[Webkit-unassigned] [Bug 85144] New: Bytecompiler should emit trivially fewer jumps in loops

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=85144

           Summary: Bytecompiler should emit trivially fewer jumps in
                    loops
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: fpizlo at apple.com


Currently for a loop like:

while (foo) bar;

We emit code like:

     jmp continueTarget
topOfLoop:
     <code for bar>
continueTarget:
     <code for foo>
breakTarget:

But it would be more efficient in almost all cases to instead emit:

     <code for foo>
     <code for bar>
continueTarget:
     <code for foo>
breakTarget:

This will lead to more bytecode, and will otherwise be natural for LLInt and Baseline.  But for DFG, it's a hearty win since it reduces the number of basic blocks and makes the data flow relationships between foo and bar more obvious.

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