[llvm-dev] Advise needed on implementing atomic operations for M68k

[John Paul Adrian Glaubitz via llvm-dev]

Hello!

One missing part in the M68k backend is currently support for atomic operations
such as atomic load and store and compare and swap which results in a backend
error [1] when trying to build code which uses atomic operations.

I have already looked into the other backends such as X86, Sparc and PowerPC trying
to understand how the implementation works but I haven't fully understood everything
yet.

>From my current understanding, we would have to define atomic_load_8/16/32 and atomic_
store_8/16/32 in the TableGen code as well as atomic_cmp_and_swap_8/16/32. For load and
store, it should be sufficient to use normal move instructions [2] and for compare_
and_swap, there are actually two instructions, CAS [3] and CAS2 [4] available
on m68k, the latter could even be used to implement a 64-bit compare_and_swap
if I'm not mistaken [5].

For compare_and_swap, it seems the necessary pattern would resemble the one used
on Sparc V9, for example:

let Predicates = [HasV9], Constraints = "$swap = $rd", asi = 0b10000000 in
  def CASrr: F3_1_asi<3, 0b111100,
                (outs IntRegs:$rd), (ins IntRegs:$rs1, IntRegs:$rs2,
                                     IntRegs:$swap),
                 "cas [$rs1], $rs2, $rd",
                 [(set i32:$rd,
                     (atomic_cmp_swap_32 iPTR:$rs1, i32:$rs2, i32:$swap))]>;

but I don't fully understand every line in this snippet of TableGen code so I was
wondering whether someone could point me at some documentation that helps making
sense of that all.

And I assume we also need a lowering function in M68kISelLowering.cpp, don't we?

Thanks,
Adrian

> [1] https://github.com/M680x0/M680x0-mono-repo/issues/13
> [2] https://github.com/M680x0/M680x0-mono-repo/issues/13#issuecomment-703313295
> [3] http://68k.hax.com/CAS
> [4] http://68k.hax.com/CAS2
> [5] https://en.wikipedia.org/wiki/Double_compare-and-swap

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 .''`.  John Paul Adrian Glaubitz
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`. `'   Freie Universitaet Berlin - glaubitz at physik.fu-berlin.de
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