Make a compact version of VariableEnvironment that UnlinkedFunctionExecutable stores and hash-cons these compact environments as we make them
https://bugs.webkit.org/show_bug.cgi?id=185329
<rdar://problem/39961536>
Reviewed by Michael Saboff.
I was made aware of a memory goof inside of JSC where we would inefficiently
use space to represent an UnlinkedFunctionExecutable's parent TDZ variables.
We did two things badly:
- We used a HashMap instead of a Vector to represent the environment. Having
a HashMap is useful when looking things up when generating bytecode, but it's
space inefficient. Because UnlinkedFunctionExecutables live a long time because
of the code cache, we should have them store this information efficiently
inside of a Vector.
- We didn't hash-cons these environments together. If you think about how
some programs are structured, hash-consing these together is hugely profitable.
Consider some code like this:
const/let V_1 = ...;
const/let V_2 = ...;
...
const/let V_n = ...;
function f_1() { ... };
function f_2() { ... };
...
function f_n() { ... };
Each f_i would store an identical hash map for its parent TDZ variables
consisting of {V_1, ..., V_n}. This was incredibly dumb. With hash-consing,
each f_i just holds onto a reference to the environment.
I benchmarked this change against an app that made heavy use of the
above code pattern and it reduced its peak memory footprint from ~220MB
to ~160MB.
- bytecode/UnlinkedFunctionExecutable.cpp:
(JSC::generateUnlinkedFunctionCodeBlock):
(JSC::UnlinkedFunctionExecutable::UnlinkedFunctionExecutable):
- bytecode/UnlinkedFunctionExecutable.h:
- parser/VariableEnvironment.cpp:
(JSC::CompactVariableEnvironment::CompactVariableEnvironment):
(JSC::CompactVariableEnvironment::operator== const):
(JSC::CompactVariableEnvironment::toVariableEnvironment const):
(JSC::CompactVariableMap::get):
(JSC::CompactVariableMap::Handle::~Handle):
- parser/VariableEnvironment.h:
(JSC::VariableEnvironmentEntry::bits const):
(JSC::VariableEnvironmentEntry::operator== const):
(JSC::VariableEnvironment::isEverythingCaptured const):
(JSC::CompactVariableEnvironment::hash const):
(JSC::CompactVariableMapKey::CompactVariableMapKey):
(JSC::CompactVariableMapKey::hash):
(JSC::CompactVariableMapKey::equal):
(JSC::CompactVariableMapKey::makeDeletedValue):
(JSC::CompactVariableMapKey::isHashTableDeletedValue const):
(JSC::CompactVariableMapKey::isHashTableEmptyValue const):
(JSC::CompactVariableMapKey::environment):
(WTF::HashTraits<JSC::CompactVariableMapKey>::emptyValue):
(WTF::HashTraits<JSC::CompactVariableMapKey>::isEmptyValue):
(WTF::HashTraits<JSC::CompactVariableMapKey>::constructDeletedValue):
(WTF::HashTraits<JSC::CompactVariableMapKey>::isDeletedValue):
(JSC::CompactVariableMap::Handle::Handle):
(JSC::CompactVariableMap::Handle::environment const):
(JSC::VariableEnvironment::VariableEnvironment): Deleted.
(JSC::VM::VM):
