alloca is slow and dangerous

[IGotD-]

On Friday, 1 January 2021 at 14:48:11 UTC, welkam wrote:
> Over the years I saw several people asking why D doesn't have 
> alloca. I just want to share a video where it states that 
> alloca is slower than static array so we have one more 
> arguments against it.
> https://youtu.be/FY9SbqTO5GQ?t=468
>
> In summary alloca is:
> 1. Hard to implement

Why is it hard to implement?

> 2. security problem

Not entirely true. You need to check so that the amount elements 
is within reasonable limits. You can have stack overflows but 
most operating systems have guard pages detecting such cases. In 
kernels it might not be the case but in kernels you need pay 
attention more than in user space programs. Keep in mind static 
arrays on the stack are prone to overflows just as much as a VLA.

In D static arrays have an additional problem and that is that D 
will initialize the array by default. For stability this is great 
but performance this takes more time. VLA can be a better option 
here as you initialize exactly the amount elements you need.

> 3. slower than static array on the stack

The reason was strangely a lot of code for just allocating on the 
stack. He said he wasn't even using -O2 optimization and with -O2 
it would be smaller. In general it shouldn't be that bad.


The fear of alloca in my opinion is exaggerated.

Doesn't D implement alloca for those who want it?
https://dlang.org/phobos/rt_alloca.html