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<a name="Multi-Alternative"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Modifiers.html#Modifiers">Modifiers</a>,
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Previous: <a rel="previous" accesskey="p" href="Simple-Constraints.html#Simple-Constraints">Simple Constraints</a>,
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Up: <a rel="up" accesskey="u" href="Constraints.html#Constraints">Constraints</a>
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<h5 class="subsubsection">6.43.3.2 Multiple Alternative Constraints</h5>
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<p><a name="index-multiple-alternative-constraints-3469"></a>
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Sometimes a single instruction has multiple alternative sets of possible
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operands. For example, on the 68000, a logical-or instruction can combine
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register or an immediate value into memory, or it can combine any kind of
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operand into a register; but it cannot combine one memory location into
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another.
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<p>These constraints are represented as multiple alternatives. An alternative
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can be described by a series of letters for each operand. The overall
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constraint for an operand is made from the letters for this operand
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from the first alternative, a comma, the letters for this operand from
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the second alternative, a comma, and so on until the last alternative.
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<!-- FIXME Is this ? and ! stuff of use in asm()? If not, hide unless INTERNAL -->
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<p>If all the operands fit any one alternative, the instruction is valid.
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Otherwise, for each alternative, the compiler counts how many instructions
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must be added to copy the operands so that that alternative applies.
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The alternative requiring the least copying is chosen. If two alternatives
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need the same amount of copying, the one that comes first is chosen.
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These choices can be altered with the ‘<samp><span class="samp">?</span></samp>’ and ‘<samp><span class="samp">!</span></samp>’ characters:
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<a name="index-g_t_0040samp_007b_003f_007d-in-constraint-3470"></a>
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<a name="index-question-mark-3471"></a>
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<dl><dt><code>?</code><dd>Disparage slightly the alternative that the ‘<samp><span class="samp">?</span></samp>’ appears in,
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as a choice when no alternative applies exactly. The compiler regards
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this alternative as one unit more costly for each ‘<samp><span class="samp">?</span></samp>’ that appears
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in it.
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<p><a name="index-g_t_0040samp_007b_0021_007d-in-constraint-3472"></a><a name="index-exclamation-point-3473"></a><br><dt><code>!</code><dd>Disparage severely the alternative that the ‘<samp><span class="samp">!</span></samp>’ appears in.
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This alternative can still be used if it fits without reloading,
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but if reloading is needed, some other alternative will be used.
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<p><a name="index-g_t_0040samp_007b_005e_007d-in-constraint-3474"></a><a name="index-caret-3475"></a><br><dt><code>^</code><dd>This constraint is analogous to ‘<samp><span class="samp">?</span></samp>’ but it disparages slightly
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the alternative only if the operand with the ‘<samp><span class="samp">^</span></samp>’ needs a reload.
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<p><a name="index-g_t_0040samp_007b_0024_007d-in-constraint-3476"></a><a name="index-dollar-sign-3477"></a><br><dt><code>$</code><dd>This constraint is analogous to ‘<samp><span class="samp">!</span></samp>’ but it disparages severely
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the alternative only if the operand with the ‘<samp><span class="samp">$</span></samp>’ needs a reload.
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</dl>
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