Changes between Version 5 and Version 6 of JavaScriptCore


Ignore:
Timestamp:
Feb 14, 2013 10:06:48 PM (9 years ago)
Author:
fpizlo@apple.com
Comment:

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  • JavaScriptCore

    v5 v6  
    2424Both the LLInt and Baseline JIT collect light-weight profiling information to enable speculative execution by the next tier of execution (the DFG).  Information collected includes recent values loaded into arguments, loaded from the heap, or loaded from a call return.  Additionally, all inline caching in the LLInt and Baseline JIT is engineered to enable the DFG to scrape type information easily: for example the DFG can detect that a heap access sometimes, often, or always sees a particular type just by looking at the current state of an inline cache; this can be used to determine the most profitable level of speculation.
    2525
    26 '''DFG JIT''' kicks in for functions that are invoked at least 60 times, or that took a loop at least 1000 times.  Again, these numbers are approximate and are subject to additional heuristics.  The DFG performs aggressive type speculation based on profiling information collected by the lower tiers.  This allows it to forward-propagate type information, eliding many type checks.  Sometimes the DFG goes further and speculates on values themselves - for example it may speculate that a value loaded from the heap is always some known function in order to enable inlining.  The DFG uses deoptimization (we call it "OSR exit") to handle cases where speculation fails.  Deoptimization may be synchronous (for example, a branch that checks that the type of a value is that which was expected) or asynchronous (for example, the runtime may observe that the shape or value of some object or variable has changed in a way that contravenes assumptions made by the DFG).  The latter is referred to as "watchpointing" in the DFG codebase.  Altogether, the Baseline JIT and the DFG JIT share a two-way OSR relationship: the Baseline JIT may OSR into the DFG when a function gets hot, and the DFG may OSR to the Baseline JIT in the case of deoptimization.  Repeated OSR exits from the DFG serve as an additional profiling hint: the DFG OSR exit machinery records the reason of the exit (including potentially the values that failed speculation) as well as the frequency with which it occurred; if an exit is taken often enough then reoptimization kicks in: execution is permanently shifted to the Baseline JIT for the affected function, more profiling is gathered, and then the DFG may be later reinvoked.  Reoptimization uses exponential back-off to defend against pathological code.  The DFG is in [http://trac.webkit.org/browser/trunk/Source/JavaScriptCore/dfg dfg/].
     26'''DFG JIT''' kicks in for functions that are invoked at least 60 times, or that took a loop at least 1000 times.  Again, these numbers are approximate and are subject to additional heuristics.  The DFG performs aggressive type speculation based on profiling information collected by the lower tiers.  This allows it to forward-propagate type information, eliding many type checks.  Sometimes the DFG goes further and speculates on values themselves - for example it may speculate that a value loaded from the heap is always some known function in order to enable inlining.  The DFG uses deoptimization (we call it "OSR exit") to handle cases where speculation fails.  Deoptimization may be synchronous (for example, a branch that checks that the type of a value is that which was expected) or asynchronous (for example, the runtime may observe that the shape or value of some object or variable has changed in a way that contravenes assumptions made by the DFG).  The latter is referred to as "watchpointing" in the DFG codebase.  Altogether, the Baseline JIT and the DFG JIT share a two-way OSR relationship: the Baseline JIT may OSR into the DFG when a function gets hot, and the DFG may OSR to the Baseline JIT in the case of deoptimization.  Repeated OSR exits from the DFG serve as an additional profiling hint: the DFG OSR exit machinery records the reason of the exit (including potentially the values that failed speculation) as well as the frequency with which it occurred; if an exit is taken often enough then reoptimization kicks in: callers are relinked to the Baseline JIT for the affected function, more profiling is gathered, and then the DFG may be later reinvoked.  Reoptimization uses exponential back-off to defend against pathological code.  The DFG is in [http://trac.webkit.org/browser/trunk/Source/JavaScriptCore/dfg dfg/].
    2727
    2828At any time, functions, eval blocks, and global code in JavaScriptCore may be executing in a mix of the LLInt, Baseline JIT, and DFG.  In the extreme case of a recursive function, there may even be multiple stack frames where one frame is in the LLInt, another is in the Baseline JIT, while another still is in the DFG; even more extreme are cases where one stack frame is executing an old DFG compilation and another is executing a new DFG compilation because recompilation kicked in but execution did not yet return to the old DFG code.  These three engines are designed to maintain identical execution semantics, and so even if multiple functions in a JavaScript program are executing in a mix of these engines the only perceptible effect ought to be execution performance.