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<h3 class="section">16.5 Output Templates and Operand Substitution</h3>
<p><a name="index-output-templates-3262"></a><a name="index-operand-substitution-3263"></a>
<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
assembler code for an instruction pattern. Most of the template is a
fixed string which is output literally. The character ‘<samp><span class="samp">%</span></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>In the simplest case, a ‘<samp><span class="samp">%</span></samp>’ followed by a digit <var>n</var> says to output
operand <var>n</var> at that point in the string.
<p>‘<samp><span class="samp">%</span></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>‘<samp><span class="samp">%c</span><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>‘<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
the constant is negated before printing.
<p>‘<samp><span class="samp">%a</span><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>‘<samp><span class="samp">%l</span><var>digit</var></samp>’ is used to substitute a <code>label_ref</code> into a jump
instruction.
<p>‘<samp><span class="samp">%=</span></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>‘<samp><span class="samp">%</span></samp>’ followed by a punctuation character specifies a substitution that
does not use an operand. Only one case is standard: ‘<samp><span class="samp">%%</span></samp>’ outputs a
‘<samp><span class="samp">%</span></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-g_t_005c-3266"></a><a name="index-backslash-3267"></a>The template may generate multiple assembler instructions. Write the text
for the instructions, with ‘<samp><span class="samp">\;</span></samp>’ between them.
<p><a name="index-matching-operands-3268"></a>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>One use of nonstandard letters or punctuation following ‘<samp><span class="samp">%</span></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><span class="samp">movel</span></samp>’ in MIT syntax is ‘<samp><span class="samp">move.l</span></samp>’ in Motorola
syntax. The same file of patterns is used for both kinds of output syntax,
but the character sequence ‘<samp><span class="samp">%.</span></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-g_t_0040code_007b_0023_007d-in-template-3269"></a>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>If the macro <code>ASSEMBLER_DIALECT</code> is defined, you can use construct
of the form ‘<samp><span class="samp">{option0|option1|option2}</span></samp>’ in the templates. These
describe multiple variants of assembler language syntax.
See <a href="Instruction-Output.html#Instruction-Output">Instruction Output</a>.
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