Re: CSE fold_rtx subreg patch

[Jeffrey A Law <law at cygnus dot com>]

  In message <199807102338.TAA14385@jwlab.FEITH.COM>you write:
  > > I note that "mode_arg" is used outside the switch statement -- do we
  > > need to fix it, or is its value correct?
  > 
  > Good point.  We probably need to fix it if the substitution is performed.
Per your later message, I'll presume we're OK on this issue.

  > > I'm also a little worried about the < to <= changes, particularly for
  > > SUBREGS since they are not optimized as well as REGs in other passes
  > > and thus "cost more".  Presumably you added it because the cases you
  > > tried had equal cost.
  > >
  > > You might consider changing RTX_COSTS for the x86 to make SUBREGS
  > > which narrow an expression cheaper than the inner expression since
  > > narrowing may allow byte/half sized instructions to be used.
  > 
  > Are you refering to RTX_COSTS in the context of the < to <= change or in
  > general?
Well, I guess what I want to know is exactly why you changed < to <=.
The rest was pure speculation about why you might have changed it and
possibilities for avoiding that change.


  > Regarding the < to <= change I don't believe that RTX_COSTS has an effect.
  > In the case of a SUBREG the COST macro calls notreg_cost which has a special
  > case for handling a SUBREG.  It basicly uses the exact same logic for a SUB REG
  > as COST uses for a REG thus guarenteeing in the case at hand that the cost  of
  > the SUBREG will always equal the cost of the REG.
Maybe we should change this -- a SUBREG which narrows an operand may
be a win on machines like the x86, m68k and maybe others when compared
to the wider register.

jeff