Re: [aarch64] Code-gen for vector initialization involving constants

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

On Mon, 13 Feb 2023 at 11:58, Prathamesh Kulkarni
<prathamesh.kulkarni@linaro.org> wrote:
>
> On Fri, 3 Feb 2023 at 12:46, Prathamesh Kulkarni
> <prathamesh.kulkarni@linaro.org> wrote:
> >
> > 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.
> >
> > Alternatively, I suppose we can remove threshold and for constants,
> > generate both sequences and check which one is more
> > efficient ?
> >
> > code-gen with patch:
> > f_s16:
> >         dup     v0.8h, w0
> >         movi    v1.4h, 0x1
> >         ins     v0.h[7], v1.h[0]
> >         ret
> >
> > The patch is lightly tested to verify that vec[t]-init-*.c tests pass
> > with bootstrap+test
> > in progress.
> > Does this look OK ?
> Hi Richard,
> ping https://gcc.gnu.org/pipermail/gcc-patches/2023-February/611243.html
Hi Richard,
ping * 2: https://gcc.gnu.org/pipermail/gcc-patches/2023-February/611243.html

Thanks,
Prathamesh
>
> Thanks,
> Prathamesh
> >
> > Thanks,
> > Prathamesh