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(gcc-295.info)Output Statement

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C Statements for Assembler Output
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   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.

   If the output control string starts with a `@', 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 (Note: Multi-Alternative.).  For
example, if a target machine has a two-address add instruction `addr'
to add into a register and another `addm' to add a register to memory,
you might write this pattern:

     (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")

   If the output control string starts with a `*', then it is not an
output template but rather a piece of C program that should compute a
template.  It should execute a `return' 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 `\'.

   The operands may be found in the array `operands', whose C data type
is `rtx []'.

   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 `INTVAL' is an
integer on the host machine.  If the host machine has more bits in an
`int' than the target machine has in the mode in which the constant
will be used, then some of the bits you get from `INTVAL' will be
superfluous.  For proper results, you must carefully disregard the
values of those bits.

   It is possible to output an assembler instruction and then go on to
output or compute more of them, using the subroutine `output_asm_insn'.
This receives two arguments: a template-string and a vector of
operands.  The vector may be `operands', or it may be another array of
`rtx' that you declare locally and initialize yourself.

   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 `which_alternative', which is the ordinal number of the
alternative that was actually satisfied (0 for the first, 1 for the
second alternative, etc.).

   For example, suppose there are two opcodes for storing zero, `clrreg'
for registers and `clrmem' for memory locations.  Here is how a pattern
could use `which_alternative' to choose between them:

     (define_insn ""
       [(set (match_operand:SI 0 "general_operand" "=r,m")
             (const_int 0))]
       ""
       "*
       return (which_alternative == 0
               ? \"clrreg %0\" : \"clrmem %0\");
       ")

   The example above, where the assembler code to generate was *solely*
determined by the alternative, could also have been specified as
follows, having the output control string start with a `@':

     (define_insn ""
       [(set (match_operand:SI 0 "general_operand" "=r,m")
             (const_int 0))]
       ""
       "@
        clrreg %0
        clrmem %0")