LookupCompileTimeConstant() is a problem for lazy bytecode. It potentially traverses along bce->parent from the innermost BytecodeEmitter all the way to the top of the BytecodeEmitter chain. But if we're lazily compiling an inner function we won't have the parent BytecodeEmitter around any more. It's there just to optimize the handling of |const| variables in switches, e.g.: const x = "foo" switch (y) { case x: ... ... } It's an old optimization, pre-dating the CVS-to-Mercurial transition in 2007. And I don't think |const| is used that much on the web, is it? bhackett said "it's antique code and if there is any need for what it's doing we can do a more effective job using TI info during mjit compilation".

I also found, in the case of huge generated JS files, that it takes like 10% of total frontend time.