210 lines
9.7 KiB
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210 lines
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HTML
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Modifiers</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Modifiers">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Constraints.html#Constraints" rel="up" title="Constraints">
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<link href="Machine-Constraints.html#Machine-Constraints" rel="next" title="Machine Constraints">
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<link href="Class-Preferences.html#Class-Preferences" rel="prev" title="Class Preferences">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Modifiers"></a>
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<div class="header">
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<p>
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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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</div>
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<hr>
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<a name="Constraint-Modifier-Characters"></a>
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<h4 class="subsection">16.8.4 Constraint Modifier Characters</h4>
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<a name="index-modifiers-in-constraints"></a>
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<a name="index-constraint-modifier-characters"></a>
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<p>Here are constraint modifier characters.
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</p>
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<dl compact="compact">
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<dd><a name="index-_003d-in-constraint"></a>
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</dd>
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<dt>‘<samp>=</samp>’</dt>
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<dd><p>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>
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<a name="index-_002b-in-constraint"></a>
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</dd>
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<dt>‘<samp>+</samp>’</dt>
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<dd><p>Means that this operand is both read and written by the instruction.
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</p>
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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>=</samp>’ identifies an operand which is only
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written; ‘<samp>+</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>
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<p>If you specify ‘<samp>=</samp>’ or ‘<samp>+</samp>’ in a constraint, you put it in the
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first character of the constraint string.
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</p>
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<a name="index-_0026-in-constraint"></a>
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<a name="index-earlyclobber-operand"></a>
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</dd>
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<dt>‘<samp>&</samp>’</dt>
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<dd><p>Means (in a particular alternative) that this operand is an
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<em>earlyclobber</em> 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>
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<p>‘<samp>&</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>&</samp>’ while others do not. See, for example, the
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‘<samp>movdf</samp>’ insn of the 68000.
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</p>
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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>mulsi3</samp>’ insn of the ARM.
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</p>
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<p>Furthermore, if the <em>earlyclobber</em> 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>
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<p>‘<samp>&</samp>’ does not obviate the need to write ‘<samp>=</samp>’ or ‘<samp>+</samp>’. As
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<em>earlyclobber</em> operands are always written, a read-only
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<em>earlyclobber</em> operand is ill-formed and will be rejected by the
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compiler.
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</p>
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<a name="index-_0025-in-constraint"></a>
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</dd>
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<dt>‘<samp>%</samp>’</dt>
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<dd><p>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>%</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>%</samp>’.
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</p>
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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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</p>
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<div class="smallexample">
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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></div>
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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.
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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>%</samp>’ to make the intended insn match.
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</p>
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<a name="index-_0023-in-constraint"></a>
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</dd>
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<dt>‘<samp>#</samp>’</dt>
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<dd><p>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>
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<a name="index-_002a-in-constraint"></a>
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</dd>
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<dt>‘<samp>*</samp>’</dt>
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<dd><p>Says that the following character should be ignored when choosing
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register preferences. ‘<samp>*</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>*</samp>’ additionally disparages slightly the alternative if the
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following character matches the operand.
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</p>
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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>*</samp>’ is used so that the ‘<samp>d</samp>’
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constraint letter (for data register) is ignored when computing
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register preferences.
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</p>
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<div class="smallexample">
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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></div>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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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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</div>
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