That's really good; clearly I haven't looked at more recent versions. The magic seems to happen in your link at SROAPass, "Scalar Replacement Of Aggregates". Very cool!

According to https://docs.hdoc.io/hdoc/llvm-project/r2E8025E445BE9CEE.htm...

> This pass takes allocations which can be completely analyzed (that is, they don't escape) and tries to turn them into scalar SSA values.

That's actually a useful hint to me. When I was trying to replace locals and macros with a struct and functions, I also used the struct directly in another struct (which was the wider source of persistence across functions), so perhaps this pass thought the struct _did_ escape. I should revisit my code and see if I can tweak it to get this optimisation applied.