On Mon, 31 Oct 2022 13:18:35 GMT, Ludovic Henry <luhe...@openjdk.org> wrote:
>> src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp line 3528:
>>
>>> 3526: vpmulld(vcoef[idx], vcoef[idx], vnext, Assembler::AVX_256bit);
>>> 3527: }
>>> 3528: jmp(LONG_VECTOR_LOOP_BEGIN);
>>
>> Calculating backward forces you to do calculating the coefficients on each
>> iteration, I think doing this normally would be better.
>
> But doing it forward requires a `reduceLane` on each iteration. It's faster
> to do it backward.
No you don't need to, the vector loop can be calculated as:
IntVector accumulation = IntVector.zero(INT_SPECIES);
for (int i = 0; i < bound; i += INT_SPECIES.length()) {
IntVector current = IntVector.load(INT_SPECIES, array, i);
accumulation =
accumulation.mul(31**(INT_SPECIES.length())).add(current);
}
return accumulation.mul(IntVector.of(31**INT_SPECIES.length() - 1, ...,
31**2, 31, 1).reduce(ADD);
Each iteration only requires a multiplication and an addition. The weight of
lanes can be calculated just before the reduction operation.
-------------
PR: https://git.openjdk.org/jdk/pull/10847
