Go and generic programming on reddit, also touches on D

[Timon Gehr]

On 09/19/2011 04:08 PM, Andrei Alexandrescu wrote:
> On 9/19/11 6:25 AM, Steven Schveighoffer wrote:
>> On Sun, 18 Sep 2011 15:34:16 -0400, Timon Gehr <timon.gehr at gmx.ch> wrote:
>>
>>> On 09/18/2011 08:28 PM, Andrei Alexandrescu wrote:
>>>> That would allow us to e.g. switch from the
>>>> pointer+length representation to the arguably better pointer+pointer
>>>> representation with ease.
>>>
>>> In what way is that representation better?
>>
>> I agree, I don't see why the representation is inherently better. Some
>> operations become higher performance (i.e. popFront), and some become
>> worse (i.e. length). Most of the others are a wash.
>
> That's where frequency of use comes into play. I'm thinking popFront
> would be used most often, and it touches two words.
>
> Andrei

Normally, each popFront comes with an accompanying empty, and the 
comparison against 0 is faster after a decrement than the comparison of 
two arbitrary pointer values.

Now a benchmark:

import std.datetime, std.stdio;

int[100000] a;

void f1(){
     for(auto ptr=a.ptr, len=a.length+1; len; ++ptr, --len){}
}
void f2(){
     for(auto b=a.ptr, e=a.ptr+a.length; b!=e; b++){}
}

void main(){
     auto b=benchmark!(f1,f2)(100000);
     writeln(b[0].to!("seconds",double)," ", b[1].to!("seconds",double));
}

On my machine: (-O -release -inline)
4.00256 4.00099

The difference is inconceivable on my oldish Core 2 Duo processor. Yet 
there is a reproducible difference that indicates that you were right 
about the pointer-pointer representation being slightly faster for that 
use case.