[Webkit-unassigned] [Bug 74908] New: Temporary GPR should not be lazily allocated in DFG JIT on X86

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=74908

           Summary: Temporary GPR should not be lazily allocated in DFG
                    JIT on X86
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          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


On X86, we used to allocate a temporary GPR lazily when it's really used rather than defined. This may cause potential issues of allocating registers inside control flow and result in problems in subsequent code generation, for example the DFG JIT may think an operand already being spilled (to satisfy the allocation request) and generate code to read the data from memory, but the allocation and spilling are in a branch which is not taken at runtime, so the generated code is incorrect.

Although current DFG JIT code doesn't have this problematic pattern, it's better to cut-off the root to avoid any potential issues in the future.

Patch forthcoming.

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