Implement function-sections for Haskell code, #8405

This adds a flag -split-sections that does similar things to
-split-objs, but using sections in single object files instead of
relying on the Satanic Splitter and other abominations. This is very
similar to the GCC flags -ffunction-sections and -fdata-sections.

The --gc-sections linker flag, which allows unused sections to actually
be removed, is added to all link commands (if the linker supports it) so
that space savings from having base compiled with sections can be
realized.

Supported both in LLVM and the native code-gen, in theory for all
architectures, but really tested on x86 only.

In the GHC build, a new SplitSections variable enables -split-sections
for relevant parts of the build.

Test Plan: validate with both settings of SplitSections

Reviewers: dterei, Phyx, austin, simonmar, thomie, bgamari

Reviewed By: simonmar, thomie, bgamari

Subscribers: hsyl20, erikd, kgardas, thomie

Differential Revision: https://phabricator.haskell.org/D1242

GHC Trac Issues: #8405

4 files changed, 20 insertions, 2 deletions

diff --git a/compiler/main/DriverPipeline.hs b/compiler/main/DriverPipeline.hs
index 33770b92f6..a1d36a6b54 100644
--- a/compiler/main/DriverPipeline.hs
+++ b/compiler/main/DriverPipeline.hs
@@ -1908,6 +1908,10 @@ linkBinary' staticLink dflags o_files dep_packages = do
                           then ["-Wl,-read_only_relocs,suppress"]
                           else [])
 
+                      ++ (if sLdIsGnuLd mySettings
+                          then ["-Wl,--gc-sections"]
+                          else [])
+
                       ++ o_files
                       ++ lib_path_opts)
                       ++ extra_ld_inputs
diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index b8705603c7..39f4a0487f 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -391,6 +391,7 @@ data GeneralFlag
    | Opt_EagerBlackHoling
    | Opt_NoHsMain
    | Opt_SplitObjs
+   | Opt_SplitSections
    | Opt_StgStats
    | Opt_HideAllPackages
    | Opt_PrintBindResult
@@ -1283,7 +1284,10 @@ wayUnsetGeneralFlags _ WayDyn      = [-- There's no point splitting objects
                                       -- when we're going to be dynamically
                                       -- linking. Plus it breaks compilation
                                       -- on OSX x86.
-                                      Opt_SplitObjs]
+                                      Opt_SplitObjs,
+                                      -- If splitobjs wasn't useful for this,
+                                      -- assume sections aren't either.
+                                      Opt_SplitSections]
 wayUnsetGeneralFlags _ WayProf     = []
 wayUnsetGeneralFlags _ WayEventLog = []
 
@@ -2326,6 +2330,15 @@ dynamic_flags = [
                 then setGeneralFlag Opt_SplitObjs
                 else addWarn "ignoring -fsplit-objs"))
 
+  , defGhcFlag "split-sections"
+      (noArgM (\dflags -> do
+        if platformHasSubsectionsViaSymbols (targetPlatform dflags)
+          then do addErr $
+                    "-split-sections is not useful on this platform " ++
+                    "since it always uses subsections via symbols."
+                  return dflags
+          else return (gopt_set dflags Opt_SplitSections)))
+
         -------- ghc -M -----------------------------------------------------
   , defGhcFlag "dep-suffix"               (hasArg addDepSuffix)
   , defGhcFlag "dep-makefile"             (hasArg setDepMakefile)
diff --git a/compiler/main/HscMain.hs b/compiler/main/HscMain.hs
index 53c6f626e5..a6d263781c 100644
--- a/compiler/main/HscMain.hs
+++ b/compiler/main/HscMain.hs
@@ -1434,7 +1434,7 @@ doCodeGen hsc_env this_mod data_tycons
     -- we generate one SRT for the whole module.
     let
      pipeline_stream
-      | gopt Opt_SplitObjs dflags
+      | gopt Opt_SplitObjs dflags || gopt Opt_SplitSections dflags
         = {-# SCC "cmmPipeline" #-}
           let run_pipeline us cmmgroup = do
                 let (topSRT', us') = initUs us emptySRT
diff --git a/compiler/main/SysTools.hs b/compiler/main/SysTools.hs
index 879b035d03..303e8dee90 100644
--- a/compiler/main/SysTools.hs
+++ b/compiler/main/SysTools.hs
@@ -791,6 +791,7 @@ getLinkerInfo' dflags = do
           -- GNU ld specifically needs to use less memory. This especially
           -- hurts on small object files. Trac #5240.
           -- Set DT_NEEDED for all shared libraries. Trac #10110.
+          -- TODO: Investigate if these help or hurt when using split sections.
           return (GnuLD $ map Option ["-Wl,--hash-size=31",
                                       "-Wl,--reduce-memory-overheads",
                                       -- ELF specific flag