Having a bad time has a really awful time when it runs at the same time as the JIT
https://bugs.webkit.org/show_bug.cgi?id=151882
rdar://problem/23547038
Reviewed by Geoffrey Garen.
The DFG's use of watchpoints for havingABadTime goes back a long time. We introduced this feature
when we first introduced watchpoints. That left it open to a lot of bitrot. On top of that, this
code doesn't get tested much because having a bad time is not something that is really supposed to
happen.
Well, now I've got reports that it does happen - or at least, we know that it is because of
crashes in an assertion that could only be triggered if a bad time was had. In the meantime, we
added two new features without adequately testing havingABadTime: concurrent JIT and FTL.
Concurrency means that we have to worry about the havingABadTime watchpoint triggering during
compilation. FTL means that we have new code and new optimizations that needs to deal with this
feature correctly.
The bug can arise via race condition or just goofy profiling. As in the newly added test, we could
first profile an allocation thinking that it will allocate sane arrays. Then we might have a bad
time, and then compile that function with the FTL. The ByteCodeParser will represent the
allocation with a NewArray node that has a sane indexingType(). But when we go to lower the Node,
we observe that the Structure* that the JSGlobalObject tells us to use has a different indexing
type. This is a feature of havingABadTime that the DFG knew about, but the FTL didn't. The FTL
didn't know about it because we didn't have adequate tests, and this code rarely gets triggered in
the wild. So, the FTL had a silly assertion that the indexing types match. They absolutely don't
have to match.
There is another bug, a race condition, that remains even if we remove the bad assertion. We set
the havingABadTime watchpoint late in compilation, and we do it based on whether the watchpoint is
still OK. This means that we could parse a function before we have a bad time and then do
optimizations (for example in AbstractInterpreter) like proving that the structure set associated
with the value returned by the NewArray is the one with a sane indexing type. Then, after those
optimizations have already been done, we will go to set the watchpoint. But just as we are doing
this, we could haveABadTime on the main thread. Currently this sort of almost works because
having a bad time requires doing a GC, and we can't GC until the watchpoint collection phase. But
that feels too fragile. So, this phase moves the setting of the watchpoint to the FixupPhase. This
is consistent with our long-term goal of removing the WatchpointCollectionPhase. Moving this to
FixupPhase means that we set the watchpoint before doing any optimizations. So, if having a bad
time happens before the FixupPhase then all optimizations will agree that we're having a bad time
and so everything is fine; if we have a bad time after FixupPhase then we will cancel the
compilation anyway.
* dfg/DFGByteCodeParser.cpp:
(JSC::DFG::ByteCodeParser::handleConstantInternalFunction):
* dfg/DFGFixupPhase.cpp:
(JSC::DFG::FixupPhase::fixupNode):
(JSC::DFG::FixupPhase::watchHavingABadTime):
(JSC::DFG::FixupPhase::createToString):
* dfg/DFGNode.h:
(JSC::DFG::Node::hasIndexingType):
(JSC::DFG::Node::indexingType):
* dfg/DFGWatchpointCollectionPhase.cpp:
(JSC::DFG::WatchpointCollectionPhase::handle):
* ftl/FTLLowerDFGToLLVM.cpp:
(JSC::FTL::DFG::LowerDFGToLLVM::compileNewArray):
(JSC::FTL::DFG::LowerDFGToLLVM::compileNewArrayBuffer):
* tests/stress/ftl-has-a-bad-time.js: Added.
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@193470 268f45cc-cd09-0410-ab3c-d52691b4dbfc
6 files changed
