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<title>Multi-Alternative - GNU Compiler Collection (GCC) Internals</title>
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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="Class-Preferences.html#Class-Preferences">Class Preferences</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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<h4 class="subsection">16.8.2 Multiple Alternative Constraints</h4>
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<p><a name="index-multiple-alternative-constraints-3341"></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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Here is how it is done for fullword logical-or on the 68000:
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<pre class="smallexample"> (define_insn "iorsi3"
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[(set (match_operand:SI 0 "general_operand" "=m,d")
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(ior:SI (match_operand:SI 1 "general_operand" "%0,0")
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(match_operand:SI 2 "general_operand" "dKs,dmKs")))]
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...)
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</pre>
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<p>The first alternative has ‘<samp><span class="samp">m</span></samp>’ (memory) for operand 0, ‘<samp><span class="samp">0</span></samp>’ for
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operand 1 (meaning it must match operand 0), and ‘<samp><span class="samp">dKs</span></samp>’ for operand
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2. The second alternative has ‘<samp><span class="samp">d</span></samp>’ (data register) for operand 0,
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‘<samp><span class="samp">0</span></samp>’ for operand 1, and ‘<samp><span class="samp">dmKs</span></samp>’ for operand 2. The ‘<samp><span class="samp">=</span></samp>’ and
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‘<samp><span class="samp">%</span></samp>’ in the constraints apply to all the alternatives; their
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meaning is explained in the next section (see <a href="Class-Preferences.html#Class-Preferences">Class Preferences</a>).
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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-3342"></a>
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<a name="index-question-mark-3343"></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-3344"></a><a name="index-exclamation-point-3345"></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-3346"></a><a name="index-caret-3347"></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-3348"></a><a name="index-dollar-sign-3349"></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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<p>When an insn pattern has multiple alternatives in its constraints, often
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the appearance of the assembler code is determined mostly by which
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alternative was matched. When this is so, the C code for writing the
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assembler code can use the variable <code>which_alternative</code>, which is
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the ordinal number of the alternative that was actually satisfied (0 for
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the first, 1 for the second alternative, etc.). See <a href="Output-Statement.html#Output-Statement">Output Statement</a>.
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</body></html>
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