNU Manual

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

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  33. <link href="Option-Index.html#Option-Index" rel="index" title="Option Index">

  34. <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">

  35. <link href="Mode-Iterators.html#Mode-Iterators" rel="up" title="Mode Iterators">

  36. <link href="Examples.html#Examples" rel="next" title="Examples">

  37. <link href="Defining-Mode-Iterators.html#Defining-Mode-Iterators" rel="prev" title="Defining Mode Iterators">

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  70. <a name="Substitutions"></a>

  71. <div class="header">

  72. <p>

  73. Next: <a href="Examples.html#Examples" accesskey="n" rel="next">Examples</a>, Previous: <a href="Defining-Mode-Iterators.html#Defining-Mode-Iterators" accesskey="p" rel="prev">Defining Mode Iterators</a>, Up: <a href="Mode-Iterators.html#Mode-Iterators" accesskey="u" rel="up">Mode Iterators</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="Substitution-in-Mode-Iterators"></a>

  77. <h4 class="subsubsection">17.23.1.2 Substitution in Mode Iterators</h4>

  78. <a name="index-define_005fmode_005fattr"></a>

  79. 

  80. <p>If an <samp>.md</samp> file construct uses mode iterators, each version of the

  81. construct will often need slightly different strings or modes.  For

  82. example:

  83. </p>

  84. <ul>

  85. <li> When a <code>define_expand</code> defines several <code>add<var>m</var>3</code> patterns

  86. (see <a href="Standard-Names.html#Standard-Names">Standard Names</a>), each expander will need to use the

  87. appropriate mode name for <var>m</var>.

  88. 

  89. </li><li> When a <code>define_insn</code> defines several instruction patterns,

  90. each instruction will often use a different assembler mnemonic.

  91. 

  92. </li><li> When a <code>define_insn</code> requires operands with different modes,

  93. using an iterator for one of the operand modes usually requires a specific

  94. mode for the other operand(s).

  95. </li></ul>

  96. 

  97. <p>GCC supports such variations through a system of &ldquo;mode attributes&rdquo;.

  98. There are two standard attributes: <code>mode</code>, which is the name of

  99. the mode in lower case, and <code>MODE</code>, which is the same thing in

  100. upper case.  You can define other attributes using:

  101. </p>

  102. <div class="smallexample">

  103. <pre class="smallexample">(define_mode_attr <var>name</var> [(<var>mode1</var> &quot;<var>value1</var>&quot;) &hellip; (<var>moden</var> &quot;<var>valuen</var>&quot;)])

  104. </pre></div>

  105. 

  106. <p>where <var>name</var> is the name of the attribute and <var>valuei</var>

  107. is the value associated with <var>modei</var>.

  108. </p>

  109. <p>When GCC replaces some <var>:iterator</var> with <var>:mode</var>, it will scan

  110. each string and mode in the pattern for sequences of the form

  111. <code>&lt;<var>iterator</var>:<var>attr</var>&gt;</code>, where <var>attr</var> is the name of a

  112. mode attribute.  If the attribute is defined for <var>mode</var>, the whole

  113. <code>&lt;&hellip;&gt;</code> sequence will be replaced by the appropriate attribute

  114. value.

  115. </p>

  116. <p>For example, suppose an <samp>.md</samp> file has:

  117. </p>

  118. <div class="smallexample">

  119. <pre class="smallexample">(define_mode_iterator P [(SI &quot;Pmode == SImode&quot;) (DI &quot;Pmode == DImode&quot;)])

  120. (define_mode_attr load [(SI &quot;lw&quot;) (DI &quot;ld&quot;)])

  121. </pre></div>

  122. 

  123. <p>If one of the patterns that uses <code>:P</code> contains the string

  124. <code>&quot;&lt;P:load&gt;\t%0,%1&quot;</code>, the <code>SI</code> version of that pattern

  125. will use <code>&quot;lw\t%0,%1&quot;</code> and the <code>DI</code> version will use

  126. <code>&quot;ld\t%0,%1&quot;</code>.

  127. </p>

  128. <p>Here is an example of using an attribute for a mode:

  129. </p>

  130. <div class="smallexample">

  131. <pre class="smallexample">(define_mode_iterator LONG [SI DI])

  132. (define_mode_attr SHORT [(SI &quot;HI&quot;) (DI &quot;SI&quot;)])

  133. (define_insn &hellip;

  134.   (sign_extend:LONG (match_operand:&lt;LONG:SHORT&gt; &hellip;)) &hellip;)

  135. </pre></div>

  136. 

  137. <p>The <code><var>iterator</var>:</code> prefix may be omitted, in which case the

  138. substitution will be attempted for every iterator expansion.

  139. </p>

  140. <hr>

  141. <div class="header">

  142. <p>

  143. Next: <a href="Examples.html#Examples" accesskey="n" rel="next">Examples</a>, Previous: <a href="Defining-Mode-Iterators.html#Defining-Mode-Iterators" accesskey="p" rel="prev">Defining Mode Iterators</a>, Up: <a href="Mode-Iterators.html#Mode-Iterators" accesskey="u" rel="up">Mode Iterators</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>

  144. </div>

  145. 

  146. 

  147. 

  148. </body>

  149. </html>