On Saturday, 24 July 2021 at 09:45:01 UTC, Jim wrote:
In that case, what should we use to check functions called from
main are not using the garbage collector?
When compiling, you can pass -vgc to dmd and it will tell you where GC allocations are possible.
Because it seems like we can't use classes with
@nogc. We can write
.deinit() for all our classes but what about the existing ones?
You're trying to shoehorn a GC-based feature into a no-GC world. It's possible, but it takes consideration of your overall architecture. If you're wanting to prevent GC usage in the entire program, then the easiest way is to avoid features that normally rely on the GC.
Consider if you really need classes. Can you just use malloced structs instead? If you do need classes, perhaps for inheritance, then scoped classes can be allocated on the stack where possible; then you get automatic destruction like structs. If you want to allocate them from the heap as in your example, then you can wrap instances in a ref-counted struct that calls a custom destructor (like your .deinit). You could also try marking your classes as extern(C++), then the GC isn't involved anyway.
There are many options, but in bypassing D's built-in automatic memory management, you're going to have to work for each of them to one degree or another.
Personally, I think
@nogc on main is a bad idea.
@nogc should be used as far down the call stack as you can put it. The higher it is, the more difficulty you're going to run into. I recommend you apply it only on functions that run in the hottest parts of a program. Those are the areas where GC allocations triggering collections can obviously become an issue. So start there.
If, down the road, you find that you've got some GC performance troubles outside of @nogc code, -dvst can help you find areas where you may be calling it unexpectedly and you can refactor those parts as needed and still do so without applying @nogc.
You can also look into disabling the GC at specific points and renabling later, or manually run collections at certain points. These are tools that are available that may or may not help.
You might get some ideas from the GC series on the blog:
But if you really want to zap GC from the entire program, you're better of with -betterC. You aren't going to be using any GC-based features anyway, so why worry about @nogc? Everything is @nogc by default in betterC as the GC doesn't exist.