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| <a name="Modifiers"></a> |
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| Next: <a href="Machine-Constraints.html#Machine-Constraints" accesskey="n" rel="next">Machine Constraints</a>, Previous: <a href="Class-Preferences.html#Class-Preferences" accesskey="p" rel="prev">Class Preferences</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> |
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| <hr> |
| <a name="Constraint-Modifier-Characters"></a> |
| <h4 class="subsection">16.8.4 Constraint Modifier Characters</h4> |
| <a name="index-modifiers-in-constraints"></a> |
| <a name="index-constraint-modifier-characters"></a> |
| |
| <p>Here are constraint modifier characters. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-_003d-in-constraint"></a> |
| </dd> |
| <dt>‘<samp>=</samp>’</dt> |
| <dd><p>Means that this operand is written to by this instruction: |
| the previous value is discarded and replaced by new data. |
| </p> |
| <a name="index-_002b-in-constraint"></a> |
| </dd> |
| <dt>‘<samp>+</samp>’</dt> |
| <dd><p>Means that this operand is both read and written by the instruction. |
| </p> |
| <p>When the compiler fixes up the operands to satisfy the constraints, |
| it needs to know which operands are read by the instruction and |
| which are written by it. ‘<samp>=</samp>’ identifies an operand which is only |
| written; ‘<samp>+</samp>’ identifies an operand that is both read and written; all |
| other operands are assumed to only be read. |
| </p> |
| <p>If you specify ‘<samp>=</samp>’ or ‘<samp>+</samp>’ in a constraint, you put it in the |
| first character of the constraint string. |
| </p> |
| <a name="index-_0026-in-constraint"></a> |
| <a name="index-earlyclobber-operand"></a> |
| </dd> |
| <dt>‘<samp>&</samp>’</dt> |
| <dd><p>Means (in a particular alternative) that this operand is an |
| <em>earlyclobber</em> operand, which is written before the instruction is |
| finished using the input operands. Therefore, this operand may not lie |
| in a register that is read by the instruction or as part of any memory |
| address. |
| </p> |
| <p>‘<samp>&</samp>’ applies only to the alternative in which it is written. In |
| constraints with multiple alternatives, sometimes one alternative |
| requires ‘<samp>&</samp>’ while others do not. See, for example, the |
| ‘<samp>movdf</samp>’ insn of the 68000. |
| </p> |
| <p>A operand which is read by the instruction can be tied to an earlyclobber |
| operand if its only use as an input occurs before the early result is |
| written. Adding alternatives of this form often allows GCC to produce |
| better code when only some of the read operands can be affected by the |
| earlyclobber. See, for example, the ‘<samp>mulsi3</samp>’ insn of the ARM. |
| </p> |
| <p>Furthermore, if the <em>earlyclobber</em> operand is also a read/write |
| operand, then that operand is written only after it’s used. |
| </p> |
| <p>‘<samp>&</samp>’ does not obviate the need to write ‘<samp>=</samp>’ or ‘<samp>+</samp>’. As |
| <em>earlyclobber</em> operands are always written, a read-only |
| <em>earlyclobber</em> operand is ill-formed and will be rejected by the |
| compiler. |
| </p> |
| <a name="index-_0025-in-constraint"></a> |
| </dd> |
| <dt>‘<samp>%</samp>’</dt> |
| <dd><p>Declares the instruction to be commutative for this operand and the |
| following operand. This means that the compiler may interchange the |
| two operands if that is the cheapest way to make all operands fit the |
| constraints. ‘<samp>%</samp>’ applies to all alternatives and must appear as |
| the first character in the constraint. Only read-only operands can use |
| ‘<samp>%</samp>’. |
| </p> |
| <p>This is often used in patterns for addition instructions |
| that really have only two operands: the result must go in one of the |
| arguments. Here for example, is how the 68000 halfword-add |
| instruction is defined: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_insn "addhi3" |
| [(set (match_operand:HI 0 "general_operand" "=m,r") |
| (plus:HI (match_operand:HI 1 "general_operand" "%0,0") |
| (match_operand:HI 2 "general_operand" "di,g")))] |
| …) |
| </pre></div> |
| <p>GCC can only handle one commutative pair in an asm; if you use more, |
| the compiler may fail. Note that you need not use the modifier if |
| the two alternatives are strictly identical; this would only waste |
| time in the reload pass. The modifier is not operational after |
| register allocation, so the result of <code>define_peephole2</code> |
| and <code>define_split</code>s performed after reload cannot rely on |
| ‘<samp>%</samp>’ to make the intended insn match. |
| </p> |
| <a name="index-_0023-in-constraint"></a> |
| </dd> |
| <dt>‘<samp>#</samp>’</dt> |
| <dd><p>Says that all following characters, up to the next comma, are to be |
| ignored as a constraint. They are significant only for choosing |
| register preferences. |
| </p> |
| <a name="index-_002a-in-constraint"></a> |
| </dd> |
| <dt>‘<samp>*</samp>’</dt> |
| <dd><p>Says that the following character should be ignored when choosing |
| register preferences. ‘<samp>*</samp>’ has no effect on the meaning of the |
| constraint as a constraint, and no effect on reloading. For LRA |
| ‘<samp>*</samp>’ additionally disparages slightly the alternative if the |
| following character matches the operand. |
| </p> |
| <p>Here is an example: the 68000 has an instruction to sign-extend a |
| halfword in a data register, and can also sign-extend a value by |
| copying it into an address register. While either kind of register is |
| acceptable, the constraints on an address-register destination are |
| less strict, so it is best if register allocation makes an address |
| register its goal. Therefore, ‘<samp>*</samp>’ is used so that the ‘<samp>d</samp>’ |
| constraint letter (for data register) is ignored when computing |
| register preferences. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_insn "extendhisi2" |
| [(set (match_operand:SI 0 "general_operand" "=*d,a") |
| (sign_extend:SI |
| (match_operand:HI 1 "general_operand" "0,g")))] |
| …) |
| </pre></div> |
| </dd> |
| </dl> |
| |
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