Base sets Printexc.record_backtrace by default

[jberdine]

The module initialization code of base calls Backtrace.initialize_module, which (unless OCAMLRUNPARAM is set) calls Printexc.record_backtrace true.

This is a significant performance foot-gun, and as far as I can see, not documented anywhere outside the code.

Aside: It's not clear to me that libraries have any business setting global state like this, but presumably there was an argument for this being the right default.

[yminsky]

Seems like a reasonable point. I feel pretty strongly that recording backtraces is the right default, but I agree it is odd to have this enforced at the library level, and surprising for users of Base.

FWIW, I believe it's only a performance issue when you're using exceptions a lot, which I think would mostly arise when you're using exceptions for fine-grained control flow. This is not a good pattern most of the time, and when you do need it, you can use Exn.raise_without_backtrace

[jberdine]

Yes, this is only going to hurt when exceptions are used a lot, and especially when they are raised from deeply-recursive functions.

I'm not sure that control-flow versus handlable-crash is an easy distinction to make for libraries though. For instance, Map.find_exn could go either way depending on usage (either a missing key is used to represent the map sparsely, or it's a failure mode, the library can't know). Probably a good eventual situation is for libraries to use raise_notrace for their internal control flow, and expose external interfaces so that exceptions cross the boundary only in failure situations. But at the moment there is a lot of code that predates raise_notrace.

Also note that since 4.04, the overhead of recording backtraces seems to apply even on code compiled without -g: ocaml/ocaml#8780.

[yminsky]

This doesn't line up with my experience. Code that uses exceptions for control flow typically is doing so explicitly, rather than relying on exceptions thrown by Map.find_exn. There are tradeoffs, but I think a pretty good compromise is having slower-but-more-informative exceptions by default, with the ability to raise uninformative exceptions when needed.

That said, having this choice forced upon you by Base isn't great.