unspec expressions

[Jason R Thorpe]

On Fri, Aug 30, 2002 at 12:50:03AM -0700, Ranjit Singh wrote:

 > 1. How are these expresions used and How does
 > construction and matching take place in this
 > case (if they occur in define_insns)?
 > An example would be helpful.

There is a patch awaiting review for the ARM target which uses unspec
in a way which might be helpful as an example to you, pasted here:

 -- snip --

;; Generate a sequence of instructions to determine if the processor is
;; in 26-bit or 32-bit mode, and return the appropriate return address
;; mask.

(define_expand "return_addr_mask"
  [(set (match_dup 1)
      (compare:CC_NOOV (unspec [(const_int 0)] UNSPEC_CHECK_ARCH)
                       (const_int 0)))
   (set (match_operand:SI 0 "s_register_operand" "")
      (if_then_else:SI (eq (match_dup 1) (const_int 0))
                       (const_int -1)
                       (const_int 67108860)))] ; 0x03fffffc
  ""
  "
  operands[1] = gen_rtx_REG (CC_NOOVmode, 24);
  ")

(define_insn "*check_arch2"
  [(set (match_operand:CC_NOOV 0 "cc_register" "")
      (compare:CC_NOOV (unspec [(const_int 0)] UNSPEC_CHECK_ARCH)
                       (const_int 0)))]
  ""
  "teq\\t%|r0, %|r0\;teq\\t%|pc, %|pc"
  [(set_attr "length" "8")
   (set_attr "conds" "set")]
)

 -- snip --

UNSPEC_CHECK_ARCH is the index, defined earlier in the file in a
"define_constants" block.

-- 
        -- Jason R. Thorpe <thorpej@wasabisystems.com>