Does DMD currently avoid range checks in array slice expressions such as

f(x[0..$/2])
f(x[$/2..$])

typically found in divide-and-conquer algorithms such as quicksort?

If not, what would it require to implement it?

Nordlöw:

> Does DMD currently avoid range checks in array slice expressions such as
>
> f(x[0..$/2])
> f(x[$/2..$])

You have two simple ways to answer your question: try to go past the slices and look if D raises an error, and look at the asm produced with various compiler switches.

Bye,
bearophile

On Sun, 30 Mar 2014 15:40:43 -0400, Nordlöw <per.nordlow@gmail.com> wrote:

> Does DMD currently avoid range checks in array slice expressions such as
>
> f(x[0..$/2])
> f(x[$/2..$])
>
> typically found in divide-and-conquer algorithms such as quicksort?

If they are range-checked, it would be a good addition to the optimizer to remove them. My guess is that it is range-checked.

> If not, what would it require to implement it?

As a hack, you can use the pointer, which will not be range checked:

f(x.ptr[0..x.length/2]) // note you can't use $ because the pointer doesn't have length
f(x.ptr[x.length/2..x.length])

-Steve

> If they are range-checked, it would be a good addition to the optimizer to remove them. My guess is that it is range-checked.
>

$/n is of course always within range if n is positive integer >= 1.

But what about in the general indexing/slicing case? In that case it would be useful if we could reuse value range propagation (VRP) in DMD, to figure out which other expressions that don't need range checking.

>
>> If not, what would it require to implement it?

That would be an interesting task to fix :)

DMD source references anyone?