[PATCH] tree-optimization: [PR101540] Simplify CONSTRUCTOR for vector(1) to be VCE

[Richard Biener]

On Mon, Nov 29, 2021 at 1:57 AM Andrew Pinski via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Sun, Nov 28, 2021 at 12:25 PM Jeff Law via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> >
> >
> > On 11/28/2021 10:56 AM, apinski--- via Gcc-patches wrote:
> > > From: Andrew Pinski <apinski@marvell.com>
> > >
> > > This just adds a simplification to simplify_vector_constructor for
> > > vector of 1 element to be VCE which should reduce memory usage in
> > > the compiler and maybe allow for some more optimizations.
> > >
> > > OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.
> > >
> > >       PR tree-optimization/101540
> > >
> > > gcc/ChangeLog:
> > >
> > >       * tree-ssa-forwprop.c (simplify_vector_constructor):
> > >       Simplify constructor of vector of 1 element to just
> > >       be a VIEW_CONVERT_EXPR.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > >       * gcc.dg/tree-ssa/pr101540-1.c: New test.
> > So why generate a VCE here if the type conversion is useless?  Why not
> > just a NOP_EXPR?  Is there something special about converting between
> > the element type and the outer vector type that requires VCE rather than
> > NOP_EXR?  Neither an ACK or NAK, just trying to understand it a bit better.
>
>
> Because right now tree-cfg.c has this check for vector types for NOP_EXPR:
>         /* Allow conversions between vectors with the same number of elements,
>            provided that the conversion is OK for the element types too.  */
>         if (VECTOR_TYPE_P (lhs_type)
>             && VECTOR_TYPE_P (rhs1_type)
>             && known_eq (TYPE_VECTOR_SUBPARTS (lhs_type),
>                          TYPE_VECTOR_SUBPARTS (rhs1_type)))
>           {
>             lhs_type = TREE_TYPE (lhs_type);
>             rhs1_type = TREE_TYPE (rhs1_type);
>           }
>         else if (VECTOR_TYPE_P (lhs_type) || VECTOR_TYPE_P (rhs1_type))
>           {
>             error ("invalid vector types in nop conversion");
>             debug_generic_expr (lhs_type);
>             debug_generic_expr (rhs1_type);
>             return true;
>           }
>
> We can change this check here for NOP_EXPR and vector types but VCE is
> still a nop in most cases and handled as such really. But I wonder if
> the rest of the compiler is ready for it though.

It's definitely not a NOP, I think the original patch is OK.

Thanks,
Richard.

>
> Thanks,
> Andrew Pinski
>
> >
> > Jeff
> >
> >