tree 6c1e1bfedb4383fc3c357759e7c35e88a017c277
parent 735a73943db7ce5a18078f5d98bd463d145386ff
author Alexander Markov <alexmarkov@google.com> 1736349601 -0800
committer Commit Queue <dart-scoped@luci-project-accounts.iam.gserviceaccount.com> 1736349601 -0800

[vm/aot,dart2wasm,tfa] Infer null after comparison with null only if value is potentially nullable

Previously, TFA was building a summary for the comparison
'v == null' assuming type of 'v' is Null on the true branch.

As a result, after joining data flow Null contaminates value of 'v'
further down:

v0 = ...
t1 = IsNull(v0)         // Condition
t2 = Null               // On true branch
t3 = NarrowNotNull(v0)  // On false branch
v1 = Join(t2, t3)       // This value is nullable even if v0 was not.
...
use(v1)

This change adds a condition to the Null value so it is evaluated to
Empty type if original value turns out to be non-nullable:

t1 = IsNull(v0)
t2 = Move(Null) {t1}    // Empty if t1 is false.
t3 = NarrowNotNull(v0)
v1 = Join(t2, t3)


TEST=pkg/vm/testcases/transformations/type_flow/transformer/regress_59845.dart
Fixes https://github.com/dart-lang/sdk/issues/59845

Change-Id: I0f78e31f9eaf6507cc78e393de2fc12a9cf29f4a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/403200
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>