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0 pointsfpoling1y ago0 comments

Explicit language support is very nice as the compiler can and do a lot of optimizations and free the space as early as possible on all control paths and ensure full memory safety.

For example, when a function returns a thing on the second stack the compiler can arrange that, before returning, the thing is moved to the second stack position that was at the start of the function. This releases the memory that was used for other things by the callee. Then the caller knows the second stack depth after the call and can release its second stack usage just as well.

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alexvitkov1y ago

Doing frees/moves/copies on the second stack makes it harder to track the lifetime of allocations, and restricts what you can use it for - to free a block on the stack you necessarily have to free everything after it.

Most programs have a point where you know nothing on it is used and it's convenient (and very performant) to free the entire thing, and that makes it way easier to reason about - when you alloc from it you know your block of memory it's valid until a <RESET> point:

  - For a video game you can <RESET> at the start of every frame
  - For a web server you can alloc a new stack for every incoming request and <RESET> after the request is over
  - For a short-lived program you may not even need to <RESET> at all

toolslive1y ago

The <reset> point is indeed what you need to look for but why does it need to be a stack ? I guess it's just a naming thing. I've always looked at it as a scratchpad (that was heap allocated)

nmilo1y ago

You can just pop the second stack at each function return, though that does limit its use to only really scratch-space for dynamically sized objects. Like this: https://nullprogram.com/blog/2023/09/27/. “Arenas” are passed by value so their end pointer will auto reset when the function returns.

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