(gcc.info) Constant Attributes

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 Constant Attributes
 -------------------
 
    A special form of `define_attr', where the expression for the
 default value is a `const' expression, indicates an attribute that is
 constant for a given run of the compiler.  Constant attributes may be
 used to specify which variety of processor is used.  For example,
 
      (define_attr "cpu" "m88100,m88110,m88000"
       (const
        (cond [(symbol_ref "TARGET_88100") (const_string "m88100")
               (symbol_ref "TARGET_88110") (const_string "m88110")]
              (const_string "m88000"))))
      
      (define_attr "memory" "fast,slow"
       (const
        (if_then_else (symbol_ref "TARGET_FAST_MEM")
                      (const_string "fast")
                      (const_string "slow"))))
 
    The routine generated for constant attributes has no parameters as it
 does not depend on any particular insn.  RTL expressions used to define
 the value of a constant attribute may use the `symbol_ref' form, but
 may not use either the `match_operand' form or `eq_attr' forms
 involving insn attributes.
 

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