D outperformed by C++, what am I doing wrong?

[Johan Engelen via Digitalmars-d-learn]

On Sunday, 13 August 2017 at 09:08:14 UTC, Petar Kirov 
[ZombineDev] wrote:
>
> This instantiation:
>
> sum_subranges(std.range.iota!(int, int).iota(int, int).Result, 
> uint)
>
> of the following function:
>
> auto sum_subranges(T)(T input, uint range)
> {
>     import std.range : chunks, ElementType, array;
>     import std.algorithm : map;
>     return input.chunks(range).map!(sum);
> }
>
> gets optimized with LDC to:
>  [snip]

> I.e. the compiler turned a O(n) algorithm to O(1), which is 
> quite neat. It is also quite surprising to me that it looks 
> like even dmd managed to do a similar optimization:
>  [snip]

Execution of sum_subranges is already O(1), because the 
calculation of the sum is delayed: the return type of the 
function is not `uint`, it is `MapResult!(sum, <range>)` which 
does a lazy evaluation of the sum.

- Johan