feb7beaf70
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:3398744a61/llvm/lib/IR/Instruction.cpp (L638-L676)3398744a61/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
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