Summary:
Previously, at each function call, we added a `WrittenTo` attribute for applying the address of the actuals. However, this results in mistakenly considering each function application that inspects its argument as impure. Instead, we should only propagate `WrittenTo` if the actuals have already `WrittenTo` attributes.
For instance, for the following functions
```
public static boolean is_null(Byte a) {
return a == null;
}
public static boolean call_is_null(Byte a) {
return is_null(a);
}
```
We used to get the following pulse summary for `call_is_null` (showing only one of the disjuncts):
```
#0: PRE:
{ roots={ &a=v1 };
mem ={ v1 -> { * -> v2 } };
attrs={ v1 -> { MustBeValid },
v2 -> { Arith =null, BoItv ([max(0, v2), min(0, v2)]) } };}
POST:
{ roots={ &a=v1, &return=v8 };
mem ={ v1 -> { * -> v2 }, v8 -> { * -> v4 } };
attrs={ v2 -> { Arith =null,
BoItv ([max(0, v2), min(0, v2)]),
WrittenTo-----------WRONG },
v4 -> { Arith =1,
BoItv (1),
Invalid ConstantDereference(is the constant 1),
WrittenTo-----------WRONG },
v8 -> { WrittenTo } };}
SKIPPED_CALLS: { }
```
where we mistakenly recorded a `WrittenTo` for `v2` (what `a` points to). As a result, we considered `call_is_null` as impure :( This diff fixes that since the callee `is_null` doesn't have any `WrittenTo` attributes for its parameter `a`. So, we don't propagate `WrittenTo` and get the following summary
```
#0: PRE:
{ roots={ &a=v1 };
mem ={ v1 -> { * -> v2 } };
attrs={ v1 -> { MustBeValid },
v2 -> { Arith =null, BoItv ([max(0, v2), min(0, v2)]) } };}
POST:
{ roots={ &a=v1, &return=v8 };
mem ={ v1 -> { * -> v2 }, v8 -> { * -> v4 } };
attrs={ v2 -> { Arith =null, BoItv ([max(0, v2), min(0, v2)]) },
v4 -> { Arith =1,
BoItv (1),
Invalid ConstantDereference(is the constant 1) },
v8 -> { WrittenTo } };}
SKIPPED_CALLS: { }
```
Reviewed By: skcho
Differential Revision: D20490102
fbshipit-source-id: 253d8ef64
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