[all-commits] [llvm/llvm-project] feb7be: [mlir][LLVM] Model side effects of volatile and at...

[Markus Böck via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: feb7beaf70bace1c3ffafc7f732c8fadca5e8c9d
      https://github.com/llvm/llvm-project/commit/feb7beaf70bace1c3ffafc7f732c8fadca5e8c9d
  Author: Markus Böck <markus.boeck02 at gmail.com>
  Date:   2023-09-08 (Fri, 08 Sep 2023)

  Changed paths:
    M mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td
    M mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp
    M mlir/test/Dialect/LLVMIR/canonicalize.mlir

  Log Message:
  -----------
  [mlir][LLVM] Model side effects of volatile and atomic load-store (#65730)

According to the LLVM language reference, both volatile memory
operations and atomic operations (except unordered) do not simply read
memory but also perform write operations on arbitrary memory[0][1].

In the case of volatile memory operations, this is the case due to the
read possibly having target specific properties. A common real-world
situation where this happens is reading memory mapped registers on an
MCU for example. Atomic operations are more special. They form a kind of
memory barrier which from the perspective of the optimizer/lang-ref
makes writes from other threads visible in the current thread. Any kind
of synchronization can therefore conservatively be modeled as a
write-effect.

This PR therefore adjusts the side effects of `llvm.load` and
`llvm.store` to add unknown global read and write effects if they are
either atomic or volatile.

Regarding testing: I am not sure how to best test this change for
`llvm.store` and the "globalness" of the effect that isn't just a unit
test checking that the output matches exactly. For the time being, I
added a test making sure that `llvm.load` does not get DCEd in
aforementioned cases.

Related logic in LLVM proper:

https://github.com/llvm/llvm-project/blob/3398744a6106c83993611bd3c5e79ec6b94417dc/llvm/lib/IR/Instruction.cpp#L638-L676


https://github.com/llvm/llvm-project/blob/3398744a6106c83993611bd3c5e79ec6b94417dc/llvm/include/llvm/IR/Instructions.h#L258-L262

[0] https://llvm.org/docs/LangRef.html#volatile-memory-accesses
[1] https://llvm.org/docs/Atomics.html#monotonic