On Tue, 25 Apr 2023 at 16:29, Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
> > Hi Richard,
> > While digging thru aarch64_expand_vector_init, I noticed it gives
> > priority to loading a constant first:
> >  /* Initialise a vector which is part-variable.  We want to first try
> >      to build those lanes which are constant in the most efficient way we
> >      can.  */
> >
> > which results in suboptimal code-gen for following case:
> > int16x8_t f_s16(int16_t x)
> > {
> >   return (int16x8_t) { x, x, x, x, x, x, x, 1 };
> > }
> >
> > code-gen trunk:
> > f_s16:
> >         movi    v0.8h, 0x1
> >         ins     v0.h[0], w0
> >         ins     v0.h[1], w0
> >         ins     v0.h[2], w0
> >         ins     v0.h[3], w0
> >         ins     v0.h[4], w0
> >         ins     v0.h[5], w0
> >         ins     v0.h[6], w0
> >         ret
> >
> > The attached patch tweaks the following condition:
> > if (n_var == n_elts && n_elts <= 16)
> >   {
> >     ...
> >   }
> >
> > to pass if maxv >= 80% of n_elts, with 80% being an
> > arbitrary "high enough" threshold. The intent is to dup
> > the most repeating variable if it it's repetition
> > is "high enough" and insert constants which should be "better" than
> > loading constant first and inserting variables like in the above case.
>
> I'm not too keen on the 80%.  Like you say, it seems a bit arbitrary.
>
> The case above can also be handled by relaxing n_var == n_elts to
> n_var >= n_elts - 1, so that if there's just one constant element,
> we look for duplicated variable elements.  If there are none
> (maxv == 1), but there is a constant element, we can duplicate
> the constant element into a register.
>
> The case when there's more than one constant element needs more thought
> (and testcases :-)).  E.g. after a certain point, it would probably be
> better to load the variable and constant parts separately and blend them
> using TBL.  It also matters whether the constants are equal or not.
>
> There are also cases that could be handled using EXT.
>
> Plus, if we're inserting many variable elements that are already
> in GPRs, we can probably do better by coalescing them into bigger
> GPR values and inserting them as wider elements.
>
> Because of things like that, I think we should stick to the
> single-constant case for now.
Hi Richard,
Thanks for the suggestions. The attached patch only handles the single
constant case.
Bootstrap+test in progress on aarch64-linux-gnu.
Does it look OK ?

Thanks,
Prathamesh
>
> Thanks,
> Richard