Allow -dead_strip linking on platforms with .subsections_via_symbols

Summary:
This allows to link objects produced with the llvm code generator to be linked with -dead_strip. This applies to at least the iOS cross compiler and OS X compiler.

Signed-off-by: Moritz Angermann <moritz@lichtzwerge.de>

Test Plan: Create a ffi library and link it with -dead_strip. If the resulting binary does not crash, the patch works as advertised.

Reviewers: rwbarton, simonmar, hvr, dterei, mzero, ezyang, austin

Reviewed By: dterei, ezyang, austin

Subscribers: thomie, mzero, simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D206

5 files changed, 26 insertions, 19 deletions

diff --git a/compiler/NOTES b/compiler/NOTES
new file mode 100644
index 0000000000..14a1f80b41
--- /dev/null
+++ b/compiler/NOTES
@@ -0,0 +1,16 @@
+Note [Subsections Via Symbols]
+
+If we are using the .subsections_via_symbols directive
+(available on recent versions of Darwin),
+we have to make sure that there is some kind of reference
+from the entry code to a label on the _top_ of of the info table,
+so that the linker will not think it is unreferenced and dead-strip
+it. That's why the label is called a DeadStripPreventer (_dsp).
+
+The LLVM code gen already creates `iTableSuf` symbols, where
+the X86 would generate the DeadStripPreventer (_dsp) symbol.
+Therefore all that is left for llvm code gen, is to ensure
+that all the `iTableSuf` symbols are marked as used.
+As of this writing the documentation regarding the
+.subsections_via_symbols and -dead_stip can be found at
+<https://developer.apple.com/library/mac/documentation/DeveloperTools/Reference/Assembler/040-Assembler_Directives/asm_directives.html#//apple_ref/doc/uid/TP30000823-TPXREF101>
\ No newline at end of file
diff --git a/compiler/llvmGen/LlvmCodeGen/Ppr.hs b/compiler/llvmGen/LlvmCodeGen/Ppr.hs
index 9c6a719613..80e8949ac8 100644
--- a/compiler/llvmGen/LlvmCodeGen/Ppr.hs
+++ b/compiler/llvmGen/LlvmCodeGen/Ppr.hs
@@ -114,12 +114,18 @@ pprInfoTable count info_lbl stat
   = do (ldata, ltypes) <- genLlvmData (Text, stat)
 
        dflags <- getDynFlags
+       platform <- getLlvmPlatform
        let setSection (LMGlobal (LMGlobalVar _ ty l _ _ c) d) = do
              lbl <- strCLabel_llvm info_lbl
              let sec = mkLayoutSection count
                  ilabel = lbl `appendFS` fsLit iTableSuf
                  gv = LMGlobalVar ilabel ty l sec (llvmInfAlign dflags) c
-                 v = if l == Internal then [gv] else []
+                 -- See Note [Subsections Via Symbols]
+                 v = if (platformHasSubsectionsViaSymbols platform
+                         && l == ExternallyVisible)
+                        || l == Internal
+                     then [gv]
+                     else []
              funInsert ilabel ty
              return (LMGlobal gv d, v)
            setSection v = return (v,[])
diff --git a/compiler/nativeGen/PPC/Ppr.hs b/compiler/nativeGen/PPC/Ppr.hs
index e62a1c4557..6851769132 100644
--- a/compiler/nativeGen/PPC/Ppr.hs
+++ b/compiler/nativeGen/PPC/Ppr.hs
@@ -73,12 +73,7 @@ pprNatCmmDecl proc@(CmmProc top_info lbl _ (ListGraph blocks)) =
          -- elimination, it might be the target of a goto.
             (if platformHasSubsectionsViaSymbols platform
              then
-             -- If we are using the .subsections_via_symbols directive
-             -- (available on recent versions of Darwin),
-             -- we have to make sure that there is some kind of reference
-             -- from the entry code to a label on the _top_ of of the info table,
-             -- so that the linker will not think it is unreferenced and dead-strip
-             -- it. That's why the label is called a DeadStripPreventer (_dsp).
+             -- See Note [Subsections Via Symbols]
                       text "\t.long "
                   <+> ppr info_lbl
                   <+> char '-'
diff --git a/compiler/nativeGen/SPARC/Ppr.hs b/compiler/nativeGen/SPARC/Ppr.hs
index c734687420..e9941b81ff 100644
--- a/compiler/nativeGen/SPARC/Ppr.hs
+++ b/compiler/nativeGen/SPARC/Ppr.hs
@@ -77,12 +77,7 @@ pprNatCmmDecl proc@(CmmProc top_info lbl _ (ListGraph blocks)) =
          -- elimination, it might be the target of a goto.
             (if platformHasSubsectionsViaSymbols platform
              then
-             -- If we are using the .subsections_via_symbols directive
-             -- (available on recent versions of Darwin),
-             -- we have to make sure that there is some kind of reference
-             -- from the entry code to a label on the _top_ of of the info table,
-             -- so that the linker will not think it is unreferenced and dead-strip
-             -- it. That's why the label is called a DeadStripPreventer (_dsp).
+             -- See Note [Subsections Via Symbols]
                       text "\t.long "
                   <+> ppr info_lbl
                   <+> char '-'
diff --git a/compiler/nativeGen/X86/Ppr.hs b/compiler/nativeGen/X86/Ppr.hs
index 2b3711751c..ddd75c83f6 100644
--- a/compiler/nativeGen/X86/Ppr.hs
+++ b/compiler/nativeGen/X86/Ppr.hs
@@ -78,12 +78,7 @@ pprNatCmmDecl proc@(CmmProc top_info lbl _ (ListGraph blocks)) =
          -- elimination, it might be the target of a goto.
             (if platformHasSubsectionsViaSymbols platform
              then
-             -- If we are using the .subsections_via_symbols directive
-             -- (available on recent versions of Darwin),
-             -- we have to make sure that there is some kind of reference
-             -- from the entry code to a label on the _top_ of of the info table,
-             -- so that the linker will not think it is unreferenced and dead-strip
-             -- it. That's why the label is called a DeadStripPreventer (_dsp).
+             -- See Note [Subsections Via Symbols]
                       text "\t.long "
                   <+> ppr info_lbl
                   <+> char '-'