[AutoDiff] Fix differentiation for non-wrt inout parameters.
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dan-zheng
commented
Aug 5, 2020
lib/SIL/IR/SILFunctionType.cpp
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| // FIXME(TF-1305): The `getResults().empty()` condition is a hack. | ||
| // | ||
| // Currently, an `inout` parameter can either be: | ||
| // 1. Both a differentiability parameter and a differentiability result. | ||
| // 2. `@noDerivative`: neither a differentiability parameter nor a | ||
| // differentiability result. | ||
| // However, there is no way to represent an `inout` parameter that: | ||
| // 3. Is a differentiability parameter but not a differentiability result. | ||
| // 4. Is a differentiability result but not a differentiability parameter. | ||
| // | ||
| // See TF-1305 for solution ideas. For now, `@noDerivative` `inout` | ||
| // parameters are not treated as differentiability results, unless the | ||
| // original function has no formal results, which case all `inout` | ||
| // parameters are treated as differentiability results. |
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Quick paste of solution ideas from TF-1305:
- Add new SILParameterDifferentiability kinds, e.g.
InoutParameterNotDifferentiableParameter(case 3) andInoutParameterNotDifferentiableResult(case 4).- Consider whether this needs to be exposed in AST
@differentiablefunction types.
- Consider whether this needs to be exposed in AST
- Find some way to store parameter/result differentiability in
SILFunctionTypeinstead of individualSILParameterInfo/SILResultInfo.
Fix SIL differential function type calculation to handle non-wrt `inout` parameters. Patch `SILFunctionType::getDifferentiabilityResultIndices` to prevent returning empty result indices for `@differentiable` function types with no formal results where all `inout` parameters are `@noDerivative`. TF-1305 tracks a robust fix. Resolves SR-13305. Exposes TF-1305: parameter/result differentiability hole for `inout` parameters.
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@swift-ci Please test |
dan-zheng
commented
Aug 5, 2020
lib/SIL/IR/SILFunctionType.cpp
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| // 4. Is a differentiability parameter but not a differentiability result. | ||
| // This case does not yet have clear use cases, so supporting it is a | ||
| // non-goal. |
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@rxwei mentioned a nuanced point, which is that there are no clear use cases for inout parameters that are differentiability parameters but not differentiability results (case 4).
That case is indeed not currently expressible, so we can deprioritize support for it: I clarified that in the comment. Supporting that case would involve something like adding a results: clause to @differentiable declaration attribute:
// Pseudo-syntax for supporting case 4:
// `inout` parameter that is a differentiability parameter but not a differentiability result.
@differentiable(wrt: inoutParam, results: return)
func foo(_ inoutParam: inout Float) -> Float {
return inoutParam
}The focus of this PR is supporting inout parameters that are differentiability results but that are not differentiability parameters (case 3), which is currently expressible and reasonable to support.
// Case 3: `inout` parameter that is a differentiability result but not a differentiability parameter.
@differentiable(wrt: x)
func foo(_ x: Float, _ inoutParam: inout Float) {
inoutParam = x * x
}… but not diff. results. There are no clear use cases for `inout` parameters that are differentiability parameters but not differentiability results, so we can de-prioritize support for it. Clarify this in the comment regarding TF-1305.
rxwei
approved these changes
Aug 5, 2020
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@swift-ci Please test |
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Fix SIL differential function type calculation to handle non-wrt
inoutparameters.
Patch
SILFunctionType::getDifferentiabilityResultIndicesto prevent returningempty result indices for
@differentiablefunction types with no formal resultswhere all
inoutparameters are@noDerivative. TF-1305 tracks a robust fix.Resolves SR-13305.
Exposes TF-1305: parameter/result differentiability hole for
inoutparameters.