Re: Combine four insns

[Jeff Law <law at redhat dot com>]

 On 08/06/10 13:19, Bernd Schmidt wrote:
> > What does the code look like, that is optimized so much better with 4
> > insns to combine? It looks like code that could come from bitfield
> > manipulations, which is an area where it's well-known that GCC does
> > not always optimize too well.
> Here are some typical examples.  Some of it is bitfield operations.
> PR42172 is.  Also, from gimplify.s:
It's also worth noting that some ports have hacks to encourage 4->1 or 
4->2 combinations.  Basically they have patterns which represent an 
intermediate step in a 4->1 or 4->2 combination even if there is no 
machine instruction which implements the insn appearing in the 
intermediate step.  I've used (and recommended) this trick numerous 
times through the years, so I suspect these patterns probably exist in 
many ports.


Here's an example from h8300.md:


;; This is a "bridge" instruction.  Combine can't cram enough insns
;; together to crate a MAC instruction directly, but it can create
;; this instruction, which then allows combine to create the real
;; MAC insn.
;;
;; Unfortunately, if combine doesn't create a MAC instruction, this
;; insn must generate reasonably correct code.  Egad.
(define_insn ""
  [(set (match_operand:SI 0 "register_operand" "=a")
        (mult:SI
          (sign_extend:SI
            (mem:HI (post_inc:SI (match_operand:SI 1 "register_operand" 
"r"))))
          (sign_extend:SI
            (mem:HI (post_inc:SI (match_operand:SI 2 "register_operand" 
"r"))))))]
  "TARGET_MAC"
  "clrmac\;mac  @%2+,@%1+"
  [(set_attr "length" "6")
   (set_attr "cc" "none_0hit")])

(define_insn ""
  [(set (match_operand:SI 0 "register_operand" "=a")
        (plus:SI (mult:SI
          (sign_extend:SI (mem:HI
            (post_inc:SI (match_operand:SI 1 "register_operand" "r"))))
          (sign_extend:SI (mem:HI
            (post_inc:SI (match_operand:SI 2 "register_operand" "r")))))
              (match_operand:SI 3 "register_operand" "0")))]
  "TARGET_MAC"
  "mac  @%2+,@%1+"
  [(set_attr "length" "4")
   (set_attr "cc" "none_0hit")])