[Webkit-unassigned] [Bug 186765] [Armv7] Linkbuffer: executableOffsetFor() fails for location 2

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=186765

--- Comment #4 from Mark Lam <mark.lam at apple.com> ---
(In reply to Mark Lam from comment #3)
> (In reply to Guillaume Emont from comment #0)
> > On THUMB2, instructions can be 2 bytes long, and therefore are not
> > guaranteed to be 4-aligned. This is the case for jump origins and targets,
> > which means that the parameter of executableOffsetFor() can be the value 2,
> > in which case executableOffsetFor() returns a value taken from before the
> > start of the buffer. 
> 
> Please explain how an offset of 2 translates to "a value taken from before
> the start of the buffer".  Can you point to some specific code where this
> happens?

I see what you meant here.  The code at the end of executableOffsetFor() is where this can happen if the location is 2.  However, that doesn't mean the change you made is valid.

2 things you should do:
1. Find out how the location value of 2 came to be, and determine if it's a valid value or not.
2. If a location of 2 is valid, then the code in executableOffsetFor() and recordLinkOffsets() might need a different implementation for ARMv7.

The answers to the above depends on how the branch compaction feature (guarded by ENABLE(BRANCH_COMPACTION)) works.

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