[Webkit-unassigned] [Bug 71684] New: DFG 32_64 - registers cannot be reused arbitrarily if speculation failures are possible

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=71684

           Summary: DFG 32_64 - registers cannot be reused arbitrarily if
                    speculation failures are possible
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: yuqiang.xian at intel.com
                CC: barraclough at apple.com, fpizlo at apple.com


Currently in DFG JIT, we try to reuse the physical register of an operand for temporary usage if the current use of the operand is the last use. But sometimes this can be wrong, for example if there are possible speculation failures and we need to fallback to baseline JIT, the value of the operand which is supposed to be hold in the physical register can be modified by register reusing. The fixes the last inspector failures in layout test on Mac 32-bit if switching on DFG. (BTW, I didn't observe the failure in Gtk+ port on Linux ia32 as the inspector tests are skipped in Gtk layout test, and only w/ a 32-bit Mac environment setup today can I notice this failure...)

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