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137 lines
7.6 KiB
HTML
137 lines
7.6 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Output Template - GNU Compiler Collection (GCC) Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Machine-Desc.html#Machine-Desc" title="Machine Desc">
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<link rel="prev" href="RTL-Template.html#RTL-Template" title="RTL Template">
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<link rel="next" href="Output-Statement.html#Output-Statement" title="Output Statement">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Output-Template"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Output-Statement.html#Output-Statement">Output Statement</a>,
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Previous: <a rel="previous" accesskey="p" href="RTL-Template.html#RTL-Template">RTL Template</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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<hr>
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</div>
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<h3 class="section">16.5 Output Templates and Operand Substitution</h3>
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<p><a name="index-output-templates-3262"></a><a name="index-operand-substitution-3263"></a>
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<a name="index-g_t_0040samp_007b_0025_007d-in-template-3264"></a><a name="index-percent-sign-3265"></a>The <dfn>output template</dfn> is a string which specifies how to output the
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assembler code for an instruction pattern. Most of the template is a
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fixed string which is output literally. The character ‘<samp><span class="samp">%</span></samp>’ is used
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to specify where to substitute an operand; it can also be used to
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identify places where different variants of the assembler require
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different syntax.
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<p>In the simplest case, a ‘<samp><span class="samp">%</span></samp>’ followed by a digit <var>n</var> says to output
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operand <var>n</var> at that point in the string.
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<p>‘<samp><span class="samp">%</span></samp>’ followed by a letter and a digit says to output an operand in an
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alternate fashion. Four letters have standard, built-in meanings described
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below. The machine description macro <code>PRINT_OPERAND</code> can define
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additional letters with nonstandard meanings.
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<p>‘<samp><span class="samp">%c</span><var>digit</var></samp>’ can be used to substitute an operand that is a
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constant value without the syntax that normally indicates an immediate
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operand.
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<p>‘<samp><span class="samp">%n</span><var>digit</var></samp>’ is like ‘<samp><span class="samp">%c</span><var>digit</var></samp>’ except that the value of
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the constant is negated before printing.
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<p>‘<samp><span class="samp">%a</span><var>digit</var></samp>’ can be used to substitute an operand as if it were a
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memory reference, with the actual operand treated as the address. This may
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be useful when outputting a “load address” instruction, because often the
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assembler syntax for such an instruction requires you to write the operand
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as if it were a memory reference.
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<p>‘<samp><span class="samp">%l</span><var>digit</var></samp>’ is used to substitute a <code>label_ref</code> into a jump
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instruction.
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<p>‘<samp><span class="samp">%=</span></samp>’ outputs a number which is unique to each instruction in the
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entire compilation. This is useful for making local labels to be
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referred to more than once in a single template that generates multiple
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assembler instructions.
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<p>‘<samp><span class="samp">%</span></samp>’ followed by a punctuation character specifies a substitution that
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does not use an operand. Only one case is standard: ‘<samp><span class="samp">%%</span></samp>’ outputs a
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‘<samp><span class="samp">%</span></samp>’ into the assembler code. Other nonstandard cases can be
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defined in the <code>PRINT_OPERAND</code> macro. You must also define
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which punctuation characters are valid with the
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<code>PRINT_OPERAND_PUNCT_VALID_P</code> macro.
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<p><a name="index-g_t_005c-3266"></a><a name="index-backslash-3267"></a>The template may generate multiple assembler instructions. Write the text
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for the instructions, with ‘<samp><span class="samp">\;</span></samp>’ between them.
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<p><a name="index-matching-operands-3268"></a>When the RTL contains two operands which are required by constraint to match
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each other, the output template must refer only to the lower-numbered operand.
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Matching operands are not always identical, and the rest of the compiler
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arranges to put the proper RTL expression for printing into the lower-numbered
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operand.
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<p>One use of nonstandard letters or punctuation following ‘<samp><span class="samp">%</span></samp>’ is to
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distinguish between different assembler languages for the same machine; for
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example, Motorola syntax versus MIT syntax for the 68000. Motorola syntax
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requires periods in most opcode names, while MIT syntax does not. For
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example, the opcode ‘<samp><span class="samp">movel</span></samp>’ in MIT syntax is ‘<samp><span class="samp">move.l</span></samp>’ in Motorola
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syntax. The same file of patterns is used for both kinds of output syntax,
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but the character sequence ‘<samp><span class="samp">%.</span></samp>’ is used in each place where Motorola
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syntax wants a period. The <code>PRINT_OPERAND</code> macro for Motorola syntax
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defines the sequence to output a period; the macro for MIT syntax defines
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it to do nothing.
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<p><a name="index-g_t_0040code_007b_0023_007d-in-template-3269"></a>As a special case, a template consisting of the single character <code>#</code>
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instructs the compiler to first split the insn, and then output the
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resulting instructions separately. This helps eliminate redundancy in the
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output templates. If you have a <code>define_insn</code> that needs to emit
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multiple assembler instructions, and there is a matching <code>define_split</code>
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already defined, then you can simply use <code>#</code> as the output template
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instead of writing an output template that emits the multiple assembler
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instructions.
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<p>If the macro <code>ASSEMBLER_DIALECT</code> is defined, you can use construct
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of the form ‘<samp><span class="samp">{option0|option1|option2}</span></samp>’ in the templates. These
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describe multiple variants of assembler language syntax.
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See <a href="Instruction-Output.html#Instruction-Output">Instruction Output</a>.
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</body></html>
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