# Borrow checker with GC fallback?

**URL:** <https://discourse.julialang.org/t/borrow-checker-with-gc-fallback/111798>\
**Category:** Internals & Design\
**Created:** [March 19, 2024, 3:21am UTC](https://discourse.julialang.org/t/borrow-checker-with-gc-fallback/111798 "2024-03-19T03:21:18Z")\
**Posts on this page:** 1\
**Showing post:** 3

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**Author:** ![foobar\_lv2](https://avatars.discourse-cdn.com/v4/letter/f/ee59a6/32.png) [@foobar\_lv2](https://discourse.julialang.org/u/foobar_lv2)\
**Post date:** [March 19, 2024, 9:05am UTC](https://discourse.julialang.org/t/borrow-checker-with-gc-fallback/111798/3 "2024-03-19T09:05:53Z")

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I don’t think we do?

We do SROA, i.e. scalar-replacement-of-aggregates, and the scalar replacement lives in registers and maybe spills to the stack.

Afaiu we never do stack-alloc, where we know that the lifetime is bounded by the stackframe and we allocate a fully formed object on the stack (please correct me if if I’m wrong or this changed!).

Afaiu we don’t do eager deterministic de-alloc, i.e. the compiler never generates a matching `free` to the `malloc` / `ijl_gc_pool_alloc`.

An example is the following:

```julia
julia> @noinline g(r) = (r[] += 1);
julia> f(i)=begin r = Ref(i); g(r); r[] end

```

In a perfect world, `g` would have inferred effects that imply that `r` doesn’t leak. But we requested the function `@noinline`, so we cannot use scalar replacement.

So `f` could place the `Ref` on the stack instead of the heap; but it would still require an object header.

Or `f` could allocate the `Ref` on the heap, using `jl_gc_pool_alloc` and then free it upon return from `g`. But we don’t do that. (we are already paying the price of expensive allocations for non-compacting GC; might as well reap the benefits of opportunistic early free)

Related [Feature request: unsafe\_free!](https://discourse.julialang.org/t/feature-request-unsafe-free/111767)

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