Auto initialization for in-place operations

Status

REJECTED

Discussed by: Paul Dubs, Alex Black, raver119 on 25. November 2019

Context

Some operations work in-place on the inputs that they are given in ND4J, but in SameDiff the same operations will generate an array from a given shape. Examples for this include BernoulliDistribution, and other random ops, that effectively initialize the array that they are given.

From a consistency point of view, it would be nice if both API's would support both ways of using those ops.

Decision

We introduce an option to mark inputs as inPlace = true to make it clear that this input is going to be changed in-place. In addition we introduce an option supportsInPlaceInit = true to mark an input as initialize-able. If the supportsInPlaceInit option is enabled, two signatures for the Op will be created, one that takes an input, and one that takes the appropriate shape and data type information in its stead.

Consequences

Advantages

• We get support for both in-place and initialization use-cases
• Support for this use-case is explicitly defined in the DSL instead of implicit support by the code generator

Disadvantages

• Codegen becomes more complex, as it requires us to generate more signatures

Reasons for Rejection

This feature would only come in handy on legacy ops. However, those ops will not be exposed to frontend languages anymore starting from the next release.

For the ops that replace them ("Custom Ops"), it is always possible to pass an output array, when used in NDArray programming mode. Thereby making the in-place initialization support a side-effect of that general design.

For support of both op(out) and op(dataType, shape) see ADR 0005 "Optional parameters and signatures".