Checking if used fwd or reverse mode AD?

General
1

Is there a way to check (in cmdstanr) whether your Stan model was run using reverse-mode or forward-mode autodiff?

Furthermore is there a way to force one method or the other?

2

Stan’s algorithms all use reverse mode. There’s currently no way to use forward mode instead, and not all of the functions supported by Stan are currently useable in forward mode

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3

To put a slightly finer point on it, we have forward mode implemented in Stan, but it’s not nearly as efficiently coded. We only use it for higher-order derivatives. Unless your problems is very small dimensional, reverse mode is much more efficient. Turing.jl in Julia lets you swap between forward- and reverse-mode autodiff.

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