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

1 files changed, 13 insertions, 2 deletions

diff --git a/compiler/nativeGen/Dwarf.hs b/compiler/nativeGen/Dwarf.hs
index 35ee9c90ab..6bf49f0e0d 100644
--- a/compiler/nativeGen/Dwarf.hs
+++ b/compiler/nativeGen/Dwarf.hs
@@ -83,11 +83,22 @@ dwarfGen df modLoc us blocks = do
                  pprDwarfFrame (debugFrame framesU procs)
 
   -- .aranges section: Information about the bounds of compilation units
-  let aranges = dwarfARangesSection $$
-                pprDwarfARange (DwarfARange lowLabel highLabel unitU)
+  let aranges' | gopt Opt_SplitSections df = map mkDwarfARange procs
+               | otherwise                 = [DwarfARange lowLabel highLabel]
+  let aranges = dwarfARangesSection $$ pprDwarfARanges aranges' unitU
 
   return (infoSct $$ abbrevSct $$ lineSct $$ frameSct $$ aranges, us'')
 
+-- | Build an address range entry for one proc.
+-- With split sections, each proc needs its own entry, since they may get
+-- scattered in the final binary. Without split sections, we could make a
+-- single arange based on the first/last proc.
+mkDwarfARange :: DebugBlock -> DwarfARange
+mkDwarfARange proc = DwarfARange start end
+  where
+    start = dblCLabel proc
+    end = mkAsmTempEndLabel start
+
 -- | Header for a compilation unit, establishing global format
 -- parameters
 compileUnitHeader :: Unique -> SDoc