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(gcc-300.info)Constant Definitions

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Constant Definitions
====================

   Using literal constants inside instruction patterns reduces
legibility and can be a maintenance problem.

   To overcome this problem, you may use the `define_constants'
expression.  It contains a vector of name-value pairs.  From that point
on, wherever any of the names appears in the MD file, it is as if the
corresponding value had been written instead.  You may use
`define_constants' multiple times; each appearance adds more constants
to the table.  It is an error to redefine a constant with a different
value.

   To come back to the a29k load multiple example, instead of

     (define_insn ""
       [(match_parallel 0 "load_multiple_operation"
          [(set (match_operand:SI 1 "gpc_reg_operand" "=r")
                (match_operand:SI 2 "memory_operand" "m"))
           (use (reg:SI 179))
           (clobber (reg:SI 179))])]
       ""
       "loadm 0,0,%1,%2")

   You could write:

     (define_constants [
         (R_BP 177)
         (R_FC 178)
         (R_CR 179)
         (R_Q  180)
     ])
     
     (define_insn ""
       [(match_parallel 0 "load_multiple_operation"
          [(set (match_operand:SI 1 "gpc_reg_operand" "=r")
                (match_operand:SI 2 "memory_operand" "m"))
           (use (reg:SI R_CR))
           (clobber (reg:SI R_CR))])]
       ""
       "loadm 0,0,%1,%2")

   The constants that are defined with a define_constant are also output
in the insn-codes.h header file as #defines.