ymandel marked an inline comment as done.
ymandel added inline comments.
================
Comment at: clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp:79
+ for (BoolValue *Constraint : Env1.getFlowConditionConstraints()) {
+ Expr1 = &Env1.makeAnd(*Expr1, *Constraint);
+ }
----------------
xazax.hun wrote:
> ymandel wrote:
> > sgatev wrote:
> > > xazax.hun wrote:
> > > > Hmm, interesting.
> > > > I think we view every boolean formula at a certain program point
> > > > implicitly as `FlowConditionAtThatPoint && Formula`. And the flow
> > > > condition at a program point should already be a disjunction of its
> > > > predecessors.
> > > >
> > > > So it would be interpreted as: `(FlowConditionPred1 ||
> > > > FlowConditionPred2) && (FormulaAtPred1 || FormulaAtPred2)`.
> > > > While this is great, this is not the strongest condition we could
> > > > derive.
> > > > `(FlowConditionPred1 && FormulaAtPred1) || (FormulaAtPred2 &&
> > > > FlowConditionPred2)` created by this code snippet is stronger which is
> > > > great.
> > > >
> > > > My main concern is whether we would end up seeing an exponential
> > > > explosion in the size of these formulas in the number of branches
> > > > following each other in a sequence.
> > > >
> > > Yeap, I agree this is suboptimal and I believe I'm the one to blame for
> > > introducing it downstream.
> > >
> > > I wonder if we can represent the flow condition of each environment using
> > > a bool atom and have a mapping of bi-conditionals between flow condition
> > > atoms and flow condition constraints. Something like:
> > >
> > > ```
> > > FC1 <=> C1 ^ C2
> > > FC2 <=> C2 ^ C3 ^ C4
> > > FC3 <=> (FC1 v FC2) ^ C5
> > > ...
> > > ```
> > >
> > > We can use that to simplify the formulas here and in `joinConstraints`.
> > > The mapping can be stored in `DataflowAnalysisContext`. We can track
> > > dependencies between flow conditions (e.g. above `FC3` depends on `FC1`
> > > and `FC2`) and modify `flowConditionImplies` to construct a formula that
> > > includes the bi-conditionals for all flow condition atoms in the
> > > transitive set before invoking the solver.
> > >
> > > I suggest putting the optimization in its own patch. I'd love to look
> > > into it right after this patch is submitted if both of you think it makes
> > > sense on a high level.
> > This sounds good to me. That said, I'm not sure how often we'd expect this
> > to be an issue in practice, since, IIUC, this specialized merge only occurs
> > when the value is handled differently in the two branches. So, a series of
> > branches alone won't trigger the exponential behavior.
> Fair point. It might never cause a problem in practice. Since we already have
> one proposal how to try to fix this if a problem ever surfaces, I'd love to
> have a comment about this in the code. But I think this is something that
> probably does not need to be addressed in the near future.
Good idea -- I reformatted essentially sgatev's response above as a comment in
the code.
Repository:
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https://reviews.llvm.org/D122838/new/
https://reviews.llvm.org/D122838
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