Fix #5636: Use clang as assembler on OSX when LLVM >= 3.0

LLVM doesn't support the OS X system assembler anymore so we must use
their assembler through clang. Also improved error messages when various
LLVM tools can't be run.

compiler/main/DriverPipeline.hs

@@ -1172,7 +1172,21 @@ runPhase SplitMangle input_fn dflags
 
 runPhase As input_fn dflags
   = do
-        let as_opts =  getOpts dflags opt_a
+        -- LLVM from version 3.0 onwards doesn't support the OS X system
+        -- assembler, so we use clang as the assembler instead. (#5636)
+        let whichAsProg | hscTarget dflags == HscLlvm &&
+                          platformOS (targetPlatform dflags) == OSDarwin
+                        = do
+                            llvmVer <- io $ figureLlvmVersion dflags
+                            return $ case llvmVer of
+                                Just n | n >= 30 -> SysTools.runClang
+                                _                -> SysTools.runAs
+
+                        | otherwise
+                        = return SysTools.runAs
+
+        as_prog <- whichAsProg
+        let as_opts = getOpts dflags opt_a
         let cmdline_include_paths = includePaths dflags
 
         next_phase <- maybeMergeStub
@@ -1182,7 +1196,7 @@ runPhase As input_fn dflags
         -- might be a hierarchical module.
         io $ createDirectoryHierarchy (takeDirectory output_fn)
 
-        io $ SysTools.runAs dflags
+        io $ as_prog dflags
                        (map SysTools.Option as_opts
                        ++ [ SysTools.Option ("-I" ++ p) | p <- cmdline_include_paths ]
 

compiler/main/DynFlags.hs

@@ -448,6 +448,7 @@ data DynFlags = DynFlags {
   ghcMode               :: GhcMode,
   ghcLink               :: GhcLink,
   hscTarget             :: HscTarget,
+  settings              :: Settings,
   hscOutName            :: String,      -- ^ Name of the output file
   extCoreName           :: String,      -- ^ Name of the .hcr output file
   verbosity             :: Int,         -- ^ Verbosity level: see Note [Verbosity levels]
@@ -518,8 +519,6 @@ data DynFlags = DynFlags {
   pluginModNames        :: [ModuleName],
   pluginModNameOpts     :: [(ModuleName,String)],
 
-  settings              :: Settings,
-
   --  For ghc -M
   depMakefile           :: FilePath,
   depIncludePkgDeps     :: Bool,

compiler/main/SysTools.lhs

@@ -21,6 +21,7 @@ module SysTools (
         runWindres,
         runLlvmOpt,
         runLlvmLlc,
+        runClang,
         figureLlvmVersion,
         readElfSection,
 
@@ -473,6 +474,22 @@ runLlvmLlc dflags args = do
   let (p,args0) = pgm_lc dflags
   runSomething dflags "LLVM Compiler" p (args0++args)
 
+-- | Run the clang compiler (used as an assembler for the LLVM
+-- backend on OS X as LLVM doesn't support the OS X system
+-- assembler)
+runClang :: DynFlags -> [Option] -> IO ()
+runClang dflags args = do
+  -- we simply assume its available on the PATH
+  let clang = "clang"
+  Exception.catch (do
+        runSomething dflags "Clang (Assembler)" clang args
+    )
+    (\(err :: SomeException) -> do
+        putMsg dflags $ text $ "Error running clang! you need clang installed"
+                            ++ " to use the LLVM backend"
+        throw err
+    )
+
 -- | Figure out which version of LLVM we are running this session
 figureLlvmVersion :: DynFlags -> IO (Maybe Int)
 figureLlvmVersion dflags = do
@@ -504,9 +521,13 @@ figureLlvmVersion dflags = do
              return $ Just v
             )
             (\err -> do
-                putMsg dflags $ text $ "Error (" ++ show err ++ ")"
-                putMsg dflags $ text "Warning: Couldn't figure out LLVM version!"
-                putMsg dflags $ text "Make sure you have installed LLVM"
+                debugTraceMsg dflags 2
+                    (text "Error (figuring out LLVM version):" <+>
+                     text (show err))
+                putMsg dflags $ vcat
+                    [ text "Warning:", nest 9 $
+                          text "Couldn't figure out LLVM version!" $$
+                          text "Make sure you have installed LLVM"]
                 return Nothing)
   return ver