Re: possible gcse failure: not able to eliminate redundant loads

[Toon Moene <toon at moene dot indiv dot nluug dot nl>]

Richard Henderson wrote:

> On Mon, Dec 16, 2002 at 11:46:57PM +0100, Toon Moene wrote:
>
> > Yes, that's impossible to express in Fortran (90/95/F2K), hence we don't 
> > have to worry about it.
> >
> > [ The way to do this in Fortran is to write:
> >
> >        IF (ASSOCIATED(P)) THEN
> >            .... USE P
> >        ELSE
> >            .... BETTER KEEP YOUR PAWS OFF ....
> >        ENDIF
> Um, isn't this still exactly what I talked about?  The use of P
> is protected by a conditional.
Yes, it might well be that I do not understand the purpose of the 
prohibition of this optimization in GCC.  Obviously (he says now :-) I 
expect optimizations to conserve the "correct program" property, i.e. if 
A is a program in Fortran then opt(A) also is a program in Fortran (note 
that this property is - much - weaker than: encodes the same computation).

If we do not conserve this property, then all bets are off:

      DIMENSION A(10), B(11)
      ...
      DO I = 1, 11
         IF (I .GT. 10) THEN
            B(I) = 0.0 ! Default value
         ELSE
            B(I) = A(I)
         ENDIF
      ENDDO
      END

Of course this is not a valid Fortran program if the compiler moved 
B=A(I) out of the IF .. THEN .. ELSE .. ENDIF construct.

--
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
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