[clang] [AVR] make the AVR ABI Swift compatible (PR #72298)

[Carl Peto via cfe-commits]

carlos4242 wrote:

Thanks, that sounds like it's worth looking into and might avoid issues with AVR.

I'm still nowhere near enough of an LLVM expert to follow all the aspects of the discussion. Although from our perspective, I've never seen an issue that I know, using the above patch for the last few years in our LLVM back end for the swift compile (not in our build of clang). The code produced by the back end has always followed the AVR GCC ABI calling convention (https://gcc.gnu.org/wiki/avr-gcc ...roughly... registers assigned to parameters starting at r24 and working backward r22, r20, etc. to r8 then the rest on stack) when I have examined the generated assembly. But maybe that is because, until now, only our swift code has been emitted with the swift calling convention, until #71986 is merged, clang can't emit code with that calling convention and that's the issue?

Stupid question... Is it possible that our AVR LLVM back end is basically ignoring the swift calling convention when it actually emits our code? Doesn't usually the front end (clang/swift) just specify parameter types and the back end decides whether to put something in registers or on the stack? Sorry if I'm being dumb!

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It might be that I just don't understand enough about LLVM and clang to really understand what all the surrounding code is doing properly, and maybe the right answer is for me to close this PR and wait for someone more experienced to come along when the time is right. We can always maintain this as a local patch in our builds instead, as we have for the past few years? I don't want to lay down issues for anyone using this in the future.

Or as an alternative, we can merge this PR as-is and deal with AVR problems later as an optimisation. I am neutral about what is the best approach and happy to take the advice of all the experts on here. (At the very least, the discussion has been enlightening for me!)

https://github.com/llvm/llvm-project/pull/72298