DriverPipeline: Fix 'unused arguments' warnings from Clang

When using Clang as the C compiler, over 100 tests were failing
due to Clang reporting that some command line arguments were not
being used. These warnings only occur when Clang is compiling
assembler files which happens in two places, one of which already
conditionally adding `-Qunused-arguments` to the command line when
the compiler was Clang. This fixes the other.

Test Plan: validate with clang as the C compiler

Reviewers: bgamari, hvr, austin, rwbarton

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1998

GHC Trac Issues: #11684

1 files changed, 24 insertions, 8 deletions

diff --git a/compiler/main/DriverPipeline.hs b/compiler/main/DriverPipeline.hs
index c384248ba1..41a7c5bb42 100644
--- a/compiler/main/DriverPipeline.hs
+++ b/compiler/main/DriverPipeline.hs
@@ -1574,16 +1574,32 @@ mkExtraObj dflags extn xs
  = do cFile <- newTempName dflags extn
       oFile <- newTempName dflags "o"
       writeFile cFile xs
-      let rtsDetails = getPackageDetails dflags rtsUnitId
-          pic_c_flags = picCCOpts dflags
+      ccInfo <- liftIO $ getCompilerInfo dflags
       SysTools.runCc dflags
-                     ([Option        "-c",
-                       FileOption "" cFile,
-                       Option        "-o",
-                       FileOption "" oFile]
-                      ++ map (FileOption "-I") (includeDirs rtsDetails)
-                      ++ map Option pic_c_flags)
+                ([Option        "-c",
+                  FileOption "" cFile,
+                  Option        "-o",
+                  FileOption "" oFile]
+                 ++ if extn /= "s"
+                        then cOpts
+                        else asmOpts ccInfo)
       return oFile
+    where
+      -- Pass a different set of options to the C compiler depending one whether
+      -- we're compiling C or assembler. When compiling C, we pass the usual
+      -- set of include directories and PIC flags.
+      cOpts = map Option (picCCOpts dflags)
+                    ++ map (FileOption "-I")
+                            (includeDirs $ getPackageDetails dflags rtsUnitId)
+
+      -- When compiling assembler code, we drop the usual C options, and if the
+      -- compiler is Clang, we add an extra argument to tell Clang to ignore
+      -- unused command line options. See trac #11684.
+      asmOpts ccInfo =
+            if any (ccInfo ==) [Clang, AppleClang, AppleClang51]
+                then [Option "-Qunused-arguments"]
+                else []
+
 
 -- When linking a binary, we need to create a C main() function that
 -- starts everything off.  This used to be compiled statically as part