D 50% slower than C++. What I'm doing wrong?

[Dmitry Olshansky]

On 15.04.2012 12:29, Joseph Rushton Wakeling wrote:
> On 15/04/12 09:23, ReneSac wrote:
>>> What really amazes me is the difference between g++, DMD and GDC in
>>> size of
>>> the executable binary. 100 orders of magnitude!
>> I have remarked it in another topic before, with a simple "hello
>> world". I need
>> to update there, now that I got DMD working. BTW, it is 2 orders of
>> magnitude.
>
> Ack. Yes, 2 orders of magnitude, 100 _times_ larger. That'll teach me to
> send comments to a mailing list at an hour of the morning so late that
> it can be called early. ;-)
>
>>> Sounds stupid as the C stuff should be fastest, but I've been surprised
>>> sometimes at how using idiomatic D formulations can improve things.
>>
>> Well, it may indeed be faster, especially the IO that is dependent on
>> things
>> like buffering and so on. But for this I just wanted something as
>> close as the
>> C++ code as possible.
>
> Fair dos. Seems worth trying the idiomatic alternatives before assuming
> the speed difference is always going to be so great, though.
>
>> Yeah, I don't know. I just did just throw those qualifiers against the
>> compiler,
>> and saw what sticks. And I was testing the decode speed specially to
>> see easily
>> if the output was corrupted. But maybe it haven't corrupted because
>> the compiler
>> don't optimize based on "pure" yet... there was no speed difference
>> too.. so...
>
> I think it's because although there's mutability outside the function,
> those variables are still internal to the struct. e.g. if you try and
> compile this code:
>
> int k = 23;
>
> pure int twotimes(int a)
> {
> auto b = 2*a;
> auto c = 2*k;
> return b+c;
> }
>
> ... it'll fail, but if you try and compile,
>
> struct TwoTimes
> {
> int k = 23;
>
> pure int twotimes(int a)
> {
> auto b = 2*a;
> auto c = 2*k;
> return b+c;
> }
> }
>

Simple - it's called weakly pure. (it can modify this for instance)
It's allowed so that you can do:
pure int f(){
	Twotimes t;
	return t.towtime(1,1);
}

which is obviously normal/strongly pure. Of course, the compiler knows 
what kind of purity each function has.

> ... the compiler accepts it. Whether that's because it's acceptably
> pure, or because the compiler just doesn't detect this case of impurity,
> is another matter. The int k is certainly mutable from outside the scope
> of the function, so AFAICS it _should_ be disallowed.


-- 
Dmitry Olshansky