alt Hacker NewsThe nice thing about godbolt is that it can show you that clang not only can but do it in theory but also does it in practice:
https://aoco.compiler-explorer.com/#g:!((g:!((g:!((h:codeEdi...
The ability of turning stack allocated variables into locals(which can be then put in registers) is one of the most important passes of modern compilers.
Since compilers use SSA, where locals are immutable while lots of languages, like C have mutable variables, some compiler frontends put locals onto the stack, and let the compiler figure out what can be put into locals and how.
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.