Question about vectorization limit

[Jakub Jelinek]

On Fri, May 31, 2013 at 03:21:51PM +0200, Toon Moene wrote:
> SUBROUTINE XYZ(A, B, N)
> DIMENSION A(N), B(N)
> DO I = 1, N
>    IF (A(I) > 0.0) THEN
>       A(I) = B(I) / A(I)
>    ELSE
>       A(I) = B(I)
>    ENDIF
> ENDDO
> END

Well, in this case (with -Ofast) it is just the case that ifcvt
or earlier passes did a poor job at moving the load from B(I)
before the conditional, which, if we ignore exceptions, should be possible,
as both branches read from the same memory.
The store to A(I) is already hoisted by cselim out of the conditional.

If you rewrite the above into:
SUBROUTINE XYZ(A, B, N)
DIMENSION A(N), B(N)
DO I = 1, N
   C = B(I)
   IF (A(I) > 0.0) THEN
      A(I) = C / A(I)
   ELSE
      A(I) = C
   ENDIF
ENDDO
END

then it is vectorized just fine.  Similarly even if this optimization
isn't performed, with masked loads it should be optimizable.
See http://gcc.gnu.org/ml/gcc-patches/2012-11/msg00202.html
though we probably just want a better infrastructure for that.

	Jakub