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