This is the first step in a sequence of CLs to support type promotion
based on local boolean variables
(https://github.com/dart-lang/language/issues/1274). To do this, we
will need a way to reliably tell whether a variable's value has
changed from one point in program execution to another, and to
associate additional information with a particular value of a
variable. We'll accomplish both of these tasks by associating each
variable with a pointer to an "SSA node". This pointer is updated to
point to a fresh node whenever the variable is written to, or two
different possible values come together at a control flow join.
Note that when a variable is write captured, it becomes impossible to
track when/if its value might change Previously we tracked this using
a `writeCaptured` boolean; now we track it by setting the SSA node
pointer to `null`.
This CL just lays the groundwork infrastructure and unit tests it;
there is no user-visible change.
Bug: https://github.com/dart-lang/language/issues/1274
Change-Id: Id729390655c9371cba264816b418f6c0463e1758
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/176180
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Paul Berry
1eb147d8af