[JSC] We should consider moving UnlinkedFunctionExecutable::m_parentScopeTDZVariables to RareData
https://bugs.webkit.org/show_bug.cgi?id=194944
Reviewed by Keith Miller.
JSTests:
- stress/verify-bytecode-generator-cached-variables-under-tdz.js: Added.
Source/JavaScriptCore:
Based on profile data collected on JetStream2, Speedometer 2 and
other benchmarks, it is very rare having non-empty
UnlinkedFunctionExecutable::m_parentScopeTDZVariables.
- Data collected from Speedometer2
Total number of UnlinkedFunctionExecutable: 39463
Total number of non-empty parentScopeTDZVars: 428 (~1%)
- Data collected from JetStream2
Total number of UnlinkedFunctionExecutable: 83715
Total number of non-empty parentScopeTDZVars: 5285 (~6%)
We also collected numbers on 6 of top 10 Alexia sites.
- Data collected from youtube.com
Total number of UnlinkedFunctionExecutable: 29599
Total number of non-empty parentScopeTDZVars: 97 (~0.3%)
- Data collected from twitter.com
Total number of UnlinkedFunctionExecutable: 23774
Total number of non-empty parentScopeTDZVars: 172 (~0.7%)
- Data collected from google.com
Total number of UnlinkedFunctionExecutable: 33209
Total number of non-empty parentScopeTDZVars: 174 (~0.5%)
- Data collected from amazon.com:
Total number of UnlinkedFunctionExecutable: 15182
Total number of non-empty parentScopeTDZVars: 166 (~1%)
- Data collected from facebook.com:
Total number of UnlinkedFunctionExecutable: 54443
Total number of non-empty parentScopeTDZVars: 269 (~0.4%)
- Data collected from netflix.com:
Total number of UnlinkedFunctionExecutable: 39266
Total number of non-empty parentScopeTDZVars: 97 (~0.2%)
Considering such numbers, this patch is moving m_parentScopeTDZVariables
to RareData. This decreases sizeof(UnlinkedFunctionExecutable) by
16 bytes. With this change, now UnlinkedFunctionExecutable constructors
receives an Optional<VariableEnvironmentMap::Handle> and only stores
it when value != WTF::nullopt. We also changed
UnlinkedFunctionExecutable::parentScopeTDZVariables() and it returns
VariableEnvironment() whenever the Executable doesn't have RareData,
or VariableEnvironmentMap::Handle is unitialized. This is required
because RareData is instantiated when any of its field is stored and
we can have an unitialized Handle even on cases when parentScopeTDZVariables
is WTF::nullopt.
Results on memory usage on JetStrem2 is neutral.
Mean of memory peak on ToT: 4258633728 bytes (confidence interval: 249720072.95)
Mean of memory peak on Changes: 4367325184 bytes (confidence interval: 321285583.61)
- builtins/BuiltinExecutables.cpp:
(JSC::BuiltinExecutables::createExecutable):
- bytecode/UnlinkedFunctionExecutable.cpp:
(JSC::UnlinkedFunctionExecutable::UnlinkedFunctionExecutable):
- bytecode/UnlinkedFunctionExecutable.h:
- bytecompiler/BytecodeGenerator.cpp:
(JSC::BytecodeGenerator::getVariablesUnderTDZ):
BytecodeGenerator::getVariablesUnderTDZ now also caches if m_cachedVariablesUnderTDZ
is empty, so we can properly return WTF::nullopt without the
reconstruction of a VariableEnvironment to check if it is empty.
- bytecompiler/BytecodeGenerator.h:
(JSC::BytecodeGenerator::makeFunction):
- parser/VariableEnvironment.h:
(JSC::VariableEnvironment::isEmpty const):
(JSC::CachedCompactVariableMapHandle::decode const):
It returns an unitialized Handle when there is no
CompactVariableEnvironment. This can happen when RareData is ensured
because of another field.
(JSC::CachedFunctionExecutableRareData::encode):
(JSC::CachedFunctionExecutableRareData::decode const):
(JSC::CachedFunctionExecutable::encode):
(JSC::CachedFunctionExecutable::decode const):
(JSC::UnlinkedFunctionExecutable::UnlinkedFunctionExecutable):
Instead of creating a dummyVariablesUnderTDZ, we simply pass
WTF::nullopt.
(JSC::CodeCache::getUnlinkedGlobalFunctionExecutable):
