[Webkit-unassigned] [Bug 64582] New: JSC JIT does not inline GC allocation fast paths

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=64582

           Summary: JSC JIT does not inline GC allocation fast paths
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: fpizlo at apple.com


The JSC GC now has easy-to-inline allocation fast paths.  But the JSC JIT still emits out-of-line stub calls for allocation.  This introduces two forms of overhead.  First, every allocation pays the price of call overhead, even if the allocation just involves a load, a branch, and a handful of stores.  Second, an inlined JIT-generated fast path can directly locate and access the appropriate SizeClass, which should simplify the fast path even more than would be possible with ahead-of-time compilation with a C++ compiler (since the C++ compiler does not statically know the address of the SizeClass and must instead find it by first loading the global data and then adding a constant).  The JSC JIT should inline the GC allocation fast path to alleviate these overheads.

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