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Re: Combine or peephole?

From: Jeff Law <law at redhat dot com>
To: Frank Isamov <frank dot isamov at gmail dot com>
Cc: gcc at gcc dot gnu dot org
Date: Mon, 19 Apr 2010 08:54:15 -0600
Subject: Re: Combine or peephole?
References: <u2ub64cfdca1004190558w94981583j2d9cf57a2b3dc620@mail.gmail.com>

On 04/19/10 06:58, Frank Isamov wrote:

Hi,

My architecture supports instructions with two parallel side effects.
For example, addition and subtraction can be done in parallel:

(define_insn "assi6"
   [(parallel [
      (set (match_operand:SI 0 "register_operand" "=r")
           (minus:SI (match_operand:SI 1 "register_operand" "r")
                     (match_operand:SI 2 "register_operand" "r")))
      (set (match_operand:SI 3 "register_operand" "=r")
           (plus:SI (match_operand:SI 4 "register_operand" "r")
                    (match_operand:SI 5 "register_operand" "r")))
   ])]
   ""
   "as\t%5, %4, %3, %2, %1, %0 %!"
)

This instruction is not chosen at ‘combine’ time even if ‘plus’ and ‘minus’ instructions are located one after other. That it is, probably, there is no data dependency between them.

combine requires a data dependency, so for this situation, combine isn't going to help. The easy solution is to create a peephole. You can also create a machine dependent reorg pass to detect more of these opportunities. Jeff

References:
- Combine or peephole?
  - From: Frank Isamov

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