Re: md description for intruction that modifies multiple operands

[Richard Earnshaw <rearnsha at arm dot com>]

>  So two insn matches that insns of the form

Err, I should re-read what I write before I post :-(

Take that as:

So two insn matchers that match insns of the form:

> 
> insn_a: (set x (unspec [(y) (z)] 0))
> insn_b: (set a (unspec [(b)] 0))
> 
> is not illegal, though it is bad style.
> 

Note also that it is regarded as good style now do define constants in the 
MD file for unspec operations.  Hence on ARM we now have

(define_constants
  [(UNSPEC_SIN       0) ; `sin' operation (MODE_FLOAT):
                        ;   operand 0 is the result,
                        ;   operand 1 the parameter.
   (UNPSEC_COS       1) ; `cos' operation (MODE_FLOAT):
                        ;   operand 0 is the result,
                        ;   operand 1 the parameter.
   (UNSPEC_PUSH_MULT 2) ; `push multiple' operation:
                        ;   operand 0 is the first register,
                        ;   subsequent registers are in parallel (use ...)
                        ;   expressions.
   (UNSPEC_PIC_SYM   3) ; A symbol that has been treated properly for pic
                        ;   usage, that is, we will add the pic_register
                        ;   value to it before trying to dereference it.
   (UNSPEC_PIC_BASE  4) ; Adding the PC value to the offset to the
                        ;   GLOBAL_OFFSET_TABLE.  The operation is fully
                        ;   described by the RTL but must be wrapped to
                        ;   prevent combine from trying to rip it apart.
   (UNSPEC_PRLG_STK  5) ; A special barrier that prevents frame accesses 
                        ;   being scheduled before the stack adjustment 
insn.
...
])

Then 

(define_insn "pic_load_addr_arm"
  [(set (match_operand:SI 0 "s_register_operand" "=r")
        (unspec:SI [(match_operand:SI 1 "" "mX")] UNSPEC_PIC_SYM))]
  "TARGET_ARM && flag_pic"
  "ldr%?\\t%0, %1"
  [(set_attr "type" "load")
   (set (attr "pool_range")     (const_int 4096))
   (set (attr "neg_pool_range") (const_int 4084))]
)

This reduces the risk of getting two unspecs that conflict in their 
discriminator field.

R.