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custom derivative doc prototype #2189

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custom derivative doc prototype #2189

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3f3ee94
prototype
ZuseZ4 Dec 6, 2024
2dd80bf
adressing feedback
ZuseZ4 Dec 6, 2024
c7545e8
add fwd mode example
ZuseZ4 Dec 7, 2024
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22 changes: 22 additions & 0 deletions enzyme/Enzyme/CApi.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -340,6 +340,21 @@ void EnzymeRegisterAllocationHandler(char *Name, CustomShadowAlloc AHandle,
};
}


/// This is the main entry point to register a custom derivative for language frontends.
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/// It should be prefered over trying to register custom-derivatives in the llvm-ir module.
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/// The main reason is that these rules can handle non-default activity cases, e.g.
/// a function call where a pointer or a float scalar is marked as const.
/// To get a better low-level understanding, the code in AdjointGenerator.h can be read.
///
/// This Function will only handle using ReverseMode AD (either split or combined).
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currently this only supports split mode. Well kind of. A combined mode call will call the "forward pass" generator from here and then also the "reverse pass" generator as well. This is also the case for split mode as well. Of course with the full functionality of gradientutils/etc available, one could check if they are in combined mode by checking if gutils->mode == ReverseModeCombined.

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@ZuseZ4 ZuseZ4 Dec 6, 2024 •
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Thank you. Can you please also answer how the last three arguments should be used and how to get them?

/// As a high-level example, assume we want to register a custom derivative for a vector resize function.
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I don't think this is a good example (far too complicated, and I'm not confident your algirthm for the derivative is correct). Perhaps explain with a simple pow(x, y) example where x or y could be differentiable or not and you want to handle both easily.

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@ZuseZ4 ZuseZ4 Dec 6, 2024 •
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Can you please explain why it is not correct?

/// We pass the mangled name as first argument.
/// resizing is a simple enough to be handled in the fwd pass, so we just pass a nullptr as RevHandle.
/// For the forward pass, we will need to resize the shadow of the input (if duplicated), so the CI
/// is a CallInst of resize on the shadow argument. The IRBuilder B, the shadow argument, and gutils
/// should all be provided available in the frontend. The last three arguments ...
///
void EnzymeRegisterCallHandler(char *Name,
CustomAugmentedFunctionForward FwdHandle,
CustomFunctionReverse RevHandle) {
Expand All @@ -363,6 +378,13 @@ void EnzymeRegisterCallHandler(char *Name,
};
}

/// This is the main entry point to register a custom derivative for language frontends.
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/// It should be prefered over trying to register custom-derivatives in the llvm-ir module.
/// The main reason is that these rules can handle non-default activity cases, e.g.
/// a function call where a pointer or a float scalar is marked as const.
/// To get a better low-level understanding, the code in AdjointGenerator.h can be read.
///
/// This Function will only handle using ForwardMode AD.
void EnzymeRegisterFwdCallHandler(char *Name, CustomFunctionForward FwdHandle) {
auto &pair = customFwdCallHandlers[Name];
pair = [=](IRBuilder<> &B, CallInst *CI, GradientUtils &gutils,
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