The IR data flow library now supports virtual dispatch with a library
that's similar to `security.TaintTracking`. In particular, it should
have the same performance characteristics. The main difference is that
non-recursive callers of `flowsFrom` now pass `_` instead of `true` for
`boolean allowFromArg`. This change allows flow through `return` to
actually work.
This is just good enough to cause no performance regressions and pass
the virtual-dispatch tests we have for `security.TaintTracking`. In
particular, it fixes the tests for `UncontrolledProcessOperation.ql`
when enabling `DefaultTaintTracking.qll`.
The data flow library conflates pointers and their objects in some
places but not others. For example, a member function call `x.f()` will
cause flow from `x` of type `T` to `this` of type `T*` inside `f`. It
might be ideal to avoid that conflation, but that's not realistic
without using the IR.
We've had good experience in the taint tracking library with conflating
pointers and objects, and it improves results for field flow, so perhaps
it's time to try it out for all data flow.
The `toString` for IR data-flow nodes are now similar to AST data-flow
nodes. This should make it easier to use the IR as a drop-in replacement
in the future. There are still differences because the IR data flow
library takes conversions into account.
I did not attempt to align the new nodes we use for field flow. That can
come later, when we add field flow to IR data flow.
This string looked out of place compared to `ExplicitParameterNode`,
whose string is simply the name of the parameter and therefore
indistinguishable from an access to the parameter without looking at the
location also. This has not been a problem so far, and if we want to
distinguish more clearly between initial values and accesses at some
point, we should do it for `ExplicitParameterNode` and
`UninitializedNode` too.
These partial defs don't do any harm, but they could hurt performance.
In typical C++ snapshots, between 5% and 20% of all calls are to `const`
functions.
