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<a name="Multi_002dAlternative"></a>
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<a name="Multiple-Alternative-Constraints"></a>
<h4 class="subsubsection">6.47.3.2 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.
All operands for a single instruction must have the same number of 
alternatives.
</p>
<p>So the first alternative for the 68000&rsquo;s logical-or could be written as 
<code>&quot;+m&quot; (output) : &quot;ir&quot; (input)</code>.  The second could be <code>&quot;+r&quot; 
(output): &quot;irm&quot; (input)</code>.  However, the fact that two memory locations 
cannot be used in a single instruction prevents simply using <code>&quot;+rm&quot; 
(output) : &quot;irm&quot; (input)</code>.  Using multi-alternatives, this might be 
written as <code>&quot;+m,r&quot; (output) : &quot;ir,irm&quot; (input)</code>.  This describes
all the available alternatives to the compiler, allowing it to choose 
the most efficient one for the current conditions.
</p>
<p>There is no way within the template to determine which alternative was 
chosen.  However you may be able to wrap your <code>asm</code> statements with 
builtins such as <code>__builtin_constant_p</code> to achieve the desired results.
</p>


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