DFG should be able to emit effectful structure checks
authorfpizlo@apple.com <fpizlo@apple.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc>
Mon, 29 Oct 2012 04:02:08 +0000 (04:02 +0000)
committerfpizlo@apple.com <fpizlo@apple.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc>
Mon, 29 Oct 2012 04:02:08 +0000 (04:02 +0000)
commit342c3846eb6ed17874aa88b72c72c1dfad1546a7
tree886ecc3017e2af41d3c728b8648e8330c3ceead0
parent29ba6f8e7b1563782151ab1677b665611aa45595
DFG should be able to emit effectful structure checks
https://bugs.webkit.org/show_bug.cgi?id=99260

Reviewed by Oliver Hunt.

This change allows us to find out if an array access that has gone polymorphic
is operating over known structures - i.e. the primordial array structures of the
global object that the code block containing the array access belongs to. We
term this state "OriginalArray" for short. The fact that the access has gone
polymorphic means that the array profile will not be able to report the set of
structures it had seen - but if it can tell us that all of the structures were
primordial then it just so happens that we can deduce what the structure set
would have been by just querying the code block's global object. This allows us
to emit an ArrayifyToStructure instead of an Arrayify if we find that we need to
do conversions. The fast path of an ArrayifyToStructure is exactly like the fast
path of a CheckStructure and is mostly subject to the same optimizations. It
also burns one fewer registers.

Essentially the notion of OriginalArray is a super cheap way of getting the
array profile to tell us a structure set instead of a singleton structure.
Currently, the array profile can only tell us the structure seen at an array
access if there was exactly one structure. If there were multiple structures, it
won't tell us anything other than the array modes and other auxiliary profiling
data (whether there were stores to holes, for example). With OriginalArray, we
cheaply get a structure set if all of the structures were primordial for the
code block's global object, since in that case the array mode set (ArrayModes)
can directly tell us the structure set. In the future, we might consider adding
complete structure sets to the array profiles, but I suspect that we would hit
diminishing returns if we did so - it would only help if we have array accesses
that are both polymorphic and are cross-global-object accesses (rare) or if the
arrays had named properties or other structure transitions that are unrelated to
indexing type (also rare).

This also does away with Arrayify (and the new ArrayifyToStructure) returning
the butterfly pointer. This turns out to be faster and easier to CSE.

And, this also changes constant folding to be able to eliminate CheckStructure,
ForwardCheckStructure, and ArrayifyToStructure in addition to being able to
transform them into structure transition watchpoints. This is great for
ArrayifyToStructure because then CSE and CFA know that there is no side effect.
Converting CheckStructure and ForwardCheckStructure to also behave this way is
just a matter of elegance.

This has no performance impact right now. It's intended to alleviate some of the
regressions seen in the early implementation of
https://bugs.webkit.org/show_bug.cgi?id=98606.

* bytecode/ArrayProfile.cpp:
(JSC::ArrayProfile::computeUpdatedPrediction):
* bytecode/ArrayProfile.h:
(JSC):
(JSC::ArrayProfile::ArrayProfile):
(ArrayProfile):
(JSC::ArrayProfile::usesOriginalArrayStructures):
* bytecode/CodeBlock.cpp:
(JSC::CodeBlock::updateAllPredictionsAndCountLiveness):
* dfg/DFGAbstractState.cpp:
(JSC::DFG::AbstractState::execute):
* dfg/DFGArrayMode.cpp:
(JSC::DFG::ArrayMode::fromObserved):
(JSC::DFG::ArrayMode::alreadyChecked):
(JSC::DFG::arrayClassToString):
* dfg/DFGArrayMode.h:
(JSC::DFG::ArrayMode::withProfile):
(JSC::DFG::ArrayMode::isJSArray):
(ArrayMode):
(JSC::DFG::ArrayMode::isJSArrayWithOriginalStructure):
(JSC::DFG::ArrayMode::supportsLength):
(JSC::DFG::ArrayMode::arrayModesWithIndexingShape):
* dfg/DFGByteCodeParser.cpp:
(JSC::DFG::ByteCodeParser::getArrayMode):
(JSC::DFG::ByteCodeParser::getArrayModeAndEmitChecks):
(JSC::DFG::ByteCodeParser::handleGetByOffset):
* dfg/DFGCSEPhase.cpp:
(JSC::DFG::CSEPhase::checkStructureElimination):
(JSC::DFG::CSEPhase::structureTransitionWatchpointElimination):
(JSC::DFG::CSEPhase::getPropertyStorageLoadElimination):
(JSC::DFG::CSEPhase::checkArrayElimination):
(JSC::DFG::CSEPhase::getScopeRegistersLoadElimination):
* dfg/DFGConstantFoldingPhase.cpp:
(JSC::DFG::ConstantFoldingPhase::foldConstants):
* dfg/DFGFixupPhase.cpp:
(JSC::DFG::FixupPhase::fixupNode):
(JSC::DFG::FixupPhase::checkArray):
* dfg/DFGNode.h:
(JSC::DFG::Node::hasStructure):
(JSC::DFG::Node::hasArrayMode):
(JSC::DFG::Node::arrayMode):
* dfg/DFGNodeType.h:
(DFG):
* dfg/DFGPredictionPropagationPhase.cpp:
(JSC::DFG::PredictionPropagationPhase::propagate):
* dfg/DFGSpeculativeJIT.cpp:
(JSC::DFG::SpeculativeJIT::jumpSlowForUnwantedArrayMode):
(JSC::DFG::SpeculativeJIT::arrayify):
* dfg/DFGSpeculativeJIT.h:
(SpeculativeJIT):
* dfg/DFGSpeculativeJIT32_64.cpp:
(JSC::DFG::SpeculativeJIT::compile):
* dfg/DFGSpeculativeJIT64.cpp:
(JSC::DFG::SpeculativeJIT::compile):
* runtime/JSGlobalObject.h:
(JSC::JSGlobalObject::isOriginalArrayStructure):
* runtime/Structure.cpp:
(JSC::Structure::nonPropertyTransition):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@132759 268f45cc-cd09-0410-ab3c-d52691b4dbfc
20 files changed:
Source/JavaScriptCore/ChangeLog
Source/JavaScriptCore/bytecode/ArrayProfile.cpp
Source/JavaScriptCore/bytecode/ArrayProfile.h
Source/JavaScriptCore/bytecode/CodeBlock.cpp
Source/JavaScriptCore/dfg/DFGAbstractState.cpp
Source/JavaScriptCore/dfg/DFGArrayMode.cpp
Source/JavaScriptCore/dfg/DFGArrayMode.h
Source/JavaScriptCore/dfg/DFGByteCodeParser.cpp
Source/JavaScriptCore/dfg/DFGCSEPhase.cpp
Source/JavaScriptCore/dfg/DFGConstantFoldingPhase.cpp
Source/JavaScriptCore/dfg/DFGFixupPhase.cpp
Source/JavaScriptCore/dfg/DFGNode.h
Source/JavaScriptCore/dfg/DFGNodeType.h
Source/JavaScriptCore/dfg/DFGPredictionPropagationPhase.cpp
Source/JavaScriptCore/dfg/DFGSpeculativeJIT.cpp
Source/JavaScriptCore/dfg/DFGSpeculativeJIT.h
Source/JavaScriptCore/dfg/DFGSpeculativeJIT32_64.cpp
Source/JavaScriptCore/dfg/DFGSpeculativeJIT64.cpp
Source/JavaScriptCore/runtime/JSGlobalObject.h
Source/JavaScriptCore/runtime/Structure.cpp