[Patch combine] Don't create vector mode ZERO_EXTEND from subregs

[James Greenhalgh]

On Sun, Dec 17, 2017 at 03:14:08AM +0000, Segher Boessenkool wrote:
> Hi!
>
> On Mon, Dec 11, 2017 at 02:18:53PM +0000, James Greenhalgh wrote:
> >
> > In simplify_set we try transforming the paradoxical subreg expression:
> >
> >   (set FOO (subreg:M (mem:N BAR) 0))
> >
> > in to:
> >
> >   (set FOO (zero_extend:M (mem:N BAR)))
> >
> > However, this code does not consider the case where M is a vector
> > mode, allowing it to construct (for example):
> >
> >   (zero_extend:V4SI (mem:SI))
> >
> > This would clearly have the wrong semantics, but fortunately we fail long
> > before then in expand_compound_operation. As we really don't want a vector
> > zero_extend of a scalar value.
> >
> > We need to explicitly reject vector modes from this transformation.
>
> It does not consider any other modes either.  Both modes involved are
> required to be SCALAR_INT_MODE_P (for zero_extend to be valid at all and
> to be equivalent to the subreg); could you test for that instead please?

Makes sense. I've committed this as obvious after an AArch64 bootstrap and
test as revision 255945.

Thanks,
James

---
gcc/

2017-12-21  James Greenhalgh  <james.greenhalgh@arm.com>

	* combine.c (simplify_set): Do not transform subregs to zero_extends
	if the destination is not a scalar int mode.
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