[PATCH] D138582: [AVR] Do not use R0/R1 on avrtiny

[Ayke via Phabricator via llvm-commits]

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This patch makes sure the compiler uses R16/R17 <https://reviews.llvm.org/source/LLVM.org/> on avrtiny (attiny10 etc) instead of R0/R1 <https://reviews.llvm.org/diffusion/L/>.

Some notes:

- This patch also modifies the interrupt prologue/epilogue code to not save/clear/restore the zero register, because that's not needed on avrtiny. The zero register is at all times zero (unlike on non-avrtiny where it is clobbered by the multiplication instruction).
- For the NEGW and ROLB instructions, it relies on the fact that R1 <https://reviews.llvm.org/diffusion/L/> is always zero on avrtiny. This is slightly ugly, but it should be safe.
- Not entirely sure about putting all tests in features/avr-tiny.ll, but it doesn't seem like the "target-cpu"="attiny10" attribute works.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D138582

Files:
  llvm/lib/Target/AVR/AVRExpandPseudoInsts.cpp
  llvm/lib/Target/AVR/AVRFrameLowering.cpp
  llvm/lib/Target/AVR/AVRISelLowering.cpp
  llvm/lib/Target/AVR/AVRISelLowering.h
  llvm/lib/Target/AVR/AVRInstrInfo.td
  llvm/lib/Target/AVR/AVRRegisterInfo.cpp
  llvm/lib/Target/AVR/AVRSubtarget.h
  llvm/test/CodeGen/AVR/features/avr-tiny.ll

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