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Previously, we were inserting za.enable/disable intrinsics for functions with the "arm_za" attribute (at the MLIR level), rather than using the backend attributes. This was done to avoid a dependency on the SME ABI functions from compiler-rt (which have only recently been implemented). Doing things this way did have correctness issues, for example, calling a streaming-mode function from another streaming-mode function (both with ZA enabled) would lead to ZA being disabled after returning to the caller (where it should still be enabled). Fixing issues like this would require re-doing the ABI work already done in the backend within MLIR. Instead, this patch switches to use the "arm_new_za" (backend) attribute for enabling ZA for an MLIR function. For the integration tests, this requires some way of linking the SME ABI functions. This is done via the `%arm_sme_abi_shlib` lit substitution. By default, this expands to a stub implementation of the SME ABI functions, but this can be overridden by providing the `ARM_SME_ABI_ROUTINES_SHLIB` CMake cache variable (pointing it at an alternative implementation). For now, the ArmSME integration tests pass with just stubs, as we don't make use of nested ZA-enabled calls. A future patch may add an option to compiler-rt to build the SME builtins into a standalone shared library to allow easily building/testing with the actual implementation.
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