Re: [boost] unordered_map 32bit VS 64bit

Subject: Re: [boost] unordered_map 32bit VS 64bit
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2012-01-26 02:11:54

On Thu, Jan 26, 2012 at 3:35 AM, Daniel James <dnljms_at_[hidden]> wrote:
> On 25 January 2012 19:05, Grund, Holger (ISGT)
> <Holger.Grund_at_[hidden]> wrote:
>>>
>> My guess would be that there is 64-bit division for transforming the hash to a bucket index, which is killing perf
>> (though one would wonder why there are no peeps for an instructions like MOD(1,x) in the codegen).
>
> You're right. I tried removing all the modulus calculations from the
> container (fine in this case since it's only ever looking for hash
> values < 4 - I'm also using a modified benchmark which looks up i % 4
> rather than 1, to stop things getting optimised away), and the
> difference became much smaller. I didn't realise how expensive they
> are on 64 bit computers, I think I might change the design of the
> container to reduce the number of times they're used, although that
> will make some other things a bit slower.

Is it possible to always use power-of-2 number of buckets and bitwise
operations instead of modulus division?