Is turning off Array bound checking possible?

Help & Support
1

Rust allows you to turn off array index bounds checks like this:

let mut k = *some_array.get_unchecked(j);
...........
*some_array.get_unchecked_mut(j) = k - kn;

This ccurs in a code segment marked as unsafe, but unsafe in Rust really just means the programmer has manual control, and better know what’s s/he’s doing, which I do for this code segment.

I’m doing this because this is part of an inner loop in this algorithm and significantly increases performance compared to the normal bounds-checked array version.

I was wondering is something like this possible in Crystal too?

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2

This is what Array(T) - Crystal 1.1.1 is for.

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3

Great! But is this only available for >= 1.1.1?

4

No? That’s just the the latest version so the API docs default to it.

5

Is there anything comparable to an array.unsafe_write[]?

6

Not technically, but I think it would just be like arr.to_unsafe[idx] = val.

7

OK, here’s the deal.

In this code for reading, these didn’t have any performance difference.

x = ary.unsafe_fetch(j)
or
x = ary.to_unsafe[j]

But writing had a substantial effect: ary.to_unsafe[j] = x

Here are results for 2 large inputs, the second input is 2.5x greater than the first.

   Before   After
    121s     117s
    318s     308s

A really quick check of the docs didn’t find anything about .to_unsafe.
I think this needs to be documented at least under performance Tips & Tricks.

Now to see where else I can use this in my code that will make a difference.

Thanks @Blacksmoke16.

8

That’s expected, because they’re exactly identical.

Here you go: Array(T) - Crystal 1.1.1

Indexable::Mutable is probably going to introduce a unsafe put method, then you don’t need to write to the buffer pointer directly.

2 Likes
9

Thanks again for the info.
I learn something new almost everyday from the forums. :slightly_smiling_face: