swift

e242a882 - [AutoDiff] Revamp and fix variedness propagation in activity analysis. (#28155)

[AutoDiff] Revamp and fix variedness propagation in activity analysis. (#28155) Varied values are those that depend on (specific) independent variables, i.e. function arguments. For addresses: all projections of a varied address should be varied. This has special support: `DifferentiableActivityInfo::propagateVariedInwardsThroughProjections`. Previously: - Variedness was propagated by iterating through all instructions in dominance order. This is not efficient because irrelevant instructions may be visited. - For varied addresses, `propagateVariedInwardsThroughProjections` propagated variedness one step to projections, but not recursively to users of the projections. This caused some values to incorrectly be marked as non-varied. Now: - Variedness is propagated by following use-def chains, starting from independent variables (function arguments). This is handled by the following helpers: - `setVariedAndPropagateToUsers(SILValue, unsigned)`: marks a value as varied and propagates variedness to users. - `propagateVaried(Operand *operand, unsigned)`: propagates variedness for the given operand to its user's results. - `DifferentiableActivityInfo::propagateVariedInwardsThroughProjections` now calls `setVariedAndPropagateToUsers` to propagate variedness recursively through projections, which is desired. Resolves various control flow differentiation correctness issues: TF-681, TF-781, TF-933. All of these issues involve incorrect variedness propagation for initially non-varied addresses. TF-947 tracks further improvements to varied propagation. TF-949 tracks similar revamping for usefulness propagation.