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If a user promotes a field and then subsequently changes the variable
used to access it, the promotion must be discarded. For example, in
this code:

    class C {
      int? _i;
    test(C c1, C c2) {
      C c = c1;           // (1)
      if (c._i != null) { // (2)
        print(c._i + 1);  // (3)
        c = c2;           // (4)
        print(c._i + 1);  // (5)

The test at (2) promotes `c._i` to non-null, so (3) is ok. But since
`c` is reassigned at (4), (5) should be a compile-time error.

Previously, flow analysis used one promotion key to track `c` and one
promotion key to track `c._i`. The `PromotionKeyStore` associated each
promotion key with a map containing the promotion keys of all of its
properties. So, for example, if the promotion key for `c` was 10 and
the promotion key for `c._i` was 11, the `PromotionKeyStore` would
associate promotion key 10 with the map `{'_i': 11}`, so that each
time `c._i` was accessed, promotion key 11 would be found. In order to
detect the compile-time error at (5), it had to keep track of the fact
that keys 10 and 11 were related, so that the assignment to `c` at (4)
could invalidate the promotion of `c._i`. It accomplished this by
linking together all the related promotion keys in a circularly linked
list, which it would walk at the time of any variable assignment.

This worked, but it required a lot of complex bookkeeping. Also, it
posed problems for integrating field promotion with cascades, for
example, in the following code:

    class B {
      void f([_]) { ... }
    class C {
      B? _b;
    test(C c1, C c2) {
      C c = c1;           // (6)
      if (c._b != null) { // (7)
        c.._b.f(          // (8)
              c = c2,     // (9)
	      c._b.f(),   // (10)
         .._b.f();        // (11)

The cascaded access `.._b.f` at (8) should be ok, since `c._b` has
been promoted. But since there is an assignment to `c` at (9), the
promotion should not carry over to (10), and an error should be
reported. However, no error should be reported at (11) because the
cascaded access to `.._b.f` at that location is using the old value of
`c` that was captured at the beginning of the cascade, prior to the
assignment. There's no way to achieve this by invalidation alone,
since the code locations at which the promotion is valid ((8) and
(11), but not (10)) aren't even contiguous.

The solution to the problem is to store property promotion keys in
`SsaNode`s used by flow analysis, rather than in the
`PromotionKeyStore`. Since a fresh `SsaNode` is allocated each time a
variable is assigned, this automatically invalidates any previous
property promotions without the need for any extra bookkeeping. So, in
the first example, at (1), an `SsaNode` is allocated and associated
with the promotion key for `c`. At (2), a promotion key is created for
`c._i` and stored in `c`'s `SsaNode`, and the flow model is updated to
indicate that that key has been promoted to non-null. At (3), that
promotion key is reacalled from the `c`'s `SsaNode`, so the promotion
is still in effect. At (4), a fresh `SsaNode` is associated with
`c`. Since that `SsaNode` doesn't contain any promotion keys yet, at
(5), the access to `c._i` causes a fresh promotion key to be
allocated, with no associated promotions. So the invalidation happens
automatically due to the fact that a new `SsaNode` was created.

Flow analysis doesn't yet support cascades, but here's how the second
example is intended to work: as before, at (6), an `SsaNode` is
allocated and associated with the promotion key for `c`. At (7), a
promotion key is created for `c._b` and stored in `c`'s `SsaNode`, and
the flow model is updated to indicate that `c._b` has been promoted to
non-null. At (8), the `SsaNode` for `c` is captured and saved for
later use. At (9), a fresh `SsaNode` is created and associated with
`c`. That fresh `SsaNode` is consulted at (10), so `c._b` is not
promoted at this point. However, at (11), the previously stored
`SsaNode` is used, so the promotion of `._b` still works.

Change-Id: I64519fbcb2368a37aa18adf35cee0ffd290db9b9
Commit-Queue: Paul Berry <>
Reviewed-by: Konstantin Shcheglov <>


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