Re: Rust frontend patches v3

[Arthur Cohen <arthur.cohen@embecosm.com>]

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Hi David,

On 10/26/22 23:15, David Malcolm wrote:
> On Wed, 2022-10-26 at 10:17 +0200, arthur.cohen@embecosm.com wrote:
>> This is the fixed version of our previous patch set for gccrs - We've
>> adressed
>> the comments raised in our previous emails.
> 
> [...snip...]
> 
> (Caveat: I'm not a global reviewer)
> 
> Sorry if this is answered in the docs in the patch kit, but a high-
> level question: what's the interaction between gccrs and gcc's garbage
> collector?  Are the only GC-managed objects (such as trees) either (a)
> created near the end of the gccrs, or (b) common globals created at
> initialization and with GTY roots? 

We only create trees at the last point of our compilation pipeline, 
before directly writing them to the backend. This then calls a 
`write_global_definitions` method, that we ported over directly from the 
Go frontend. Among other things, this method has the role of preserving 
trees from the GC using `go_preserve_from_gc()` (or 
`rust_preserve_from_gc()` in our case).

Elsewhere in our pipeline, we never call any garbage-collection routines 
or GC-related functions.

> Are there any points where a collection happen within gccrs?  Or is almost everything stored using
> gccrs's own data structures, and are these managed in the regular (non-
> GC) heap?

This is correct. We have an AST representation, implemented using unique 
pointers, which is then lowered to an HIR, also using unique pointers.

> I skimmed the patches and see that gccrs uses e.g. std::vector,
> std::unique_ptr, std::map, and std::string; this seems reasonable to
> me, but it got me thinking about memory management strategies.
> 
> I see various std::map<T, tree> e.g. in Rust::Compile::Context; so e.g.
> is the GC guaranteed never to collect whilst this is live?

This is a really interesting question, and I hope the answer is yes! But 
I'm unsure as to how to enforce that, as I am not too familiar with the 
GCC GC. I'm hoping someone else will weigh in. As I said, we do not do 
anything particular with the GC during the execution of our 
`CompileCrate` visitor, so hopefully it shouldn't run.

> Hope this is constructive
> Dave
> 

Thanks a lot for the input,

All the best,

Arthur





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