GPU: check that the toolchain can compile MSL 4.1 before enabling Metal

The Metal frameworks are present on every macOS, so finding them said nothing
about whether the backend can be built. It needs MSL 4.1, which arrives with
macOS 27 and its toolchain, and a library compiled as MSL 4.1 only loads on
macOS 27 and later. The deployment target has no say in this: the -std= flag is
what picks the target OS, and MACOSX_DEPLOYMENT_TARGET and -mmacosx-version-min
are both ignored by the Metal compiler.

Compiling a three-line kernel answers the question directly. AUTO now turns
Metal off on an older toolchain instead of failing somewhere in the middle of
the build, and an explicit ENABLE_METAL=ON says why it cannot be honoured.

Author: ktf

dependencies/FindO2GPU.cmake

@@ -10,7 +10,7 @@
 # or submit itself to any jurisdiction.
 
 # NOTE!!!! - Whenever this file is changed, move it over to alidist/resources
-# FindO2GPU.cmake Version 20
+# FindO2GPU.cmake Version 21
 
 set(CUDA_COMPUTETARGET_DEFAULT_FULL 80-real;86-real;89-real;120-real;75-virtual)
 set(HIP_AMDGPUTARGET_DEFAULT_FULL gfx906;gfx908)
@@ -450,12 +450,29 @@ if(ENABLE_METAL)
   find_library(COREFOUNDATION_FRAMEWORK CoreFoundation)
   find_library(FOUNDATION_FRAMEWORK Foundation)
   find_library(QUARTZCORE_FRAMEWORK QuartzCore)
-  if(METAL_FRAMEWORK AND COREFOUNDATION_FRAMEWORK AND FOUNDATION_FRAMEWORK AND QUARTZCORE_FRAMEWORK)
+  # The frameworks are there on every macOS, but the backend needs MSL 4.1, which
+  # is macOS 27 and its toolchain, so ask the compiler instead of assuming. A
+  # library built as MSL 4.1 also only loads on macOS 27 and later.
+  if(NOT DEFINED GPUCA_METAL_MSL41)
+    set(GPUCA_METAL_PROBE "${CMAKE_CURRENT_BINARY_DIR}/metal_msl41_probe.metal")
+    file(WRITE "${GPUCA_METAL_PROBE}" "#include <metal_stdlib>\nkernel void probe(device float* o [[buffer(0)]], uint i [[thread_position_in_grid]]) { o[i] = o[i] * 2.0f; }\n")
+    execute_process(COMMAND xcrun -sdk macosx metal -std=metal4.1 -c "${GPUCA_METAL_PROBE}" -o "${GPUCA_METAL_PROBE}.air"
+                    RESULT_VARIABLE GPUCA_METAL_PROBE_RESULT OUTPUT_QUIET ERROR_QUIET)
+    if(GPUCA_METAL_PROBE_RESULT EQUAL 0)
+      set(GPUCA_METAL_MSL41 ON CACHE INTERNAL "Metal toolchain compiles MSL 4.1")
+    else()
+      set(GPUCA_METAL_MSL41 OFF CACHE INTERNAL "Metal toolchain compiles MSL 4.1")
+    endif()
+  endif()
+  if(METAL_FRAMEWORK AND COREFOUNDATION_FRAMEWORK AND FOUNDATION_FRAMEWORK AND QUARTZCORE_FRAMEWORK AND GPUCA_METAL_MSL41)
     set(METAL_ENABLED ON)
     set(METAL_FRAMEWORKS ${METAL_FRAMEWORK} ${COREFOUNDATION_FRAMEWORK}
                          ${FOUNDATION_FRAMEWORK} ${QUARTZCORE_FRAMEWORK})
     message(STATUS "Found Metal frameworks")
   elseif(NOT ENABLE_METAL STREQUAL "AUTO")
+    if(NOT GPUCA_METAL_MSL41)
+      message(FATAL_ERROR "The Metal backend needs MSL 4.1: this toolchain rejects -std=metal4.1, and the result would need macOS 27 to run")
+    endif()
     message(FATAL_ERROR "Metal frameworks not available")
   else()
     set(METAL_ENABLED OFF)