SIMD implementation of dot-product. Benchmarks

[Manu]

On 18 August 2013 14:39, Ilya Yaroshenko <ilyayaroshenko at gmail.com> wrote:

> On Sunday, 18 August 2013 at 01:53:53 UTC, Manu wrote:
>
>> It doesn't look like you account for alignment.
>> This is basically not-portable (I doubt unaligned loads in this context
>> are
>> faster than performing scalar operations), and possibly inefficient on x86
>> too.
>>
>
> dotProduct uses unaligned loads (__builtin_ia32_loadups256,
> __builtin_ia32_loadupd256) and it up to 21 times faster then trivial scalar
> version.
>
> Why unaligned loads is not-portable and inefficient?


x86 is the only arch that can perform an unaligned load. And even on x86
(many implementations) it's not very efficient.


 To make it account for potentially random alignment will be awkward, but it
>> might be possible to do efficiently.
>>
>
> Did you mean use unaligned loads or prepare data for alignment loads at
> the beginning of function?
>

I mean to only use aligned loads, in whatever way that happens to work out.
The hard case is when the 2 arrays have different start offsets.

Otherwise you need to wrap your code in a version(x86) block.
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