[PATCH] D97984: ARM: Set MaxAtomicSizeInBitsSupported appropriately.

[James Y Knight via Phabricator via llvm-commits]

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This will result in larger atomic operations getting expanded to
__atomic_* libcalls via AtomicExpandPass. This is a part of a change
to similarly clean up atomics handling on all targets.

When ldrex is available, we can support lock-free atomics up to 32
bits. When ldrexd is available, 64 bits.

When neither are available, we can still support up to 32-bit
lock-free atomics on certain platforms, if they provide
kernel-assisted cmpxchg. In that case, we can emit native 32-bit loads
and stores, and emit rmw/cmpxchg via __sync_* libcalls. 64-bit atomics
are not supported similarly, because 64-bit loads/stores aren't
atomic.

Finally, refactor/simplify the remainder of the code, which no longer
needs to try to deal with oversized atomics.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D97984

Files:
  llvm/lib/Target/ARM/ARMISelLowering.cpp
  llvm/lib/Target/ARM/ARMISelLowering.h
  llvm/lib/Target/ARM/ARMSubtarget.cpp
  llvm/lib/Target/ARM/ARMSubtarget.h
  llvm/test/CodeGen/ARM/atomic-64bit.ll
  llvm/test/CodeGen/ARM/atomic-load-store.ll
  llvm/test/CodeGen/ARM/atomic-op.ll
  llvm/test/CodeGen/ARM/atomic-ops-m33.ll

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