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| <h4 class="subsection">6.40.3 Constraint Modifier Characters</h4> |
| |
| <p><a name="index-modifiers-in-constraints-2572"></a><a name="index-constraint-modifier-characters-2573"></a> |
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| Here are constraint modifier characters. |
| |
| |
| <a name="index-g_t_0040samp_007b_003d_007d-in-constraint-2574"></a> |
| <dl><dt>‘<samp><span class="samp">=</span></samp>’<dd>Means that this operand is write-only for this instruction: the previous |
| value is discarded and replaced by output data. |
| |
| <p><a name="index-g_t_0040samp_007b_002b_007d-in-constraint-2575"></a><br><dt>‘<samp><span class="samp">+</span></samp>’<dd>Means that this operand is both read and written by the instruction. |
| |
| <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><span class="samp">=</span></samp>’ identifies an output; ‘<samp><span class="samp">+</span></samp>’ |
| identifies an operand that is both input and output; all other operands |
| are assumed to be input only. |
| |
| <p>If you specify ‘<samp><span class="samp">=</span></samp>’ or ‘<samp><span class="samp">+</span></samp>’ in a constraint, you put it in the |
| first character of the constraint string. |
| |
| <p><a name="index-g_t_0040samp_007b_0026_007d-in-constraint-2576"></a><a name="index-earlyclobber-operand-2577"></a><br><dt>‘<samp><span class="samp">&</span></samp>’<dd>Means (in a particular alternative) that this operand is an |
| <dfn>earlyclobber</dfn> 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>‘<samp><span class="samp">&</span></samp>’ applies only to the alternative in which it is written. In |
| constraints with multiple alternatives, sometimes one alternative |
| requires ‘<samp><span class="samp">&</span></samp>’ while others do not. See, for example, the |
| ‘<samp><span class="samp">movdf</span></samp>’ insn of the 68000. |
| |
| <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><span class="samp">mulsi3</span></samp>’ insn of the ARM. |
| |
| <p>‘<samp><span class="samp">&</span></samp>’ does not obviate the need to write ‘<samp><span class="samp">=</span></samp>’. |
| |
| <p><a name="index-g_t_0040samp_007b_0025_007d-in-constraint-2578"></a><br><dt>‘<samp><span class="samp">%</span></samp>’<dd>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><span class="samp">%</span></samp>’ to make the intended insn match. |
| |
| <p><a name="index-g_t_0040samp_007b_0023_007d-in-constraint-2579"></a><br><dt>‘<samp><span class="samp">#</span></samp>’<dd>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-g_t_0040samp_007b_002a_007d-in-constraint-2580"></a><br><dt>‘<samp><span class="samp">*</span></samp>’<dd>Says that the following character should be ignored when choosing |
| register preferences. ‘<samp><span class="samp">*</span></samp>’ has no effect on the meaning of the |
| constraint as a constraint, and no effect on reloading. |
| |
| </dl> |
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