Differentiable Fortran with LFortran and Enzyme

Author here — I work on Tesseract at Pasteur Labs, and I wrote this up because the "what if this was possible" was bugging me for way too long :)

I was surprised by how well this worked, the LFortran + Enzyme stack seems to be a very clean way to get gradients through Fortran code via LLVM IR transformations. Pretty cool to see a 220-line Fortran heat solver turn into ~6,900-line reverse pass automatically if I dare say so.

Would be awesome to see this applied to a real scientific codebase, and I hope that the demo is enough to convince people that it’s worth trying.

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srean 2 hours ago | parent | next [-]

Very interesting. Does LFortran have the same internal array layout as the standard C runtime ?

A shared layout and a shared calling convention would be very nice.

Sorry about my naive question. Haven't touched Fortran directly in three decades I think.

EDIT: thanks for your reply. For some reason it has been flagged dead. So am responding here. You can mail dang hn at ycombinator dot co m about the flagging. He is very nice.

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kmaitreys 11 minutes ago | parent | next [-]

I would also like to know this. Fortran itself is column-major, so I would guess the internal layout isn't same for multi-dimensional arrays when compared to row-major C? I'm not sure how LFortran represents arrays internally though.

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wombatpm 2 hours ago | parent | prev [-]

Lots of scientific code in Fortran has sparse arrays, so a NxN array that only has values on 5 diagonals will store that as 5xN array to save memory allowing you to run a larger problem.

	▲	srean 2 hours ago \| parent [-]
		That's a very orthogonal issue. Sparse arrays are supported on C libraries too. I have done my time with CSC and CSR even inside Python that called out to C libraries.

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tanderson92 32 minutes ago | parent | prev [-]

When you say you 'wrote this up', you mean you had an AI write (at least) chunks of it.

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dionhaefner 24 minutes ago | parent [-]

Indeed, just like I let my compiler write (at least) chunks of my AD logic. Not great when the tool becomes a leaky abstraction, but overall net positive don't you think?

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tanderson92 22 minutes ago | parent [-]

Disagree; it irks me to read AI slop, and writing is meant to be persuasive and engaging to human readers.

	▲	dionhaefner 16 minutes ago \| parent [-]
		Happy to take the blame for the lack of persuasian and engagement then :) Thanks for the feedback, although I’d like to believe there’s a way to have that cake and eat it too.