It had massive technical debt. Both Ignition and FCG had to compile from source, and we had to be able to tier up from both of them up to TurboFan, so TurboFan

It had massive technical debt. Both Ignition and FCG had to compile from source, and we had to be able to tier up from both of them up to TurboFan, so TurboFan maintained two front ends - one to compile from bytecode, and one to compile from AST. There was all sorts of complexity then around deoptimisation point IDs, maintaining consistent suspension points for generators/async functions, maintaining consistent type feedback across Ignition and FCG. Even better, for a while we had Crankshaft in there too as an optional "alternative" top-tier (while we were working on TurboFan), in a configuration we called the "Frankenpipeline". You can see that wonderful mess on slide 26 of this presentation: https://docs.google.com/presentation/d/1OqjVqRhtwlKeKfvMdX6HaCIu9wpZsrzqpIVIwQSuiXQ/edit?usp=drivesdk https://docs.google.com/presentation/d/1OqjVqRhtwlKeKfvMdX6H... (from this talk: https://youtu.be/r5OWCtuKiAk https://youtu.be/r5OWCtuKiAk) Three tiers is perfectly fine if the complexity is handled well, I could even see us adopting a fourth tier at some point in the future (not dissimilar indeed to JSC), most likely between Sparkplug and TurboFan.