Update index.qmd

[willtebbutt]

Addresses part of #547 .

@gdalle does this read more correctly?

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[gdalle]

I think there might still be a misunderstanding here. The keyword argument AutoReverseDiff(; compile) only controls whether the tape is optimized after being recorded. It doesn't control whether a recorded tape is only used once, or reused across function calls (inducing possible control flow problems).
Tape reuse is instead determined by how the ADGradient is created, and whether or not an example vector x is passed to it. See these key lines in LogDensityProblemsAD.jl:

https://github.com/tpapp/LogDensityProblemsAD.jl/blob/e3401f21b5a065df0d5de38b37fad0e6650618f3/ext/LogDensityProblemsADADTypesExt.jl#L52-L54

https://github.com/tpapp/LogDensityProblemsAD.jl/blob/e3401f21b5a065df0d5de38b37fad0e6650618f3/ext/LogDensityProblemsADReverseDiffExt.jl#L45-L56

So the real question is:

• in which places does Turing "prepare" the ADGradient with an example x
• can the user of the library do anything about it, if they know that their loglikelihood has value-dependent control flow?

[willtebbutt]

Ah, I see. Maybe @torfjelde is the right person to ask about this? I've not dug in to exactly where to sort out RD gradient stuff yet in Turing.jl.

[torfjelde]

in which places does Turing "prepare" the ADGradient with an example x

Typically this occurs in the initial step of the sampler, i.e. once (there are exceptions, but in those cases you can't really do much better in our case).

can the user of the library do anything about it, if they know that their loglikelihood has value-dependent control flow?

Unfortunately not for ReverseDiff.jl in compiled mode (they can however just not use compiled mode).

[torfjelde]

Added some comments 👍

[gdalle]

Unfortunately not for ReverseDiff.jl in compiled mode (they can however just not use compiled mode).

@torfjelde I think that's the key confusion between us. My understanding is that, regardless of whether it is compiled or not, the tape only captures execution for one version of the control flow. Thus, the tape always becomes invalid if the control flow changes (e.g. different branch of an if statement). Compilation just speeds up the differentiation but does not affect anything else, especially not correctness. Do you have a different understanding?

Source: https://juliadiff.org/ReverseDiff.jl/dev/api/#The-AbstractTape-API

[devmotion]

The tape is purely internal and neither stored nor reused if compile = Val(false).

Tape reuse is instead determined by how the ADGradient is created, and whether or not an example vector x is passed to it

This is wrong. The tape is only reused if compile = Val(true): https://github.com/tpapp/LogDensityProblemsAD.jl/blob/e3401f21b5a065df0d5de38b37fad0e6650618f3/ext/LogDensityProblemsADReverseDiffExt.jl#L51-L56

[gdalle]

Okay then IIUC it's a matter of inconsistency between

• the "compile" kwarg here and in LogDensityProblemsAD (which determines whether a tape is reused)
• the compilation as defined by ReverseDiff, which is about speeding up an existing tape (and disconnected from the reuse issue)

[gdalle]

Note that in ADTypes, the "compile" argument to AutoReverseDiff is defined ambiguously too. So we should perhaps add more details to the struct, something like AutoReverseDiff(tape=true, compile=false)?

[gdalle]

I'd love for you to chime in here @devmotion @torfjelde: SciML/ADTypes.jl#91

[torfjelde]

Note that in ADTypes, the "compile" argument to AutoReverseDiff is defined ambiguously too. So we should perhaps add more details to the struct, something like AutoReverseDiff(tape=true, compile=false)?

I'm not too opinionated about this, as compilation without caching seems somewhat useless? Are there scenarios where you'd like to do that?

[gdalle]

I'm not too opinionated about this, as compilation without caching seems somewhat useless? Are there scenarios where you'd like to do that?

Indeed you can't compile a tape you never record in the first place. In any case, I think the ambiguous terminology was fixed by SciML/ADTypes.jl#91. It's just a shame that the word "compile" was chosen instead of "record", given how both are used in ReverseDiff's documentation. But it's a sunk cost now.

[willtebbutt]

Thank you all for raising this problem / helping to fix it -- it's good that we've gotten to the bottom of this! I'm basically happy with this. @gdalle and @torfjelde could take a final look at this, and let me know whether you are happy.

[torfjelde]

It's just a shame that the word "compile" was chosen instead of "record", given how both are used in ReverseDiff's documentation. But it's a sunk cost now.

I agree with that 😕

[torfjelde]

LGTM! Added one final comment that would be nice to shoehorn in here since we're already making alterations, but looks good:)

[willtebbutt]

@penelopeysm it looks like my PR has the potential to cause merge conflicts with #559 . Would you like me to wait until you've merged yours before merging this, or are you happy to resolve any conflicts that arise?

[penelopeysm]

I'm happy to sort it out :) It's really just the Manifest, so just an issue of letting Pkg do its job.