17.6 C Statements for Assembler Output

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 (see 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 ‘\’.

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.

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")

If you just need a little bit of C code in one (or a few) alternatives, you can use ‘*’ inside of a ‘@’ multi-alternative template:

(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")