On Fri, 30 Sept 2022 at 21:38, Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Richard Sandiford via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
> >> Sorry to ask a silly question but in which case shall we select 2nd vector ?
> >> For num_poly_int_coeffs == 2,
> >> a1 /trunc n1 == (a1 + 0x) / (n1.coeffs[0] + n1.coeffs[1]*x)
> >> If a1/trunc n1 succeeds,
> >> 0 / n1.coeffs[1] == a1/n1.coeffs[0] == 0.
> >> So, a1 has to be < n1.coeffs[0] ?
> >
> > Remember that a1 is itself a poly_int.  It's not necessarily a constant.
> >
> > E.g. the TRN1 .D instruction maps to a VEC_PERM_EXPR with the selector:
> >
> >   { 0, 2 + 2x, 1, 4 + 2x, 2, 6 + 2x, ... }
>
> Sorry, should have been:
>
>   { 0, 2 + 2x, 2, 4 + 2x, 4, 6 + 2x, ... }
Hi Richard,
Thanks for the clarifications, and sorry for late reply.
I have attached POC patch that tries to implement the above approach.
Passes bootstrap+test on x86_64-linux-gnu and aarch64-linux-gnu for VLS vectors.

For VLA vectors, I have only done limited testing so far.
It seems to pass couple of tests written in the patch for
nelts_per_pattern == 3,
and folds the following svld1rq test:
int32x4_t v = {1, 2, 3, 4};
return svld1rq_s32 (svptrue_b8 (), &v[0])
into:
return {1, 2, 3, 4, ...};
I will try to bootstrap+test it on SVE machine to test further for VLA folding.

I have a couple of questions:
1] When mask selects elements from same vector but from different patterns:
For eg:
arg0 = {1, 11, 2, 12, 3, 13, ...},
arg1 = {21, 31, 22, 32, 23, 33, ...},
mask = {0, 0, 0, 1, 0, 2, ... },
All have npatterns = 2, nelts_per_pattern = 3.

With above mask,
Pattern {0, ...} selects arg0[0], ie {1, ...}
Pattern {0, 1, 2, ...} selects arg0[0], arg0[1], arg0[2], ie {1, 11, 2, ...}
While arg0[0] and arg0[2] belong to same pattern, arg0[1] belongs to different
pattern in arg0.
The result is:
res = {1, 1, 1, 11, 1, 2, ...}
In this case, res's 2nd pattern {1, 11, 2, ...} is encoded with:
with a0 = 1, a1 = 11, S = -9.
Is that expected tho ? It seems to create a new encoding which
wasn't present in the input vector. For instance, the next elem in
sequence would be -7,
which is not present originally in arg0.
I suppose it's fine since if the user defines mask to have pattern {0,
1, 2, ...}
they intended result to have pattern with above encoding.
Just wanted to confirm if this is correct ?

2] Could you please suggest a test-case for S < 0 ?
I am not able to come up with one :/

Thanks,
Prathamesh
>
> > which is an interleaving of the two patterns:
> >
> >   { 0, 2, 4, ... }                  a0 = 0, a1 = 2, S = 2
> >   { 2 + 2x, 4 + 2x, 6 + 2x }        a0 = 2 + 2x, a1 = 4 + 2x, S = 2