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<title>Modifiers - GNU Compiler Collection (GCC) Internals</title>
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<a name="Modifiers"></a>
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Next: <a rel="next" accesskey="n" href="Machine-Constraints.html#Machine-Constraints">Machine Constraints</a>,
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Previous: <a rel="previous" accesskey="p" href="Class-Preferences.html#Class-Preferences">Class Preferences</a>,
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Up: <a rel="up" accesskey="u" href="Constraints.html#Constraints">Constraints</a>
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<h4 class="subsection">16.8.4 Constraint Modifier Characters</h4>
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<p><a name="index-modifiers-in-constraints-3353"></a><a name="index-constraint-modifier-characters-3354"></a>
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<!-- prevent bad page break with this line -->
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Here are constraint modifier characters.
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<a name="index-g_t_0040samp_007b_003d_007d-in-constraint-3355"></a>
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<dl><dt>‘<samp><span class="samp">=</span></samp>’<dd>Means that this operand is written to by this instruction:
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the previous value is discarded and replaced by new data.
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<p><a name="index-g_t_0040samp_007b_002b_007d-in-constraint-3356"></a><br><dt>‘<samp><span class="samp">+</span></samp>’<dd>Means that this operand is both read and written by the instruction.
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<p>When the compiler fixes up the operands to satisfy the constraints,
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it needs to know which operands are read by the instruction and
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which are written by it. ‘<samp><span class="samp">=</span></samp>’ identifies an operand which is only
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written; ‘<samp><span class="samp">+</span></samp>’ identifies an operand that is both read and written; all
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other operands are assumed to only be read.
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<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
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first character of the constraint string.
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<p><a name="index-g_t_0040samp_007b_0026_007d-in-constraint-3357"></a><a name="index-earlyclobber-operand-3358"></a><br><dt>‘<samp><span class="samp">&</span></samp>’<dd>Means (in a particular alternative) that this operand is an
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<dfn>earlyclobber</dfn> operand, which is written before the instruction is
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finished using the input operands. Therefore, this operand may not lie
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in a register that is read by the instruction or as part of any memory
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address.
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<p>‘<samp><span class="samp">&</span></samp>’ applies only to the alternative in which it is written. In
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constraints with multiple alternatives, sometimes one alternative
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requires ‘<samp><span class="samp">&</span></samp>’ while others do not. See, for example, the
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‘<samp><span class="samp">movdf</span></samp>’ insn of the 68000.
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<p>A operand which is read by the instruction can be tied to an earlyclobber
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operand if its only use as an input occurs before the early result is
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written. Adding alternatives of this form often allows GCC to produce
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better code when only some of the read operands can be affected by the
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earlyclobber. See, for example, the ‘<samp><span class="samp">mulsi3</span></samp>’ insn of the ARM.
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<p>Furthermore, if the <dfn>earlyclobber</dfn> operand is also a read/write
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operand, then that operand is written only after it's used.
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<p>‘<samp><span class="samp">&</span></samp>’ does not obviate the need to write ‘<samp><span class="samp">=</span></samp>’ or ‘<samp><span class="samp">+</span></samp>’. As
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<dfn>earlyclobber</dfn> operands are always written, a read-only
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<dfn>earlyclobber</dfn> operand is ill-formed and will be rejected by the
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compiler.
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<p><a name="index-g_t_0040samp_007b_0025_007d-in-constraint-3359"></a><br><dt>‘<samp><span class="samp">%</span></samp>’<dd>Declares the instruction to be commutative for this operand and the
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following operand. This means that the compiler may interchange the
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two operands if that is the cheapest way to make all operands fit the
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constraints. ‘<samp><span class="samp">%</span></samp>’ applies to all alternatives and must appear as
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the first character in the constraint. Only read-only operands can use
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‘<samp><span class="samp">%</span></samp>’.
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<p>This is often used in patterns for addition instructions
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that really have only two operands: the result must go in one of the
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arguments. Here for example, is how the 68000 halfword-add
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instruction is defined:
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<pre class="smallexample"> (define_insn "addhi3"
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[(set (match_operand:HI 0 "general_operand" "=m,r")
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(plus:HI (match_operand:HI 1 "general_operand" "%0,0")
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(match_operand:HI 2 "general_operand" "di,g")))]
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...)
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</pre>
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<p>GCC can only handle one commutative pair in an asm; if you use more,
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the compiler may fail. Note that you need not use the modifier if
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the two alternatives are strictly identical; this would only waste
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time in the reload pass. The modifier is not operational after
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register allocation, so the result of <code>define_peephole2</code>
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and <code>define_split</code>s performed after reload cannot rely on
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‘<samp><span class="samp">%</span></samp>’ to make the intended insn match.
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<p><a name="index-g_t_0040samp_007b_0023_007d-in-constraint-3360"></a><br><dt>‘<samp><span class="samp">#</span></samp>’<dd>Says that all following characters, up to the next comma, are to be
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ignored as a constraint. They are significant only for choosing
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register preferences.
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<p><a name="index-g_t_0040samp_007b_002a_007d-in-constraint-3361"></a><br><dt>‘<samp><span class="samp">*</span></samp>’<dd>Says that the following character should be ignored when choosing
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register preferences. ‘<samp><span class="samp">*</span></samp>’ has no effect on the meaning of the
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constraint as a constraint, and no effect on reloading. For LRA
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‘<samp><span class="samp">*</span></samp>’ additionally disparages slightly the alternative if the
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following character matches the operand.
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<p>Here is an example: the 68000 has an instruction to sign-extend a
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halfword in a data register, and can also sign-extend a value by
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copying it into an address register. While either kind of register is
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acceptable, the constraints on an address-register destination are
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less strict, so it is best if register allocation makes an address
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register its goal. Therefore, ‘<samp><span class="samp">*</span></samp>’ is used so that the ‘<samp><span class="samp">d</span></samp>’
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constraint letter (for data register) is ignored when computing
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register preferences.
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<pre class="smallexample"> (define_insn "extendhisi2"
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[(set (match_operand:SI 0 "general_operand" "=*d,a")
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(sign_extend:SI
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(match_operand:HI 1 "general_operand" "0,g")))]
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...)
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</pre>
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</dl>
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