[PATCH] D152063: [AVR] Support left-rotations

[Patryk Wychowaniec via Phabricator via llvm-commits]

Patryk27 created this revision.
Herald added subscribers: Jim, JDevlieghere, hiraditya, dylanmckay.
Herald added a project: All.
Patryk27 requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

Currently, trying to compile this code:

  define i8 @test(i8 %0) {
  start:
    %1 = call i8 @llvm.fshl.i8(i8 %0, i8 %0, i8 1)
    ret i8 %1
  }
  
  declare i8 @llvm.fshl.i8(i8, i8, i8)



  ./build/bin/llc -march=avr -mcpu=atmega32u4 test.ll

... will fail, saying:

  LLVM ERROR: Cannot select: t9: i8 = ROL t2
    t2: i8,ch = CopyFromReg t0, Register:i8 %0
      t1: i8 = Register %0

(spotted in the wild at https://github.com/rust-lang/rust/issues/107261)

Curiously, `fshl.u16` (and larger types) work, similarly as all `fshr`s,
which I think is related to the fact that `ROLBRd` is hard-coded to
require `GPR8:$zero` in `AVRInstrInfo.td`.

Now, I'm not really sure why it's been done this way and so I'm not 100%
sure my approach to solving this problem is correct as well - but
adjusting `ROLBRd` to work similarly as `RORBRd` seems to do the job.

Following the example from above, we now generate:

  lsl r24
  adc r24, r1

I've made sure the codegen is alright by cross-checking results using
simavr - I've checked following cases:

- 8-bit left/right rotations where rhs is constant/variable,
- 16-bit left/right rotations where rhs is constant/variable,
- 32-bit left/right rotations where rhs is constant/variable,

... and they all yielded correct results when compared with Rust's
`.rotate_left()` and `.rotate_right()`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D152063

Files:
  llvm/lib/Target/AVR/AVRExpandPseudoInsts.cpp
  llvm/lib/Target/AVR/AVRISelLowering.cpp
  llvm/lib/Target/AVR/AVRInstrInfo.td
  llvm/test/CodeGen/AVR/pseudo/ROLBrd.mir
  llvm/test/CodeGen/AVR/rotate.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D152063.528100.patch
Type: text/x-patch
Size: 9555 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20230603/e09e8de5/attachment.bin>