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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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| <a name="Output-Templates-and-Operand-Substitution"></a> |
| <h3 class="section">16.5 Output Templates and Operand Substitution</h3> |
| <a name="index-output-templates"></a> |
| <a name="index-operand-substitution"></a> |
| |
| <a name="index-_0025-in-template"></a> |
| <a name="index-percent-sign"></a> |
| <p>The <em>output template</em> is a string which specifies how to output the |
| assembler code for an instruction pattern. Most of the template is a |
| fixed string which is output literally. The character ‘<samp>%</samp>’ is used |
| to specify where to substitute an operand; it can also be used to |
| identify places where different variants of the assembler require |
| different syntax. |
| </p> |
| <p>In the simplest case, a ‘<samp>%</samp>’ followed by a digit <var>n</var> says to output |
| operand <var>n</var> at that point in the string. |
| </p> |
| <p>‘<samp>%</samp>’ followed by a letter and a digit says to output an operand in an |
| alternate fashion. Four letters have standard, built-in meanings described |
| below. The machine description macro <code>PRINT_OPERAND</code> can define |
| additional letters with nonstandard meanings. |
| </p> |
| <p>‘<samp>%c<var>digit</var></samp>’ can be used to substitute an operand that is a |
| constant value without the syntax that normally indicates an immediate |
| operand. |
| </p> |
| <p>‘<samp>%n<var>digit</var></samp>’ is like ‘<samp>%c<var>digit</var></samp>’ except that the value of |
| the constant is negated before printing. |
| </p> |
| <p>‘<samp>%a<var>digit</var></samp>’ can be used to substitute an operand as if it were a |
| memory reference, with the actual operand treated as the address. This may |
| be useful when outputting a “load address” instruction, because often the |
| assembler syntax for such an instruction requires you to write the operand |
| as if it were a memory reference. |
| </p> |
| <p>‘<samp>%l<var>digit</var></samp>’ is used to substitute a <code>label_ref</code> into a jump |
| instruction. |
| </p> |
| <p>‘<samp>%=</samp>’ outputs a number which is unique to each instruction in the |
| entire compilation. This is useful for making local labels to be |
| referred to more than once in a single template that generates multiple |
| assembler instructions. |
| </p> |
| <p>‘<samp>%</samp>’ followed by a punctuation character specifies a substitution that |
| does not use an operand. Only one case is standard: ‘<samp>%%</samp>’ outputs a |
| ‘<samp>%</samp>’ into the assembler code. Other nonstandard cases can be |
| defined in the <code>PRINT_OPERAND</code> macro. You must also define |
| which punctuation characters are valid with the |
| <code>PRINT_OPERAND_PUNCT_VALID_P</code> macro. |
| </p> |
| <a name="index-_005c"></a> |
| <a name="index-backslash"></a> |
| <p>The template may generate multiple assembler instructions. Write the text |
| for the instructions, with ‘<samp>\;</samp>’ between them. |
| </p> |
| <a name="index-matching-operands"></a> |
| <p>When the RTL contains two operands which are required by constraint to match |
| each other, the output template must refer only to the lower-numbered operand. |
| Matching operands are not always identical, and the rest of the compiler |
| arranges to put the proper RTL expression for printing into the lower-numbered |
| operand. |
| </p> |
| <p>One use of nonstandard letters or punctuation following ‘<samp>%</samp>’ is to |
| distinguish between different assembler languages for the same machine; for |
| example, Motorola syntax versus MIT syntax for the 68000. Motorola syntax |
| requires periods in most opcode names, while MIT syntax does not. For |
| example, the opcode ‘<samp>movel</samp>’ in MIT syntax is ‘<samp>move.l</samp>’ in Motorola |
| syntax. The same file of patterns is used for both kinds of output syntax, |
| but the character sequence ‘<samp>%.</samp>’ is used in each place where Motorola |
| syntax wants a period. The <code>PRINT_OPERAND</code> macro for Motorola syntax |
| defines the sequence to output a period; the macro for MIT syntax defines |
| it to do nothing. |
| </p> |
| <a name="index-_0023-in-template"></a> |
| <p>As a special case, a template consisting of the single character <code>#</code> |
| instructs the compiler to first split the insn, and then output the |
| resulting instructions separately. This helps eliminate redundancy in the |
| output templates. If you have a <code>define_insn</code> that needs to emit |
| multiple assembler instructions, and there is a matching <code>define_split</code> |
| already defined, then you can simply use <code>#</code> as the output template |
| instead of writing an output template that emits the multiple assembler |
| instructions. |
| </p> |
| <p>If the macro <code>ASSEMBLER_DIALECT</code> is defined, you can use construct |
| of the form ‘<samp>{option0|option1|option2}</samp>’ in the templates. These |
| describe multiple variants of assembler language syntax. |
| See <a href="Instruction-Output.html#Instruction-Output">Instruction Output</a>. |
| </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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