[Webkit-unassigned] [Bug 159388] New: DFG LICM needs to go all-in on the idea that some loops can't be LICMed

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=159388

            Bug ID: 159388
           Summary: DFG LICM needs to go all-in on the idea that some
                    loops can't be LICMed
    Classification: Unclassified
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: fpizlo at apple.com

Some time ago I acknowledged that LICM required loops to meet certain requirements that may get broken by the time we do LICM, like that the terminal of the pre-header is ExitOK.  It used to be that we just ignored that requirement and would hoist anyway, but since r189126 we've stopped hoisting out of loops that don't have ExitOK.  We also added tests for the case that the pre-header doesn't exist or is invalid.

It turns out that this patch didn't go far enough: even though it made LICM avoid loops that had an invalid pre-header, the part that updated the AI state in nested loops still assumed that these loops had valid pre-headers.  We would crash in null dereference in that loop if a nested loop had an invalid pre-header.

The fix is simple: don't update the AI state of nested loops that don't have pre-headers, since we won't try to hoist out of those loops anyway.

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