This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Speedup CSE by 5%


On Thu, Jan 27, 2005 at 01:12:43PM +0100, Bernd Schmidt wrote:
> >>I understand all that.  However, if you look at the first call to
> >>validate_change (inside an ASM_OPERANDS case) you'll see a case where
> >>I think we can modify x without copying it first.
> >
> >
> >I think the proposed patch is actually safe.  Consider
> [...]
> >Just to make things clear, we might want to do something like this:
> >
> >    case ASM_OPERANDS:
> >      if (insn)
> >	{
> >	  for (i = ASM_OPERANDS_INPUT_LENGTH (x) - 1; i >= 0; i--)
> >	    validate_change (insn, &ASM_OPERANDS_INPUT (x, i),
> >			     fold_rtx (ASM_OPERANDS_INPUT (x, i), insn), 0);
> >	}
> >      break;
> 
> Agreed.
> 
> There are two possible reasons for the copy_rtx I can think of: either 
> fold_rtx destructively modifies its input (which I shouldn't happen from 
> what I can see), or we have RTL sharing issues.  In the latter case, we 
> can postpone the copy until after we found that fold_rtx has made a change.
> 
> I did a bit of archaeology; the call to copy_rtx was introduced as part 
> of a larger change by kenner in
> 
> http://savannah.gnu.org/cgi-bin/viewcvs/gcc/old-gcc/cse.c.diff?r1=1.165&r2=1.166

BTW, the copy_rtx call as argument to fold_rtx in find_best_addr isn't the only
one in cse.c, there is also
    case NOT:
    case NEG:
      /* If we have (NOT Y), see if Y is known to be (NOT Z).
         If so, (NOT Y) simplifies to Z.  Similarly for NEG.  */
      new = lookup_as_function (XEXP (x, 0), code);
      if (new)
        return fold_rtx (copy_rtx (XEXP (new, 0)), insn);
and
        case MINUS:
          /* If we have (MINUS Y C), see if Y is known to be (PLUS Z C2).
             If so, produce (PLUS Z C2-C).  */
          if (const_arg1 != 0 && GET_CODE (const_arg1) == CONST_INT)
            {
              rtx y = lookup_as_function (XEXP (x, 0), PLUS);
              if (y && GET_CODE (XEXP (y, 1)) == CONST_INT)
                return fold_rtx (plus_constant (copy_rtx (y),
                                                -INTVAL (const_arg1)),
                                 NULL_RTX);
            }
At least the latter is similar to the one in find_best_addr, in that
it is called with NULL_RTX argument, therefore fold_rtx should be making
a copy instead of modifying in place according to fold_rtx documentation.

	Jakub


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]