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