Comment by HarHarVeryFunny

Yep. C was always meant to be a "close to the metal" language providing a feature set that could be mapped pretty directly to the processors it was running on. It's a "low level, high level language" where the expectation is more what you see is what you get (WYSIWYG), even though a modern optimizer might be expected to remove invariant code out of loops, etc - localized efficiency gains, but not large scale transformation.

So, optimal C targetting the 6502 is not going to look much like C targetting a modern processor. The developer still needs to be very aware of the limitations of the processor they are targetting.

One somewhat radical thing that LLVM-MOS does is to analyze the program's call graph, and for functions that are not used recursively it will assign parameters and local variables to zero page instead, both for speed of access and to avoid need for a stack frame. Even though this violates the WYSIWYG mental model, this is a nice abstraction of what the assembly language programmer would have done themself.