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| <title>Using the GNU Compiler Collection (GCC): Modifiers</title> |
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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="Multi_002dAlternative.html#Multi_002dAlternative" accesskey="p" rel="prev">Multi-Alternative</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">6.42.3 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 write-only for this instruction: the previous |
| value is discarded and replaced by output 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 inputs to the instruction and |
| which are outputs from it. ‘<samp>=</samp>’ identifies an output; ‘<samp>+</samp>’ |
| identifies an operand that is both input and output; all other operands |
| are assumed to be input only. |
| </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 modified before the instruction is |
| finished using the input operands. Therefore, this operand may not lie |
| in a register that is used as an input operand 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>An input operand 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 inputs can be affected by the earlyclobber. |
| See, for example, the ‘<samp>mulsi3</samp>’ insn of the ARM. |
| </p> |
| <p>‘<samp>&</samp>’ does not obviate the need to write ‘<samp>=</samp>’. |
| </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. |
| 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> |
| </dd> |
| </dl> |
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