This works by adding data-flow edges to skip over array expressions when
reading from arrays. On the post-update side, there was already code to
skip over array expressions when storing to arrays. That happens in
`valueToUpdate` in `AddressFlow.qll`, which needed just a small tweak to
support assignments with non-field expressions at the top-level LHS,
like `*a = ...` or `a[0] = ...`.
The new code in `AddressFlow.qll` is copy-pasted from `EscapesTree.qll`,
and there is already a note in these files saying that they share a lot
of code and must be maintained in sync.
The conflicts came from how `this` is now a parameter but not a
`Parameter` on `master`.
Conflicts:
cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll
cpp/ql/test/library-tests/dataflow/DefaultTaintTracking/defaulttainttracking.cpp
cpp/ql/test/library-tests/dataflow/DefaultTaintTracking/tainted.expected
cpp/ql/test/library-tests/dataflow/DefaultTaintTracking/test_diff.expected
cpp/ql/test/library-tests/dataflow/dataflow-tests/dataflow-ir-consistency.expected
cpp/ql/test/library-tests/dataflow/fields/ir-flow.expected
cpp/ql/test/library-tests/syntax-zoo/dataflow-ir-consistency.expected
Flow from a definition by reference of a field into its object was
working inconsistently and in a very syntax-dependent way. For a
function `f` receiving a reference, `f(a->x)` could propagate data back
to `a` via the _reverse read_ mechanism in the shared data-flow library,
but for a function `g` receiving a pointer, `g(&a->x)` would not work.
And `f((*a).x)` would not work either.
In all cases, the issue was that the shared data-flow library propagates
data backwards between `PostUpdateNode`s only, but there is no
`PostUpdateNode` for `a->x` in `g(&a->x)`. This pull request inserts
such post-update nodes where appropriate and links them to their
neighbors. In this exapmle, flow back from the output parameter of `g`
passes first to the `PostUpdateNode` of `&`, then to the (new)
`PostUpdateNode` of `a->x`, and finally, as a _reverse read_ with the
appropriate field projection, to `a`.
This case was added in dccc0f4db. The surrounding code has changed a lot
since then, and the case no longer seems to have an effect except to
create some dead ends and possibly cycles in the local flow graph.
This consistency check seems to have value for AST data flow, but I've
disabled it on the IR for now.
This commit also includes two unrelated changes that seem to fix a
semantic merge conflict.
It turns out that the evaluator will evaluate the GVN stage even when no
predicate from it is needed after optimization of the subsequent stages.
The GVN library is expensive to evaluate, and it'll become even more
expensive when we switch its implementation to IR.
This PR disables the use of GVN in `DataFlow::BarrierNode` for the AST
data-flow library, which should improve performance when evaluating a
single data-flow query on a snapshot with no cache. Precision decreases
slightly, leading to a new FP in the qltests.
There is no corresponding change for the IR data-flow library since IR
GVN is not very expensive.
