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, 5 insertions, 3 deletions

diff --git a/compiler/nativeGen/AsmCodeGen.hs b/compiler/nativeGen/AsmCodeGen.hs
index 1b57a504bd..b3988026be 100644
--- a/compiler/nativeGen/AsmCodeGen.hs
+++ b/compiler/nativeGen/AsmCodeGen.hs
@@ -373,10 +373,10 @@ cmmNativeGenStream dflags this_mod modLoc ncgImpl h us cmm_stream ngs
               dbgMap = debugToMap ndbgs
 
           -- Insert split marker, generate native code
-          let splitFlag = gopt Opt_SplitObjs dflags
+          let splitObjs = gopt Opt_SplitObjs dflags
               split_marker = CmmProc mapEmpty mkSplitMarkerLabel [] $
                              ofBlockList (panic "split_marker_entry") []
-              cmms' | splitFlag  = split_marker : cmms
+              cmms' | splitObjs  = split_marker : cmms
                     | otherwise  = cmms
           (ngs',us') <- cmmNativeGens dflags this_mod modLoc ncgImpl h dbgMap us
                                       cmms' ngs 0
@@ -388,8 +388,10 @@ cmmNativeGenStream dflags this_mod modLoc ncgImpl h us cmm_stream ngs
 
           -- Emit & clear DWARF information when generating split
           -- object files, as we need it to land in the same object file
+          -- When using split sections, note that we do not split the debug
+          -- info but emit all the info at once in finishNativeGen.
           (ngs'', us'') <-
-            if debugFlag && splitFlag
+            if debugFlag && splitObjs
             then do (dwarf, us'') <- dwarfGen dflags modLoc us ldbgs
                     emitNativeCode dflags h dwarf
                     return (ngs' { ngs_debug = []