dynamically allocating on the stack

[Nicholas Wilson]

On Saturday, 21 April 2018 at 23:47:41 UTC, Mike Franklin wrote:
> On Saturday, 21 April 2018 at 19:06:52 UTC, Steven 
> Schveighoffer wrote:
>
>> alloca is an intrinsic, and part of the language technically 
>> -- it has to be.
>
> From what I can tell `alloca` is only available in the 
> platform's C standard library (actually for Linux it appears be 
> part of libgcc as `__builtin_alloca`; `alloca` is just and 
> alias for it).
>  Of course I can use 3rd party libraries like C to do this, but 
> it seems like something useful to have in the language for 
> certain use case and optimizations.  Also, my immediate use 
> case if for bare metal microcontroller programming where I'm 
> intentionally avoid C and looking for a way to do this in 
> idiomatic D.
>
> Mike

As you have discovered alloca is a builtin: this is true for LDC, 
GDC and DMD, it's true for clang and i suspect most C/C++ 
compilers.
You're not using the C library version of it, the compiler does 
the stack space reservation inline for you. There is no way 
around this.

Use `std.experimental.allocator.building_blocks.Region` with a 
slice from `alloca`.

or if you just want the memory

scope T[] arr = (cast(T*)alloca(n*T.sizeof))[0 .. n];