commit aef81ef08e4763569e986505c9f65179cb5157f7
author Paul Berry <paulberry@google.com> Mon Aug 28 15:01:56 2023 +0000
committer Commit Queue <dart-scoped@luci-project-accounts.iam.gserviceaccount.com> Mon Aug 28 15:01:56 2023 +0000
tree 94819e122bffba1a295c67cdd49db66a2846beda
parent a675a79f3941c7cf3f6f84af2c7863ecff414691

Flow analysis: fix scrutinee promotion for fields.

Prior to this change, the SSA node stored in `PromotionModel.ssaNode`
was only correct for promotion models that represented variable
references. If a promotion model represented a promotable field, its
`ssaNode` pointed to a bogus SSA node. This had two undesirable
effects:

- It meant that `FlowModel.rebaseForward` needed to contain a hack to
  prevent it from looking at the bogus SSA node for a promotable
  field, and falsely concluding that the field's value had been
  reassigned (which is impossible for promotable fields)--see
  https://dart-review.googlesource.com/c/sdk/+/321752.

- It meant that if a promotable field was used as a scrutinee in a
  refutable pattern match, the promotion logic would look at the bogus
  SSA node for the field, and falsely conclude that its value had been
  reassigned, preventing field promotion from working during pattern
  matching.

This change ensures that the correct SSA node is always stored in
`PromotionModel.ssaNode`, and removes the hack in
`FlowModel.rebaseForward`. This required some re-ordering some of the
logic for control flow joins, to ensure that when a join creates a
fresh promotion model for a property, it has already created the
corresponding `_PropertySsaNode` (previously, it created the
`_PropertySsaNode` afterwards, but that is too late since the
`PromotionModel` class is immutable).

Unit tests and language tests are introduced to validate the newly
fixed behavior for promotable fields used as a scrutinee in a
refutable pattern match.

Also, the uses of `FlowModel.infoFor` in queries such as
`getMatchedValueType`, `isAssigned`, `isUnassigned`, and
`promotedType` were changed to simple map lookups, to prevent bogus
SSA nodes from being created and then immediately discarded. This
resulted in a fairly significant boost to CFE compilation speed:

  page-faults:u: -1.2664% +/- 0.1535% (-2531.33 +/- 306.73)
  instructions:u: -0.6210% +/- 0.0009% (-119891846.00 +/- 180585.35)
  branches:u: -0.6765% +/- 0.0014% (-26637478.67 +/- 54272.64)
  branch-misses:u: -0.9562% +/- 0.8909% (-548444.00 +/- 510991.65)

Change-Id: I30f82e8a4ba11236735258077d61d36717fa32c2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/322443
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>