Re: [PATCH] rs6000: Use NO_EXPR to cast to MMA pointer types

[Segher Boessenkool <segher@kernel.crashing.org>]

On Mon, Sep 05, 2022 at 11:25:21AM +0200, Richard Biener wrote:
> On Fri, Sep 2, 2022 at 7:24 PM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > On Fri, Sep 02, 2022 at 12:02:54PM -0500, Peter Bergner wrote:
> > > On 9/2/22 11:31 AM, Segher Boessenkool wrote:
> > > > (Did you also look at non-MMA VIEW_CONVERT_EXPR uses btw?)
> > >
> > > I did.  It seemed they were all related to pointers to vectors and I remember
> > > you mentioning that as one of the reasons for using VIEW_CONVERT_EXPR over
> > > NOP_EXPR, so I left them alone to be safe.
> >
> > Huh?  I have no idea what you mean here.
> >
> > Casting from one pointer type to another never needs it.  Casting from a
> > scalar integer type to a pointer type not either AFAIKi.  But I am not a
> > Gimple expert, all this might be wrong, it isn't documented anywbere :-(
> 
> NOP_EXPR is for conversions between types with the same kind
> (and pointer-to-integer and integer-to-pointer
> conversions when pointer and integer are of the same size).
> When used on vectors it converts the vector elements.  When you want to
> re-interpret V4SI as V4SF you need VIEW_CONVERT_EXPR (bit_cast),
> likewise V4SI interpreted as V16QI needs that.
> 
> Think of VIEW_CONVERT as bit_cast and NOP_EXPR as conversion.
> Of course for some conversions (like unsigned int to int) you can also
> use a VIEW_CONVERT since it's semantically the same.  In those cases
> we canonicalize to NOP_EXPR via folding.

Thanks for the explanation!

About that last point...  You say VIEW_CONVERT_EXPR is folded to
NOP_EXPR where possible.  Does that happen in all cases / can we depend
on that?  So that in target code like what started this thread the only
real difference is documentation of intent?  (Which never is unimportant
of course!)


Segher