Constrain valid arguments to BIT_FIELD_REF

[Richard Guenther]

On Tue, 4 Mar 2008, Andrew Pinski wrote:

> On 3/4/08, Richard Guenther <rguenther@suse.de> wrote:
> >  I suggest to make sure that bit position and size are constants, the
> >  object referenced is of integral type (BIT_FIELD_REF should not be
> >  used as a way to circumvent aliasing) and the result type is of the
> >  same type as the operand zero type (and not a bitfield type of the
> >  referenced size -- in which case the BIT_FIELD_REF_UNSIGNED would
> >  be useless).  The result would then be properly extended according
> >  to BIT_FIELD_REF_UNSIGNED.
> 
> I tried non constant bit position with BIT_FIELD_REF of vector types
> and it crashed in expand so I think this is the correct thing to do.
> Though it would be nice if we have a VEC_EXTRACT tree instead of
> overloading BIT_FIELD_REF for it that takes a non constant position so
> we can do better optimization there in some cases (yes people write
> code that extracts parts of vectors, trust me).

FWIW I agree.  After all we also have REALPART_EXPR and IMAGPART_EXPR,
a VEC_EXTRACT sounds fine (after all we already have a lot of VEC_
codes).

At least BIT_FIELD_REF should not be VIEW_CONVERT_EXPR on steroids.

Richard.