We shouldn't crash if DFG AI proved that something was unreachable on one run but then decided not to prove it on another run
https://bugs.webkit.org/show_bug.cgi?id=157379
Reviewed by Mark Lam.
Any run of DFG AI is a fixpoint that loosens the proof until it can't find any more
counterexamples to the proof. It errs on the side of loosening proofs, i.e., on the side of
proving fewer things.
We run this fixpoint multiple times since there are multiple points in the DFG optimization
pipeline when we run DFG AI. Each of those runs completes a fixpoint and produces the
tightest proof it can that did not result in counterexamples being found.
It's possible that on run K of DFG AI, we prove some property, but on run K+1, we don't prove
that property. The code could have changed between the two runs due to other phases. Other
phases may modify the code in such a way that it's less amenable to AI's analysis. Our design
allows this because DFG AI is not 100% precise. It defends itself from making unsound choices
or running forever by sometimes punting on proving some property. It must be able to do this,
and so therefore, it might sometimes prove fewer things on a later run.
Currently in trunk if the property that AI proves on run K but fails to prove on run K+1 is
the reachability of a piece of code, then run K+1 will crash on an assertion at the
Unreachable node. It will complain that it reached an Unreachable. But it might be reaching
that Unreachable because it failed to prove that something earlier was always exiting. That's
OK, see above.
So, we should remove the assertion that AI doesn't see Unreachable.
No new tests because I don't know how to make this happen. I believe that this happens in the
wild based on crash logs.
* dfg/DFGAbstractInterpreterInlines.h:
(JSC::DFG::AbstractInterpreter<AbstractStateType>::executeEffects):
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@200468 268f45cc-cd09-0410-ab3c-d52691b4dbfc
2 files changed
