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<h3 class="section">16.6 C Statements for Assembler Output</h3>
<p><a name="index-output-statements-3270"></a><a name="index-C-statements-for-assembler-output-3271"></a><a name="index-generating-assembler-output-3272"></a>
Often a single fixed template string cannot produce correct and efficient
assembler code for all the cases that are recognized by a single
instruction pattern. For example, the opcodes may depend on the kinds of
operands; or some unfortunate combinations of operands may require extra
machine instructions.
<p>If the output control string starts with a ‘<samp><span class="samp">@</span></samp>’, then it is actually
a series of templates, each on a separate line. (Blank lines and
leading spaces and tabs are ignored.) The templates correspond to the
pattern's constraint alternatives (see <a href="Multi_002dAlternative.html#Multi_002dAlternative">Multi-Alternative</a>). For example,
if a target machine has a two-address add instruction ‘<samp><span class="samp">addr</span></samp>’ to add
into a register and another ‘<samp><span class="samp">addm</span></samp>’ to add a register to memory, you
might write this pattern:
<pre class="smallexample"> (define_insn "addsi3"
[(set (match_operand:SI 0 "general_operand" "=r,m")
(plus:SI (match_operand:SI 1 "general_operand" "0,0")
(match_operand:SI 2 "general_operand" "g,r")))]
""
"@
addr %2,%0
addm %2,%0")
</pre>
<p><a name="index-g_t_0040code_007b_002a_007d-in-template-3273"></a><a name="index-asterisk-in-template-3274"></a>If the output control string starts with a ‘<samp><span class="samp">*</span></samp>’, then it is not an
output template but rather a piece of C program that should compute a
template. It should execute a <code>return</code> statement to return the
template-string you want. Most such templates use C string literals, which
require doublequote characters to delimit them. To include these
doublequote characters in the string, prefix each one with ‘<samp><span class="samp">\</span></samp>’.
<p>If the output control string is written as a brace block instead of a
double-quoted string, it is automatically assumed to be C code. In that
case, it is not necessary to put in a leading asterisk, or to escape the
doublequotes surrounding C string literals.
<p>The operands may be found in the array <code>operands</code>, whose C data type
is <code>rtx []</code>.
<p>It is very common to select different ways of generating assembler code
based on whether an immediate operand is within a certain range. Be
careful when doing this, because the result of <code>INTVAL</code> is an
integer on the host machine. If the host machine has more bits in an
<code>int</code> than the target machine has in the mode in which the constant
will be used, then some of the bits you get from <code>INTVAL</code> will be
superfluous. For proper results, you must carefully disregard the
values of those bits.
<p><a name="index-output_005fasm_005finsn-3275"></a>It is possible to output an assembler instruction and then go on to output
or compute more of them, using the subroutine <code>output_asm_insn</code>. This
receives two arguments: a template-string and a vector of operands. The
vector may be <code>operands</code>, or it may be another array of <code>rtx</code>
that you declare locally and initialize yourself.
<p><a name="index-which_005falternative-3276"></a>When an insn pattern has multiple alternatives in its constraints, often
the appearance of the assembler code is determined mostly by which alternative
was matched. When this is so, the C code can test the variable
<code>which_alternative</code>, which is the ordinal number of the alternative
that was actually satisfied (0 for the first, 1 for the second alternative,
etc.).
<p>For example, suppose there are two opcodes for storing zero, ‘<samp><span class="samp">clrreg</span></samp>’
for registers and ‘<samp><span class="samp">clrmem</span></samp>’ for memory locations. Here is how
a pattern could use <code>which_alternative</code> to choose between them:
<pre class="smallexample"> (define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r,m")
(const_int 0))]
""
{
return (which_alternative == 0
? "clrreg %0" : "clrmem %0");
})
</pre>
<p>The example above, where the assembler code to generate was
<em>solely</em> determined by the alternative, could also have been specified
as follows, having the output control string start with a ‘<samp><span class="samp">@</span></samp>’:
<pre class="smallexample"> (define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r,m")
(const_int 0))]
""
"@
clrreg %0
clrmem %0")
</pre>
<p>If you just need a little bit of C code in one (or a few) alternatives,
you can use ‘<samp><span class="samp">*</span></samp>’ inside of a ‘<samp><span class="samp">@</span></samp>’ multi-alternative template:
<pre class="smallexample"> (define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r,<,m")
(const_int 0))]
""
"@
clrreg %0
* return stack_mem_p (operands[0]) ? \"push 0\" : \"clrmem %0\";
clrmem %0")
</pre>
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