Re: [PATCH] Simplify vec_unpack of uniform_vector_p constructors in match.pd.

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Jun 6, 2022 at 11:06 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Richard Biener <richard.guenther@gmail.com> writes:
> > On Sat, May 21, 2022 at 5:31 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
> >> This patch simplifies vec_unpack_hi_expr/vec_unpack_lo_expr of a uniform
> >> constructor or vec_duplicate operand.  The motivation is from PR 105621
> >> where after optimization, we're left with:
> >>
> >>   vect_cst__21 = {c_8(D), c_8(D), c_8(D), c_8(D)};
> >>   vect_iftmp.7_4 = [vec_unpack_hi_expr] vect_cst__21;
> >>
> >> It turns out that there are no constant folding/simplification patterns
> >> in match.pd, but the above can be simplified further to the equivalent:
> >>
> >>   _20 = (long int) c_8(D);
> >>   vect_iftmp.7_4 = [vec_duplicate_expr] _20;
> >>
> >> which on x86-64 results in one less instruction, replacing pshufd $0
> >> then punpackhq, with punpcklqdq.  This transformation is also useful
> >> for helping CSE to spot that unpack_hi and unpack_lo are equivalent.
> >>
> >> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> >> and make -k check with no new failures.  Ok for mainline?
> >
> > I think we need a way to query whether the target can do a VEC_DUPLICATE_EXPR.
> > Currently we only ever have them for VL vectors and expand via
> > expand_vector_broadcast which eventually simply gives up when there's no
> > vec_duplicate or vec_init optabs suitable.
> >
> > IIRC with the VEC_PERM extension we should be able to handle
> > VEC_DUPLICATE via VEC_PERM?  (but we don't yet accept a scalar
> > input, just V1<mode>?)
>
> Yeah, should be possible.  Not sure whether it would really help though.
> A VEC_PERM_EXPR with only one scalar argument could only have one sensible
> permute mask[*], so there'd be a bit of false generality.
>
> Maybe allowing scalar arguments would be more useful for 2 distinct
> scalar arguments, but then I guess the question is: why stop at 2?
> So if we go down the route of accepting scalars, it might be more
> consistent to make VEC_PERM_EXPR support any number of operands
> and use it as a replacement for CONSTRUCTOR as well.

Discussion was hijacked by the '[PATCH]AArch64 relax predicate on load
structure load
instructions' thread btw.

Roger - your eyesopen.com mail bounces, can you fix your MAINTAINERS
entry please?

Richard.

> Thanks,
> Richard
>
> [*] At least until we support “don't care” elements.  However, like I
>     mentioned before, I'd personally prefer a “don't care” mask to be
>     a separate operand, rather than treating something like -1 as a
>     special value.  Special values like that don't really fit the
>     current encoding scheme for VL constants, but a separate mask would.
>
>     A separate don't-care mask would also work for variable permute masks.
> >
> > I see most targets have picked up vec_duplicate but sparc, but still
> > we'd need to check the specific mode.  I think we can disregart
> > vec_init checking and only apply the transforms when vec_duplicate
> > is available.
> >
> > Richard.
> >
> >>
> >> 2022-05-21  Roger Sayle  <roger@nextmovesoftware.com>
> >>
> >> gcc/ChangeLog
> >>         * match.pd (simplify vec_unpack_hi): Simplify VEC_UNPACK_*_EXPR
> >>         of uniform vector constructors and vec_duplicate.
> >>
> >> gcc/testsuite/ChangeLog
> >>         * g++.dg/vect/pr105621.cc: New test case.
> >>
> >>
> >> Thanks in advance,
> >> Roger
> >> --
> >>