[mlir][test] Make SME e2e tests require an emulator (#86489)

Integration tests for ArmSME require an emulator (there's no hardware
available). Make sure that CMake complains if `MLIR_RUN_ARM_SME_TESTS`
is set while `ARM_EMULATOR_EXECUTABLE` is empty.

I'm also adding a note in the docs for future reference.

Author: banach-space

mlir/cmake/modules/MLIRCheckHardwareFeatures.cmake

@@ -0,0 +1,101 @@
+# A collection of helper CMake functions to detect hardware capabilities. At
+# the moment these are used when configuring MLIR integration tests.
+
+# Checks whether the specified hardware capability is supported by the host
+# Linux system. This is implemented by checking auxiliary vector feature
+# provided by the Linux kernel.
+#
+# check_hwcap(
+#   hwcap_spec
+#   output_var
+# )
+#
+# hwcap_spec - HWCAP value to check - these are defined in hwcap.h in the Linux
+#              kernel.
+#
+# output_var - Output variable to use to save the results (TRUE for supported,
+#              FALSE for not supported).
+#
+# EXAMPLES:
+#
+# check_hwcap("HWCAP2_SME" SME_EMULATOR_REQUIRED)
+#
+function(check_hwcap hwcap_spec output)
+    set(hwcap_test_src
+      [====[
+      #include <asm/hwcap.h>
+      #include <sys/auxv.h>
+
+      int main(void)
+      {
+          long hwcaps = getauxval(AT_<HWCAP_VEC>);
+          return (hwcaps & <HWCAP_SPEC>) != 0;
+      }
+      ]====]
+    )
+
+    # Extract from $hwcap_spec whether this is AT_HWCAP or AT_HWCAP2
+    string(FIND ${hwcap_spec} "_" wsloc)
+    string(SUBSTRING ${hwcap_spec} 0 ${wsloc} hwcap_vec)
+
+    string(REPLACE "<HWCAP_VEC>" ${hwcap_vec} hwcap_test_src "${hwcap_test_src}")
+    string(REPLACE "<HWCAP_SPEC>" ${hwcap_spec} hwcap_test_src "${hwcap_test_src}")
+
+    set(hwcap_test_file ${CMAKE_BINARY_DIR}/${CMAKE_FILES_DIRECTORY}/hwcap_check.c)
+    file(WRITE ${hwcap_test_file} "${hwcap_test_src}")
+
+    # Compile _and_ run
+    try_run(
+        test_run_result test_compile_result
+        ${CMAKE_BINARY_DIR}
+        ${hwcap_test_file}
+    )
+    # Compilation will fail if hwcap_spec is not defined - this usually means
+    # that your Linux kernel is too old.
+    if(${test_compile_result} AND (DEFINED test_run_result))
+      message(${test_run_result})
+      message(STATUS "Checking whether ${hwcap_spec} is supported by the host system: ${test_run_result}")
+      set(${output} ${test_run_result} PARENT_SCOPE)
+    else()
+      message(STATUS "Checking whether ${hwcap_spec} is supported by the host system: FALSE")
+    endif()
+endfunction(check_hwcap)
+
+# For the given group of e2e tests (defined by the `mlir_e2e_tests` flag),
+# checks whether an emulator is required. If yes, verifies that the
+# corresponding CMake var pointing to an emulator (`emulator_exec`) has been
+# set.
+#
+# check_emulator(
+#   mlir_e2e_tests
+#   hwcap_spec
+#   emulator_exec
+# )
+#
+# mlir_e2e_tests  - MLIR CMake variables corresponding to the group of e2e tests
+#                   to check
+# hwcap_spec      - HWCAP value to check. This should correspond to the hardware
+#                   capabilities required by the tests to be checked. Possible
+#                   values are defined in hwcap.h in the Linux kernel.
+# emulator_exec   - variable the defines the emulator (ought to be set if
+#                   required, can be empty otherwise).
+#
+# EXAMPLES:
+#
+#  check_emulator(MLIR_RUN_ARM_SVE_TESTS "HWCAP_SVE" ARM_EMULATOR_EXECUTABLE)
+#
+function(check_emulator mlir_e2e_tests hwcap_spec emulator_exec)
+  if (NOT ${mlir_e2e_tests})
+    return()
+  endif()
+
+  check_hwcap(${hwcap_spec} emulator_not_required)
+  if (${emulator_not_required})
+    return()
+  endif()
+
+  if (${emulator_exec} STREQUAL "")
+    message(FATAL_ERROR "${mlir_e2e_tests} requires an emulator, but ${emulator_exec} is not set")
+  endif()
+
+endfunction()

mlir/docs/Dialects/ArmSME.md

@@ -6,14 +6,22 @@ This dialect defines custom and LLVM IR intrinsic operations that are used to
 target Arm Scalable Matrix Extension. Through the available conversion and
 ArmSME passes you can, for example, lower a
 [linalg.matmul](https://mlir.llvm.org/docs/Dialects/Linalg/#linalgmatmul-linalgmatmulop)
-opereation to Arm SME
+operation to Arm SME
 [FMOPA](https://developer.arm.com/documentation/ddi0602/2023-03/SME-Instructions/FMOPA--widening---Half-precision-floating-point-sum-of-outer-products-and-accumulate-)
 (floating-point outer product) operations. See one of the in-tree end-to-end
 integration tests for reference:
 
 * [Linalg/CPU/ArmSME/matmul.mlir](https://github.com/llvm/llvm-project/blob/main/mlir/test/Integration/Dialect/Linalg/CPU/ArmSME/matmul.mlir)
 * [Vector/CPU/ArmSME/test-outerproduct-f64.mlir](https://github.com/llvm/llvm-project/blob/main/mlir/test/Integration/Dialect/Vector/CPU/ArmSME/test-outerproduct-f64.mlir)
 
+In order to run ArmSME integration tests, include these flags in the CMake
+invocation when configuring LLVM and MLIR:
+```bash
+  -DMLIR_INCLUDE_INTEGRATION_TESTS=On
+  -DMLIR_RUN_ARM_SME_TESTS=On
+  -DARM_EMULATOR_EXECUTABLE=<path-to-emulator> 
+```
+
 These tests are run "post-commit" by the
 [clang-aarch64-sve-vla](https://lab.llvm.org/buildbot/#/builders/197) LLVM
 BuildBot worker.

mlir/test/CMakeLists.txt

@@ -1,3 +1,5 @@
+include(MLIRCheckHardwareFeatures)
+
 add_subdirectory(CAPI)
 add_subdirectory(lib)
 
@@ -39,6 +41,10 @@ if (MLIR_INCLUDE_INTEGRATION_TESTS)
   option(MLIR_RUN_ARM_SVE_TESTS "Run Arm SVE tests.")
   option(MLIR_RUN_ARM_SME_TESTS "Run Arm SME tests.")
 
+  # Check whether an emulator is required - if yes then make sure that it's
+  # been set.
+  check_emulator(MLIR_RUN_ARM_SVE_TESTS "HWCAP_SVE" ARM_EMULATOR_EXECUTABLE)
+  check_emulator(MLIR_RUN_ARM_SME_TESTS "HWCAP2_SME" ARM_EMULATOR_EXECUTABLE)
 
   # The native target may not be enabled when cross compiling, raise an error.
   if(NOT MLIR_ENABLE_EXECUTION_ENGINE)