[PATCH] D128588: [AVR] Fix expanding MOVW for overlapping registers

[Patryk Wychowaniec via Phabricator via llvm-commits]

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Sometimes the codegen emits a `COPY` with overlapping registers, such as
this one:

  $r25r24 = COPY $r24r23

Our current expansion of such `COPY` would be wrong, since it'd start
with the lower register first, with the second `mov` assuming the
registers are not modified in-between:

  mov r24, r23
  mov r25, r24

(i.e. that's effectively `mov r25, r23`, which is _ayy ayy bad_.)

This patch improves the expansion by making it detect whether the
registers are overlapping and if so, expanding from the high register
first:

  mov r25, r24
  mov r24, r23

Because our registers are always paired in descending order (e.g.
there's r25r24, but no r24r25), I think it'd be safe to go with the
high-to-low expansion always (not only if the registers overlap), but
that makes the output a bit more difficult to follow and would require
adjusting the existing tests -- so I went with the more safer & simpler
route.

In the wild, this was found here:
https://github.com/rust-lang/rust/issues/98167.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D128588

Files:
  llvm/lib/Target/AVR/AVRInstrInfo.cpp
  llvm/test/CodeGen/AVR/pseudo/COPY.mir
  llvm/test/CodeGen/AVR/rust-bug-98167.ll

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