[RS6000] reload_vsx_from_gprsf splitter

Michael Meissner meissner@linux.vnet.ibm.com
Thu Feb 11 21:56:00 GMT 2016

On Thu, Feb 11, 2016 at 07:38:15PM +0100, Ulrich Weigand wrote:
> For the most part, this patch doesn't really change anything in the
> interaction with reload as far as I can see.  The changes introduced
> by the patch do make sense to me.  In particular, replacing the two
> patterns p8_mtvsrd_1 and p8_mtvsrd_2 used to fill high and low parts
> of a TFmode register pair with a single pattern p8_mtvsrd that just
> works on any DFmode register (leaving the split into high/low to the
> caller if necessary) seems to simplify things.
> > > +  /* You might think that we could use op0 as one temp and a DF clobber
> > > +     as the other, but you'd be wrong.  These secondary_reload patterns
> > > +     don't check that the clobber is different to the destination, which
> > > +     is probably a reload bug.  */
> It's not a bug, it's deliberate behavior.  The reload registers allocated
> for secondary reload clobbers may overlap the destination, since in many
> cases you simply move the input to the reload register, and then the
> reload register to the destination (where the latter move can be a no-op
> if it is possible to allocate the reload register and the destination
> into the same physical register).  If you need two separate intermediate
> values, you need to allocate a secondary reload register of a larger
> mode (as is already done in the pattern).

This is one of the cases I wished the reload support had the ability to
allocate 2 scratch temporaries instead of 1.  As I said in my other message,
TFmode was a hack to get two registers to use.

> While this was not introduced by this patch, I'm a little bit concerned
> about the hard-coded use of REGNO here, which will crash if op0 at this
> point happens to be a SUBREG instead of a REG.  This is extremely unlikely
> at this point in reload, but not 100% impossible, e.g. if op0 for some
> reason is one of the "magic" registers like the stack pointer.
> That's why it is in general better to use simplify_gen_subreg or one of
> gen_highpart/gen_lowpart, which will handle SUBREG correctly as well.
> I'm not sure why it matters whether this is "conceptually a separate
> value" as the comment argues ...

Good catch.

Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email: meissner@linux.vnet.ibm.com, phone: +1 (978) 899-4797

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