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| <a name="Output-Statement"></a> |
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| Next: <a href="Predicates.html#Predicates" accesskey="n" rel="next">Predicates</a>, Previous: <a href="Output-Template.html#Output-Template" accesskey="p" rel="prev">Output 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="C-Statements-for-Assembler-Output"></a> |
| <h3 class="section">16.6 C Statements for Assembler Output</h3> |
| <a name="index-output-statements"></a> |
| <a name="index-C-statements-for-assembler-output"></a> |
| <a name="index-generating-assembler-output"></a> |
| |
| <p>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> |
| <p>If the output control string starts with a ‘<samp>@</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>addr</samp>’ to add |
| into a register and another ‘<samp>addm</samp>’ to add a register to memory, you |
| might write this pattern: |
| </p> |
| <div class="smallexample"> |
| <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></div> |
| |
| <a name="index-_002a-in-template"></a> |
| <a name="index-asterisk-in-template"></a> |
| <p>If the output control string starts with a ‘<samp>*</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>\</samp>’. |
| </p> |
| <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> |
| <p>The operands may be found in the array <code>operands</code>, whose C data type |
| is <code>rtx []</code>. |
| </p> |
| <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"></a> |
| <p>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"></a> |
| <p>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> |
| <p>For example, suppose there are two opcodes for storing zero, ‘<samp>clrreg</samp>’ |
| for registers and ‘<samp>clrmem</samp>’ for memory locations. Here is how |
| a pattern could use <code>which_alternative</code> to choose between them: |
| </p> |
| <div class="smallexample"> |
| <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></div> |
| |
| <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>@</samp>’: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_insn "" |
| [(set (match_operand:SI 0 "general_operand" "=r,m") |
| (const_int 0))] |
| "" |
| "@ |
| clrreg %0 |
| clrmem %0") |
| </pre></div> |
| |
| <p>If you just need a little bit of C code in one (or a few) alternatives, |
| you can use ‘<samp>*</samp>’ inside of a ‘<samp>@</samp>’ multi-alternative template: |
| </p> |
| <div class="smallexample"> |
| <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></div> |
| |
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