Switch to XCode's Metal compiler?

[maleadt]

We're currently using a custom metallib-as to downgrade LLVM IR and generate a metallib containing LLVM 5 bitcode, however, it's possible to just re-use XCode's Clang-based metal compiler for this:

$ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/metal/macos/bin/metal kernel.(ll|bc)
warning: overriding the module target triple with air64-apple-macosx12.0.0 [-Woverride-module]
1 warning generated

The problem is that we still need to downgrade the LLVM IR (this is more obvious when using textual bitcode inputs, in which case metal will complain about unsupported deferencerable arguments, spFlags MDNode contents, etc), but a tool for that could be more valuable, e.g., to support NVIDIA's NVVM compiler.

Another disadvantage is that we'd then require users to have Xcode installed, and the full thing at that (the metal compiler isn't part of the command line xcode tools). That's a multi-gigabyte download and installation, since presumably Apple doesn't allow us to redistribute those parts of the SDK.

[maleadt]

diff --git a/src/metal.jl b/src/metal.jl
index b314899..922e9be 100644
--- a/src/metal.jl
+++ b/src/metal.jl
@@ -708,18 +708,34 @@ end
                      If you think this is a bug, please file an issue and attach $(input).""")
         end
     end
+    cp(translated, "/tmp/ours.metallib"; force=true)
+
+    input_alternative = tempname(cleanup=false) * ".ll"
+    translated_alternative = tempname(cleanup=false) * ".reference.metallib"
+    strip_debuginfo!(mod)   # quick hack to support some more IR without downgrading
+    write(input_alternative, string(mod))
+    let
+        proc = run(ignorestatus(`/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/metal/macos/bin/metal -o $translated_alternative $input_alternative`))
+        if !success(proc)
+            error("""Failed to translate LLVM code to MetalLib.
+                     If you think this is a bug, please file an issue and attach $(input_alternative).""")
+        end
+    end
+    cp(translated_alternative, "/tmp/theirs.metallib"; force=true)

     output = if format == LLVM.API.LLVMObjectFile
-        read(translated)
+        read(translated_alternative)
     else
         # disassemble
         Metal_LLVM_Tools_jll.metallib_dis() do disassembler
-            read(`$disassembler -o - $translated`, String)
+            read(`$disassembler -o - $translated_alternative`, String)
         end
     end

     rm(input)
     rm(translated)
+    rm(input_alternative)
+    rm(translated_alternative)

     return output
 end