[Webkit-unassigned] [Bug 80415] Eliminate redundant Phis in DFG

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=80415





--- Comment #12 from Yuqiang Xian <yuqiang.xian at intel.com>  2012-03-06 17:53:51 PST ---
(In reply to comment #11)
> 
> Aha!  I think that the thing that our dummy Phi insertion was giving us was liveness, and you're saying that we've lost this.
> 
> This is dangerous since we need to know liveness to compute DFG OSR entry constraints.
> 
> So we have two options:
> 
> 1) Have a different way of computing liveness.  I'm OK with this if you want to do it, since we'll need it eventually!
> 
> 2) If we eliminate a Phi from a block, then have the block's references to that Phi (i.e. variablesAtHead/AtTail) refer to its replacement.
> 
> I like (2) better for this patch, since it feels like a more minimal change.
> 
> What do you think?

I agree. Then the variablesAtHead/Tail of a block may reference a node outside the block and we need to extract the type predictions from it. But it _should_ not be a problem as we now keep the abstract values of all the nodes in the graph in one AbstractState - will verify it.

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