swift

414e0293 - [AutoDiff] Revamp usefulness propagation in activity analysis. (#28225)

[AutoDiff] Revamp usefulness propagation in activity analysis. (#28225) Useful values are those that contribute to (specific) dependent variables, i.e. function results. For addresses: all projections of a useful address should be useful. This has special support: `DifferentiableActivityInfo::propagateUsefulThroughAddress`. Previously: - Usefulness was propagated by iterating through all instructions in post-dominance order. This is not efficient because irrelevant instructions may be visited. - For useful addresses, `propagateUsefulThroughAddress` propagated usefulness one step to projections, but not recursively to users of the projections. This caused some values to incorrectly not be marked useful. Now: - Usefulness is propagated by following use-def chains, starting from dependent variables (function results). This is handled by the following helpers: - `setUsefulAndPropagateToOperands(SILValue, unsigned)`: marks a value as useful and recursively propagates usefulness through defining instruction operands and basic block argument incoming values. - `propagateUseful(SILInstruction *inst, unsigned)`: propagates usefulness to the operands of the given instruction. - `DifferentiableActivityInfo::propagateUsefulThroughAddress` now calls `propagateUseful` to propagate usefulness recursively through users' operands. Effects: - More values are now (correctly) marked as useful, affecting non-differentiability diagnostics for active enum values (TF-956) and for-in loops (TF-957). Both have room for improvement. Resolves control flow differentiation correctness issue: TF-954.