Side effects sounds like the right mechanism to model this from generic pass perspective. We can indeed introduce a new resource for this purpose, which should effectively prevent undesired reorderings by generic passes. It also sounds better to have this property be represented as an attribute on the op, likely absent by default, because having side effects (which are part of the op’s semantics) depend on some pass flag sounds wrong layering-wise. Whether this attribute is a part of FastMathFlags or a separate unit attribute is debatable.
Transforming operations into opaque uninspectable function calls sounds like the exact opposite of what MLIR usually does, i.e., exposing more of the semantics to the compiler.
Side note: LLVM IR models these as intrinsics - LLVM Language Reference Manual — LLVM 16.0.0git documentation - but I’d rather not duplicate the ops in MLIR.