[Webkit-unassigned] [Bug 74480] New: DFG should infer when local variables are doubles

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=74480

           Summary: DFG should infer when local variables are doubles
           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: fpizlo at apple.com


Currently the DFG will box and unbox double variables that span basic blocks. This is expensive.  On 64-bit platforms (that use JSVALUE64) it means moving data between FPR file and the GPR file, and doing some arithmetic.  Moving data between FPR and GPR is typically expensive on x86-64.  On 32-bit systems that means clobbering three registers at a time (two GPRs and one FPR), doing twice as many loads and stores, and also doing data movement between FPRs and GPRs.

It also means that the DFG must recheck that a local variable contains a double most of the time when a basic block uses it for the first time.

The DFG should instead realize when a local variable contains a double.

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