Closures and memory allocation

[Cym13]

On Saturday, 22 June 2019 at 16:52:07 UTC, Anonymouse wrote:
> I'm looking into why my thing does so many memory allocations. 
> Profiling with kcachegrind shows _d_allocmemory being called 
> upon entering a certain function, lots and lots of times.
>
> It's a function that receives concurrency messages, so it 
> contains nested functions that close over local variables. 
> Think receiveTimeout(0.seconds, &nested1, &nested2, &nested3, 
> ...) with 13 pointers to nested functions passed.
>
> When entering the following function, does it allocate:
>
> 1. 0 times, because while there are closures defined, none is 
> ever called?
> 2. 2 times, because there are closures over two variables?
> 3. 20 times, because there are 20 unique closures?
>

Clearly this is a good time for you to learn about the tools D 
offers to profile allocations. There is the --profile=gc DMD 
argument that you can use but here there's something better: 
DMD's GC has a few hooks that are directly inside druntime and 
therefore available to any D program.

Putting your above code in test.d you can then do:

$ dmd test.d
$ ./test --DRT-gcopt=profile:1
         Number of collections:  2
         Total GC prep time:  0 milliseconds
         Total mark time:  0 milliseconds
         Total sweep time:  0 milliseconds
         Max Pause Time:  0 milliseconds
         Grand total GC time:  0 milliseconds
GC summary:    1 MB,    2 GC    0 ms, Pauses    0 ms <    0 ms

And here is your answer: two allocations. More information about 
--DRT-gcopt there: https://dlang.org/spec/garbage.html