Re: [PATCH][AArch64][1/3] Expand signed mod by power of 2 using CSNEG

[Andrew Pinski <pinskia@gmail.com>]

On Fri, Jul 24, 2015 at 3:55 AM, Kyrill Tkachov <kyrylo.tkachov@arm.com> wrote:
> Hi all,
>
> This patch implements an aarch64-specific expansion of the signed modulo by
> a power of 2.
> The proposed sequence makes use of the conditional negate instruction CSNEG.
> For a power of N, x % N can be calculated with:
> negs   x1, x0
> and    x0, x0, #(N - 1)
> and    x1, x1, #(N - 1)
> csneg  x0, x0, x1, mi
>
> So, for N == 256 this would be:
> negs   x1, x0
> and    x0, x0, #255
> and    x1, x1, #255
> csneg  x0, x0, x1, mi
>
> For comparison, the existing sequence emitted by expand_smod_pow2 in
> expmed.c is:
> asr     x1, x0, 63
> lsr     x1, x1, 56
> add     x0, x0, x1
> and     x0, x0, 255
> sub     x0, x0, x1
>
> Note that the CSNEG sequence is one instruction shorter and that the two and
> operations
> are independent, compared to the existing sequence where all instructions
> are dependent
> on the preceeding instructions.

Just FYI.

For ThunderX, this is a size win and a performance win at least in a
microbenchmark.

>
> For the special case of N == 2 we can do even better:
> cmp     x0, xzr
> and     x0, x0, 1
> csneg   x0, x0, x0, ge

This is a size win and a performance win on ThunderX.

>
> I first tried implementing this in the generic code in expmed.c but that
> didn't work
> out for a few reasons:
>
> * This relies on having a conditional-negate instruction. We could gate it
> on
> HAVE_conditional_move and the combiner is capable of merging the final
> negate into
> the conditional move if a conditional negate is available (like on aarch64)
> but on
> targets without a conditional negate this would end up emitting a separate
> negate.
>
> * The first negs has to be a negs for the sequence to be a win i.e. having a
> separate
> negate and compare makes the sequence slower than the existing one (at least
> in my
> microbenchmarking) and I couldn't get subsequent passes to combine the
> negate and combine
> into the negs (presumably due to the use of the negated result in one of the
> ands).
> Doing it in the aarch64 backend where I could just call the exact gen_*
> functions that
> I need worked much more cleanly.

I agree this does make it harder to implement in a target generic way.

Thanks,
Andrew

>
> The costing logic is updated to reflect this sequence during the
> intialisation of
> expmed.c where it calculates the smod_pow2_cheap metric.
>
> The tests will come in patch 3 of the series which are partly shared with
> the equivalent
> arm implementation.
>
> Bootstrapped and tested on aarch64.
> Ok for trunk?
>
> Thanks,
> Kyrill
>
> 2015-07-24  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>     * config/aarch64/aarch64.md (mod<mode>3): New define_expand.
>     (*neg<mode>2_compare0): Rename to...
>     (neg<mode>2_compare0): ... This.
>     * config/aarch64/aarch64.c (aarch64_rtx_costs, MOD case): Reflect
>     CSNEG sequence in MOD by power of 2 case.