If -msse is specified, pass it to the LLVM backend

compiler/main/DriverPipeline.hs

@@ -1436,7 +1436,8 @@ runPhase LlvmLlc input_fn dflags
                 ++ map SysTools.Option lc_opts
                 ++ [SysTools.Option tbaa]
                 ++ map SysTools.Option fpOpts
-                ++ map SysTools.Option abiOpts)
+                ++ map SysTools.Option abiOpts
+                ++ map SysTools.Option sseOpts)
 
     return (next_phase, output_fn)
   where
@@ -1462,6 +1463,10 @@ runPhase LlvmLlc input_fn dflags
                     ArchARM ARMv7 _ _    -> []
                     _                    -> []
 
+        sseOpts | isSse4_2Enabled dflags = ["-mattr=+sse42"]
+                | isSse2Enabled dflags   = ["-mattr=+sse2"]
+                | otherwise              = []
+
 -----------------------------------------------------------------------------
 -- LlvmMangle phase
 

compiler/main/DynFlags.hs

@@ -3410,6 +3410,10 @@ makeDynFlagsConsistent dflags
 -- -----------------------------------------------------------------------------
 -- SSE
 
+-- TODO: Instead of using a separate predicate (i.e. isSse2Enabled) to
+-- check if SSE is enabled, we might have x86-64 imply the -msse2
+-- flag.
+
 isSse2Enabled :: DynFlags -> Bool
 isSse2Enabled dflags = case platformArch (targetPlatform dflags) of
     ArchX86_64 -> -- SSE2 is fixed on for x86_64.  It would be