[ruby-core:97165] [Ruby master Feature#16029] Expose fstring related APIs to C-extensions

Issue #16029 has been updated by byroot (Jean Boussier).


>   it makes sense to add an official API as well that does not depend on dynamic invocation

It's not just about the dynamic invocation. Sure it's a bit slower but not a huge deal.

It's mostly that to get to that invocation you need to first allocate a duplicate string.

The ideal API would allow to pass a char* to avoid all this garbage.

For context our app loads a lot of static data from flat files (json / yaml / messagepack) for a total of over 5M objects (~25% of the allocations during boot). 80% of that ends up deduplicated later on by `String#-@`. 

I believe that if json / psych / messagepack had access to `rb_fstring_cstr`, I could avoid allocating all these useless objects and save a lot of GC time (~30% of our boot time is spent in GC, 25% marking and 5% sweeping).

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Feature #16029: Expose fstring related APIs to C-extensions
https://bugs.ruby-lang.org/issues/16029#change-84271

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
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As discussed with @tenderlove here: https://github.com/ruby/ruby/pull/2287#issuecomment-513865160

We'd like to update various data format parsers (JSON, MessagePack, etc) to add the possibility to deduplicate strings while parsing.

But unfortunately the `rb_fstring_*` family of functions isn't available to C-extensions, so the only available fallback is `rb_funcall(str, rb_intern("-@"))` which most parsers will likely consider too slow. So the various `rb_fstring_*` functions would need to be public.

Proposed patch: https://github.com/ruby/ruby/pull/2299



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