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<a name="Output-Template"></a>
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<div class="header">
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<p>
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Next: <a href="Output-Statement.html#Output-Statement" accesskey="n" rel="next">Output Statement</a>, Previous: <a href="RTL-Template.html#RTL-Template" accesskey="p" rel="prev">RTL Template</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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>
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<a name="Output-Templates-and-Operand-Substitution"></a>
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<h3 class="section">16.5 Output Templates and Operand Substitution</h3>
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<a name="index-output-templates"></a>
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<a name="index-operand-substitution"></a>
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<a name="index-_0025-in-template"></a>
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<a name="index-percent-sign"></a>
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<p>The <em>output template</em> 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>%</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>
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<p>In the simplest case, a ‘<samp>%</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>
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<p>‘<samp>%</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>
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<p>‘<samp>%c<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>
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<p>‘<samp>%n<var>digit</var></samp>’ is like ‘<samp>%c<var>digit</var></samp>’ except that the value of
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the constant is negated before printing.
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</p>
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<p>‘<samp>%a<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>
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<p>‘<samp>%l<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>
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<p>‘<samp>%=</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>
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<p>‘<samp>%</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>%%</samp>’ outputs a
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‘<samp>%</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>
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<a name="index-_005c"></a>
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<a name="index-backslash"></a>
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<p>The template may generate multiple assembler instructions. Write the text
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for the instructions, with ‘<samp>\;</samp>’ between them.
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</p>
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<a name="index-matching-operands"></a>
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<p>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>
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<p>One use of nonstandard letters or punctuation following ‘<samp>%</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>movel</samp>’ in MIT syntax is ‘<samp>move.l</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>%.</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>
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<a name="index-_0023-in-template"></a>
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<p>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>
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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>{option0|option1|option2}</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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Output-Statement.html#Output-Statement" accesskey="n" rel="next">Output Statement</a>, Previous: <a href="RTL-Template.html#RTL-Template" accesskey="p" rel="prev">RTL Template</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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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