As far as I know: Not really the initial "downgraded" code is normally worse in more or less any aspect. It's just emitted fastly.
The only reason I could come up with is to downgrade to them upgrade again to a differently optimized code. But then WebAssembly doesn't need to do speculative optimizations like JS needs. So I guess it's not really relevant.
In theory, you could generate better code after having observed the actual arguments to various functions and recompile while treating them as constants. Basically profile-guided (re)optimization. But the observation would slow down the initial code, it would be a large implementation effort, and the gains would be dubious imho. Especially given how much information was already lost via the initial AOT (ahead of time) optimizing compile; you might need to keep the original source around or something.
I considered this scenario as llvm does somewhat support profile-guided optimizations but it would be a fundamental change where we go from "unoptimized" to "optimized-with-tracing" instead of optimized, so that step could still have optimization upgrade hooks, so we still wouldn't need a downgrade to "base-line". I guess.
I agree that it's anyway unlikely to happen, at least in the browser.
I could imagine that some WebAssembly based cloud provider (or similar) would be potentially interested in generating profiles and then use that for better optimizations in other instances (so still no down-/upgrade ).
> you could generate better code after having observed the actual arguments to various functions and recompile while treating them as constants
This transformation is valid for pure functions; therefore you can mark it constexpr before compiling to WASM. There's the `constexpr-everything` project (among others) that uses libclang to apply constexpr to all legal functions/expressions in a codebase.
The only reason I could come up with is to downgrade to them upgrade again to a differently optimized code. But then WebAssembly doesn't need to do speculative optimizations like JS needs. So I guess it's not really relevant.