On Mon, 31 Oct 2022 at 15:27, Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> Prathamesh Kulkarni <prathamesh.kulka...@linaro.org> writes:
> > On Wed, 26 Oct 2022 at 21:07, Richard Sandiford
> > <richard.sandif...@arm.com> wrote:
> >>
> >> Sorry for the slow response. I wanted to find some time to think
> >> about this a bit more.
> >>
> >> Prathamesh Kulkarni <prathamesh.kulka...@linaro.org> writes:
> >> > On Fri, 30 Sept 2022 at 21:38, Richard Sandiford
> >> > <richard.sandif...@arm.com> wrote:
> >> >>
> >> >> Richard Sandiford via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> >> >> > Prathamesh Kulkarni <prathamesh.kulka...@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.
> >>
> >> Yeah, you're right, sorry. Going back to:
> >>
> >> (2) The explicit encoding can be used to produce a sequence of N*Ex*Px
> >> elements for any integer N. This extended sequence can be reencoded
> >> as having N*Px patterns, with Ex staying the same.
> >>
> >> I guess we need to pick an N for the selector such that each new
> >> selector pattern (each one out of the N*Px patterns) selects from
> >> the *same pattern* of the same data input.
> >>
> >> So if a particular pattern in the selector has a step S, and the data
> >> input it selects from has Pi patterns, N*S must be a multiple of Pi.
> >> N must be a multiple of least_common_multiple(S,Pi)/S.
> >>
> >> I think that means that the total number of patterns in the result
> >> (Pr from previous messages) can safely be:
> >>
> >> Ps * least_common_multiple(
> >> least_common_multiple(S[1], P[input(1)]) / S[1],
> >> ...
> >> least_common_multiple(S[Ps], P[input(Ps)]) / S[Ps]
> >> )
> >>
> >> where:
> >>
> >> Ps = the number of patterns in the selector
> >> S[I] = the step for selector pattern I (I being 1-based)
> >> input(I) = the data input selected by selector pattern I (I being
> >> 1-based)
> >> P[I] = the number of patterns in data input I
> >>
> >> That's getting quite complicated :-) If we allow arbitrary P[...]
> >> and S[...] then it could also get large. Perhaps we should finally
> >> give up on the general case and limit this to power-of-2 patterns and
> >> power-of-2 steps, so that least_common_multiple becomes MAX. Maybe that
> >> simplifies other things as well.
> >>
> >> What do you think?
> > Hi Richard,
> > Thanks for the suggestions. Yeah I suppose we can initially add support for
> > power-of-2 patterns and power-of-2 steps and try to generalize it in
> > follow up patches if possible.
> >
> > Sorry if this sounds like a silly ques -- if we are going to have
> > pattern in selector, select *same pattern from same input vector*,
> > instead of re-encoding the selector to have N * Ps patterns, would it
> > make sense for elements in selector to denote pattern number itself
> > instead of element index
> > if input vectors are VLA ?
> >
> > For eg:
> > op0 = {1, 2, 3, 4, 1, 2, 3, 5, 1, 2, 3, 6, ...}
> > op1 = {...}
> > with npatterns == 4, nelts_per_pattern == 3,
> > sel = {0, 3} should pick pattern 0 and pattern 3 from op0,
> > so, res = {1, 4, 1, 5, 1, 6, ...}
> > Not sure if this is correct tho.
>
> This wouldn't allow us to represent things like a "duplicate one
> element", or "copy the leading N elements from the first input and
> the other elements from elements N+ of the second input", which we
> can with the current scheme.
>
> The restriction about each (unwound) selector pattern selecting from the
> same input pattern only applies to case where the selector pattern is
> stepped (and only applies to the stepped part of the pattern, not the
> leading element). The restriction is also local to this code; it
> doesn't make other VEC_PERM_EXPRs invalid.
Hi Richard,
Thanks for the clarifications.
Just to clarify your approach with an eg:
Let selected input vector be:
arg0: {a0, b0, c0, d0,
a0 + S, b0 + S, c0 + S, d0 + S,
a0 + 2S, b0 + 2S, c0 + 2S, dd + 2S, ...}
where arg0 has npatterns = 4, and nelts_per_pattern = 3.
Let sel = {0, 0, 1, 2, 2, 4, ...}
where sel_npatterns = 2 and sel_nelts_per_pattern = 3
So, the first pattern in sel:
p1: {0, 1, 2, ...} which will select {a0, b0, c0, ...}
which would be incorrect, since they belong to different patterns in arg0.
So to select elements from same pattern in arg0, we need to divide p1
into at least N1 = P_arg0 / S0 = 4 distinct patterns.
Similarly for second pattern in sel:
p2: {0, 2, 4, ...}, we need to divide it into
at least N2 = P_arg0 / S1 = 2 distinct patterns.
Select N = max(N1, N2) = 4
So, the selector will be extended to N * Ps * Es = 4 * 2 * 3 == 24 elements,
and will be re-encoded with N*Ps = 8 patterns:
re-encoded sel:
{a0, b0, c0, d0, a0 + S, b0 + S, c0 + S, d0 + S,
a0 + 2S, b0 + 2S, c0 + 2S, d0 + 2S, a0 + 3S, b0 + 3S, c0 + 3S, d0 + 3S,
a0 + 4S, b0 + 4S, c0 + 4s, d0 + 4S, a0 + 5S, b0 + 5S, c0 + 5S, d0 + 5S,
...}
with 8 patterns,
p1: {a0, a0 + 2S, a0 + 4S, ...}
p2: {b0, b0 + 2S, b0 + 4S, ...}
...
which select elements from same pattern from same input vector.
Does this look correct ?
For feasibility, we can check initially that sel_npatterns, arg0_npatterns,
arg1_npatterns are powers of 2 and for each stepped pattern,
it's stepped size S is a power of 2. I suppose this will be sufficient
to ensure that sel can be re-encoded with N*Ps npatterns
such that each new pattern selects elements from same pattern
of the input vector ?
Then compute N:
N = 1;
for (every pattern p in sel)
{
op = corresponding input vector for pattern;
S = step_size (p);
N_pattern = max (S, npatterns (op)) / S;
N = max(N, N_pattern)
}
and re-encode selector with N*Ps patterns.
I guess rest of the patch will mostly stay the same.
Thanks,
Prathamesh
>
> Thanks,
> Richard
>
> >
> > Thanks,
> > Prathamesh
> >>
> >> > 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 :/
> >>
> >> svrev is one way of creating negative steps.
> >>
> >> Thanks,
> >> Richard
> >>
> >> >
> >> > 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
