Ping.
https://gcc.gnu.org/ml/gcc-patches/2015-07/msg02036.html
Thanks,
Kyrill
On 24/07/15 11:55, Kyrill Tkachov 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.
For the special case of N == 2 we can do even better:
cmp x0, xzr
and x0, x0, 1
csneg x0, x0, x0, ge
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.
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.tkac...@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.
