CheckStructure can be incorrectly subsumed by CheckStructureOrEmpty
authorsbarati@apple.com <sbarati@apple.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc>
Fri, 12 Jan 2018 20:47:44 +0000 (20:47 +0000)
committersbarati@apple.com <sbarati@apple.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc>
Fri, 12 Jan 2018 20:47:44 +0000 (20:47 +0000)
commit82c72ec37fb5af647e61720606a9f1a5f08ceb0d
treebb1e154b88c1213853c78d312536166c4d7c205e
parent4b2c8d975f352d249ff896466a285dc3212db328
CheckStructure can be incorrectly subsumed by CheckStructureOrEmpty
https://bugs.webkit.org/show_bug.cgi?id=181177
<rdar://problem/36205704>

Reviewed by Yusuke Suzuki.

JSTests:

* stress/check-structure-ir-ensures-empty-does-not-flow-through.js: Added.
(runNearStackLimit.t):
(runNearStackLimit):
(test.f):
(test):

Source/JavaScriptCore:

The semantics of CheckStructure are such that it does not allow the empty value to flow through it.
However, we may eliminate a CheckStructure if it's preceded by a CheckStructureOrEmpty. This doesn't
have semantic consequences when validation is turned off. However, with validation on, this trips up
our OSR exit machinery that says when an exit is allowed to happen.

Consider the following IR:

a: GetClosureVar // Or any other node that produces BytecodeTop
...
c: CheckStructure(Cell:@a, {s2})
d: PutByOffset(KnownCell:@a, KnownCell:@a, @value)

In the TypeCheckHoistingPhase, we may insert CheckStructureOrEmptys like this:
a: GetClosureVar
e: CheckStructureOrEmpty(@a, {s1})
...
f: CheckStructureOrEmpty(@a, {s2})
c: CheckStructure(Cell:@a, {s2})
d: PutByOffset(KnownCell:@a, KnownCell:@a, @value)

This will cause constant folding to change the IR to:
a: GetClosureVar
e: CheckStructureOrEmpty(@a, {s1})
...
f: CheckStructureOrEmpty(@a, {s2})
d: PutByOffset(KnownCell:@a, KnownCell:@a, @value)

Our mayExit analysis determines that the PutByOffset should not exit. Note
that AI will determine the only value the PutByOffset can see in @a is
the empty value. Because KnownCell filters SpecCell and not SpecCellCheck,
when lowering the PutByOffset, we reach a contradiction in AI and emit
an OSR exit. However, because mayExit said we couldn't exit, we assert.

Note that if we did not run the TypeCheckHoistingPhase on this IR, AI
would have determined we would OSR exit at the second CheckStructure.

This patch makes it so constant folding produces the following IR:
a: GetClosureVar
e: CheckStructureOrEmpty(@a, {s1})
g: AssertNotEmpty(@a)
...
f: CheckStructureOrEmpty(@a, {s2})
h: AssertNotEmpty(@a)
d: PutByOffset(KnownCell:@a, KnownCell:@a, @value)

This modification will cause AI to know we will OSR exit before even reaching
the PutByOffset. Note that in the original IR, the GetClosureVar won't
actually produce the TDZ value. If it did, bytecode would have caused us
to emit a CheckNotEmpty before the CheckStructure/PutByOffset combo. That's
why this bug is about IR bookkeeping and not an actual error in IR analysis.
This patch introduces AssertNotEmpty instead of using CheckNotEmpty to be
more congruous with CheckStructure's semantics of crashing on the empty value
as input (on 64 bit platforms).

* dfg/DFGAbstractInterpreterInlines.h:
(JSC::DFG::AbstractInterpreter<AbstractStateType>::executeEffects):
* dfg/DFGClobberize.h:
(JSC::DFG::clobberize):
* dfg/DFGConstantFoldingPhase.cpp:
(JSC::DFG::ConstantFoldingPhase::foldConstants):
* dfg/DFGDoesGC.cpp:
(JSC::DFG::doesGC):
* dfg/DFGFixupPhase.cpp:
(JSC::DFG::FixupPhase::fixupNode):
* dfg/DFGNodeType.h:
* dfg/DFGPredictionPropagationPhase.cpp:
* dfg/DFGSafeToExecute.h:
(JSC::DFG::safeToExecute):
* dfg/DFGSpeculativeJIT32_64.cpp:
(JSC::DFG::SpeculativeJIT::compile):
* dfg/DFGSpeculativeJIT64.cpp:
(JSC::DFG::SpeculativeJIT::compile):
* ftl/FTLCapabilities.cpp:
(JSC::FTL::canCompile):
* ftl/FTLLowerDFGToB3.cpp:
(JSC::FTL::DFG::LowerDFGToB3::compileNode):
(JSC::FTL::DFG::LowerDFGToB3::compileAssertNotEmpty):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@226907 268f45cc-cd09-0410-ab3c-d52691b4dbfc
15 files changed:
JSTests/ChangeLog
JSTests/stress/check-structure-ir-ensures-empty-does-not-flow-through.js [new file with mode: 0644]
Source/JavaScriptCore/ChangeLog
Source/JavaScriptCore/dfg/DFGAbstractInterpreterInlines.h
Source/JavaScriptCore/dfg/DFGClobberize.h
Source/JavaScriptCore/dfg/DFGConstantFoldingPhase.cpp
Source/JavaScriptCore/dfg/DFGDoesGC.cpp
Source/JavaScriptCore/dfg/DFGFixupPhase.cpp
Source/JavaScriptCore/dfg/DFGNodeType.h
Source/JavaScriptCore/dfg/DFGPredictionPropagationPhase.cpp
Source/JavaScriptCore/dfg/DFGSafeToExecute.h
Source/JavaScriptCore/dfg/DFGSpeculativeJIT32_64.cpp
Source/JavaScriptCore/dfg/DFGSpeculativeJIT64.cpp
Source/JavaScriptCore/ftl/FTLCapabilities.cpp
Source/JavaScriptCore/ftl/FTLLowerDFGToB3.cpp