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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

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  15. A GNU Manual

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  25. <title>Multi-Alternative (GNU Compiler Collection (GCC) Internals)</title>

  26. 

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  32. <link href="index.html#Top" rel="start" title="Top">

  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="Constraints.html#Constraints" rel="up" title="Constraints">

  36. <link href="Class-Preferences.html#Class-Preferences" rel="next" title="Class Preferences">

  37. <link href="Simple-Constraints.html#Simple-Constraints" rel="prev" title="Simple Constraints">

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  69. <body lang="en">

  70. <a name="Multi_002dAlternative"></a>

  71. <div class="header">

  72. <p>

  73. Next: <a href="Class-Preferences.html#Class-Preferences" accesskey="n" rel="next">Class Preferences</a>, Previous: <a href="Simple-Constraints.html#Simple-Constraints" accesskey="p" rel="prev">Simple 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="Multiple-Alternative-Constraints"></a>

  77. <h4 class="subsection">17.8.2 Multiple Alternative Constraints</h4>

  78. <a name="index-multiple-alternative-constraints"></a>

  79. 

  80. <p>Sometimes a single instruction has multiple alternative sets of possible

  81. operands.  For example, on the 68000, a logical-or instruction can combine

  82. register or an immediate value into memory, or it can combine any kind of

  83. operand into a register; but it cannot combine one memory location into

  84. another.

  85. </p>

  86. <p>These constraints are represented as multiple alternatives.  An alternative

  87. can be described by a series of letters for each operand.  The overall

  88. constraint for an operand is made from the letters for this operand

  89. from the first alternative, a comma, the letters for this operand from

  90. the second alternative, a comma, and so on until the last alternative.

  91. All operands for a single instruction must have the same number of 

  92. alternatives.

  93. Here is how it is done for fullword logical-or on the 68000:

  94. </p>

  95. <div class="smallexample">

  96. <pre class="smallexample">(define_insn &quot;iorsi3&quot;

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

  98.         (ior:SI (match_operand:SI 1 &quot;general_operand&quot; &quot;%0,0&quot;)

  99.                 (match_operand:SI 2 &quot;general_operand&quot; &quot;dKs,dmKs&quot;)))]

  100.   &hellip;)

  101. </pre></div>

  102. 

  103. <p>The first alternative has &lsquo;<samp>m</samp>&rsquo; (memory) for operand 0, &lsquo;<samp>0</samp>&rsquo; for

  104. operand 1 (meaning it must match operand 0), and &lsquo;<samp>dKs</samp>&rsquo; for operand

  105. 2.  The second alternative has &lsquo;<samp>d</samp>&rsquo; (data register) for operand 0,

  106. &lsquo;<samp>0</samp>&rsquo; for operand 1, and &lsquo;<samp>dmKs</samp>&rsquo; for operand 2.  The &lsquo;<samp>=</samp>&rsquo; and

  107. &lsquo;<samp>%</samp>&rsquo; in the constraints apply to all the alternatives; their

  108. meaning is explained in the next section (see <a href="Class-Preferences.html#Class-Preferences">Class Preferences</a>).

  109. </p>

  110. <p>If all the operands fit any one alternative, the instruction is valid.

  111. Otherwise, for each alternative, the compiler counts how many instructions

  112. must be added to copy the operands so that that alternative applies.

  113. The alternative requiring the least copying is chosen.  If two alternatives

  114. need the same amount of copying, the one that comes first is chosen.

  115. These choices can be altered with the &lsquo;<samp>?</samp>&rsquo; and &lsquo;<samp>!</samp>&rsquo; characters:

  116. </p>

  117. <dl compact="compact">

  118. <dd><a name="index-_003f-in-constraint"></a>

  119. <a name="index-question-mark"></a>

  120. </dd>

  121. <dt><code>?</code></dt>

  122. <dd><p>Disparage slightly the alternative that the &lsquo;<samp>?</samp>&rsquo; appears in,

  123. as a choice when no alternative applies exactly.  The compiler regards

  124. this alternative as one unit more costly for each &lsquo;<samp>?</samp>&rsquo; that appears

  125. in it.

  126. </p>

  127. <a name="index-_0021-in-constraint"></a>

  128. <a name="index-exclamation-point"></a>

  129. </dd>

  130. <dt><code>!</code></dt>

  131. <dd><p>Disparage severely the alternative that the &lsquo;<samp>!</samp>&rsquo; appears in.

  132. This alternative can still be used if it fits without reloading,

  133. but if reloading is needed, some other alternative will be used.

  134. </p>

  135. <a name="index-_005e-in-constraint"></a>

  136. <a name="index-caret"></a>

  137. </dd>

  138. <dt><code>^</code></dt>

  139. <dd><p>This constraint is analogous to &lsquo;<samp>?</samp>&rsquo; but it disparages slightly

  140. the alternative only if the operand with the &lsquo;<samp>^</samp>&rsquo; needs a reload.

  141. </p>

  142. <a name="index-_0024-in-constraint"></a>

  143. <a name="index-dollar-sign"></a>

  144. </dd>

  145. <dt><code>$</code></dt>

  146. <dd><p>This constraint is analogous to &lsquo;<samp>!</samp>&rsquo; but it disparages severely

  147. the alternative only if the operand with the &lsquo;<samp>$</samp>&rsquo; needs a reload.

  148. </p></dd>

  149. </dl>

  150. 

  151. <p>When an insn pattern has multiple alternatives in its constraints, often

  152. the appearance of the assembler code is determined mostly by which

  153. alternative was matched.  When this is so, the C code for writing the

  154. assembler code can use the variable <code>which_alternative</code>, which is

  155. the ordinal number of the alternative that was actually satisfied (0 for

  156. the first, 1 for the second alternative, etc.).  See <a href="Output-Statement.html#Output-Statement">Output Statement</a>.

  157. </p>

  158. <hr>

  159. <div class="header">

  160. <p>

  161. Next: <a href="Class-Preferences.html#Class-Preferences" accesskey="n" rel="next">Class Preferences</a>, Previous: <a href="Simple-Constraints.html#Simple-Constraints" accesskey="p" rel="prev">Simple 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>

  162. </div>

  163. 

  164. 

  165. 

  166. </body>

  167. </html>