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 <a name="Multi_002dAlternative"></a>
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 Next: <a href="Modifiers.html#Modifiers" accesskey="n" rel="next">Modifiers</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>
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 <hr>
 <a name="Multiple-Alternative-Constraints"></a>
 <h4 class="subsection">6.42.2 Multiple Alternative Constraints</h4>
 <a name="index-multiple-alternative-constraints"></a>

 <p>Sometimes a single instruction has multiple alternative sets of possible
 operands.  For example, on the 68000, a logical-or instruction can combine
 register or an immediate value into memory, or it can combine any kind of
 operand into a register; but it cannot combine one memory location into
 another.
 </p>
 <p>These constraints are represented as multiple alternatives.  An alternative
 can be described by a series of letters for each operand.  The overall
 constraint for an operand is made from the letters for this operand
 from the first alternative, a comma, the letters for this operand from
 the second alternative, a comma, and so on until the last alternative.
 </p>
 <p>If all the operands fit any one alternative, the instruction is valid.
 Otherwise, for each alternative, the compiler counts how many instructions
 must be added to copy the operands so that that alternative applies.
 The alternative requiring the least copying is chosen.  If two alternatives
 need the same amount of copying, the one that comes first is chosen.
 These choices can be altered with the &lsquo;<samp>?</samp>&rsquo; and &lsquo;<samp>!</samp>&rsquo; characters:
 </p>
 <dl compact="compact">
 <dd><a name="index-_003f-in-constraint"></a>
 <a name="index-question-mark"></a>
 </dd>
 <dt><code>?</code></dt>
 <dd><p>Disparage slightly the alternative that the &lsquo;<samp>?</samp>&rsquo; appears in,
 as a choice when no alternative applies exactly.  The compiler regards
 this alternative as one unit more costly for each &lsquo;<samp>?</samp>&rsquo; that appears
 in it.
 </p>
 <a name="index-_0021-in-constraint"></a>
 <a name="index-exclamation-point"></a>
 </dd>
 <dt><code>!</code></dt>
 <dd><p>Disparage severely the alternative that the &lsquo;<samp>!</samp>&rsquo; appears in.
 This alternative can still be used if it fits without reloading,
 but if reloading is needed, some other alternative will be used.
 </p></dd>
 </dl>


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	<a name="Multi_002dAlternative"></a>
	<div class="header">
	<p>
	Next: <a href="Modifiers.html#Modifiers" accesskey="n" rel="next">Modifiers</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>   [<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>
	</div>
	<hr>
	<a name="Multiple-Alternative-Constraints"></a>
	<h4 class="subsection">6.42.2 Multiple Alternative Constraints</h4>
	<a name="index-multiple-alternative-constraints"></a>

	<p>Sometimes a single instruction has multiple alternative sets of possible
	operands. For example, on the 68000, a logical-or instruction can combine
	register or an immediate value into memory, or it can combine any kind of
	operand into a register; but it cannot combine one memory location into
	another.
	</p>
	<p>These constraints are represented as multiple alternatives. An alternative
	can be described by a series of letters for each operand. The overall
	constraint for an operand is made from the letters for this operand
	from the first alternative, a comma, the letters for this operand from
	the second alternative, a comma, and so on until the last alternative.
	</p>
	<p>If all the operands fit any one alternative, the instruction is valid.
	Otherwise, for each alternative, the compiler counts how many instructions
	must be added to copy the operands so that that alternative applies.
	The alternative requiring the least copying is chosen. If two alternatives
	need the same amount of copying, the one that comes first is chosen.
	These choices can be altered with the ‘<samp>?</samp>’ and ‘<samp>!</samp>’ characters:
	</p>
	<dl compact="compact">
	<dd><a name="index-_003f-in-constraint"></a>
	<a name="index-question-mark"></a>
	</dd>
	<dt><code>?</code></dt>
	<dd><p>Disparage slightly the alternative that the ‘<samp>?</samp>’ appears in,
	as a choice when no alternative applies exactly. The compiler regards
	this alternative as one unit more costly for each ‘<samp>?</samp>’ that appears
	in it.
	</p>
	<a name="index-_0021-in-constraint"></a>
	<a name="index-exclamation-point"></a>
	</dd>
	<dt><code>!</code></dt>
	<dd><p>Disparage severely the alternative that the ‘<samp>!</samp>’ appears in.
	This alternative can still be used if it fits without reloading,
	but if reloading is needed, some other alternative will be used.
	</p></dd>
	</dl>



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