Bug 157379 – 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

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Filip Pizlo]

Any run of DFG AI is a fixpoint that loosens the proof until it can't fine 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.

[Filip Pizlo]

Created attachment 278177 [details]
the patch

[Mark Lam]

Comment on attachment 278177 [details]
the patch

r=me

[Filip Pizlo]

Landed in http://trac.webkit.org/changeset/200468