Stack-allocated arrays

[Andrei Alexandrescu]

dsimcha wrote:
> I've noticed that, when writing multithreaded number crunching code,
> allocating memory for temporary storage can be a huge threading bottleneck.
> Of course, the obvious, but ugly solution is to pre-allocate and recycle temp
> variables.  However, this breaks encapsulation at every place imaginable.
> 
> An alternative that I've tried recently is to use alloca to stack allocate the
> temporary arrays if they are small enough.  If the arrays being allocated are
> larger, I heap allocate, and this isn't a problem because in the large array
> case, the number crunching takes up enough time that heap allocations aren't
> much of a bottleneck anymore.  However, this is also butt ugly because I have
> to use a bunch of pointers and casts and explicitly test the size for it to
> work.  Given that this is highly useful, I think a nice feature for D would be
> to have some kind of a scheme to make this clean.  For example:
> 
> auto foo = newTemp[length];
> 
> If length is larger than some arbitrary but reasonable value, this is heap
> allocated.  If length is small, it is stack allocated.  Either way, allowing
> the array to escape the current scope is undefined behavior and the compiler
> would detect at least the simple cases of this.  Ideally, since this would
> only be used for temp variables in performance-critical code, the contents of
> the array would also not be initialized.
> 
> It would be nice to do this in a library, but there's no obvious way to do it
> without compiler support, since calling a function sets up a new stack frame.
>  I'd like to just do it with an interface like:
> 
> auto foo = newTemp!(T)(length).
> 
> Anyone know any ASM frame pointer hacks to make something like this workable?
>  Otherwise, is there enough interest in this that it might belong in the core
> language?

I, for one, am very interested and pleaded to Walter to introduce 
stack-allocation for simple arrays. I could point at an informal case 
study in which many students of mine discovered the related g++ language 
extension for C++ and used it to great effect.


Andrei