[Webkit-unassigned] [Bug 174697] New: Add the ability to change sp and pc to the ARM64 JIT probe.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=174697

            Bug ID: 174697
           Summary: Add the ability to change sp and pc to the ARM64 JIT
                    probe.
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: mark.lam at apple.com

It's not possible to change the value of the pc on ARM64 without using a gpr.  For this reason and because previous use cases of JIT probes aim first to preserve register values, this ability to change sp and pc was not supported on the ARM64 JIT probe.  

However, we'll now need this in order to use the JIT probes for implementing OSR exits.  So, we'll add it by using the lr register for the pc dispatch.  In JIT code, lr is never used for any other purpose than to modify pc anyway.  Additionally, at all OSR exit sites, we are guaranteed that lr will not contain any interesting values that we'll need to preserve.  So, it is ok to let the JIT probe use lr for changing the pc.

<rdar://problem/33436965>

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