Re: [PATCH] Promoted variable SUBREG optimizations

[Richard Henderson <rth at redhat dot com>]

On Sat, Aug 09, 2003 at 09:33:25AM -0600, Roger Sayle wrote:
> But most importantly, when generating the SUBREG it records the fact
> that source register already contains the sign or zero extended
> DImode representation of the value for the RTL optimizers to use.

Well, the thing is, at best all it says is that at one point the
value contained a sign extended representation of the value.  Even
there I would only really trust the "at one time" part to apply to 
parameters, not local variables.

> 	* simplify-rtx.c (simplify_unary_operation): Optimize sign and
> 	zero-extensions of subregs of promoted variables where the
> 	extension is identical to that used to promote the variable.

For instance, by the time we get to combine, we could have generated

(define_insn "*addsi_internal"
  [(set (match_operand:SI 0 "register_operand" "=r,r,r,r")
        (plus:SI (match_operand:SI 1 "reg_or_0_operand" "%rJ,rJ,rJ,rJ")
                 (match_operand:SI 2 "add_operand" "rI,O,K,L")))]
  ""
  "@
   addl %r1,%2,%0
   subl %r1,%n2,%0
   lda %0,%2(%r1)
   ldah %0,%h2(%r1)")

Note that the last two alternatives do *not* preserve sign extesion
to DImode.  And yet there's nothing stopping an extended subreg from
being placed here.

I think extended subregs are a fringe feature that too many parts of
the compiler don't know about, and so don't universally ensure that
its constraints are really met.  I don't think you can *realiably*
add code that depends on it.


r~