Re: [PATCH, PR middle-end/68134] Reject scalar modes in default get_mask_mode hook

[Alan Lawrence <alan dot lawrence at foss dot arm dot com>]

On 20/02/16 09:29, Ilya Enkovich wrote:
> 2016-02-19 20:36 GMT+03:00 Alan Lawrence <alan.lawrence@foss.arm.com>:
> > Mostly this is fairly straightforward, relatively little midend code is
> > required, and the backend cleans up quite a bit. However, I get stuck on the
> > case of singleton vectors (64x1). No surprises there, then...
> >
> > The PR/68134 fix, makes the 'mask mode' for comparing 64x1 vectors, into
> > BLKmode, so that we get past this in expand_vector_operations:
> >
> > /* A scalar operation pretending to be a vector one.  */
> >    if (VECTOR_BOOLEAN_TYPE_P (type)
> >        && !VECTOR_MODE_P (TYPE_MODE (type))
> >        && TYPE_MODE (type) != BLKmode)
> >      return;
> >
> > and we do the operation piecewise. (Which is what we want; there is only one
> > piece!)
> >
> > However, with my vec_cmp + vcond_mask changes, dropping vconddidi, this
> > means we look for a vcond_maskdiblk and vec_cmpdiblk. Which doesn't really
> > feel right - it feels like the 64x1 mask should be a DImode, just like other
> > 64x1 vectors.
>
> The problem here is to distinguish vector mask of one DI element and
> DI scalar mask.  We don't want to lower scalar mask manipulations
> because they are simple integer operations, not vector ones. Probably
> vector of a single DI should have V1DI mode and not pretend to be a
> scalar?  This would make things easier.

Thanks for the quick reply, Ilya.

What's the difference between, as you say, a "simple integer operation" and a 
"vector" operation of just one element?

This is why we do *not* have V1DImode in the AArch64 (or ARM) backends, but 
instead treat 64x1 vectors as DImode - the operations are the same; so keeping 
them as the same mode, enables CSE and lots of other optimizations, plus we 
don't have to have two near-identical copies (DI + V1DI) for many patterns, etc...

If the operations were on a "DI scalar mask", when would the first part of that 
test, VECTOR_BOOLEAN_TYPE_P, hold?

Thanks, Alan