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, 8 insertions, 3 deletions

diff --git a/compiler/cmm/Cmm.hs b/compiler/cmm/Cmm.hs
index 9e9bae93c6..d0564e6f68 100644
--- a/compiler/cmm/Cmm.hs
+++ b/compiler/cmm/Cmm.hs
@@ -8,7 +8,7 @@ module Cmm (
      CmmGraph, GenCmmGraph(..),
      CmmBlock,
      RawCmmDecl, RawCmmGroup,
-     Section(..), CmmStatics(..), CmmStatic(..),
+     Section(..), SectionType(..), CmmStatics(..), CmmStatic(..),
 
      -- ** Blocks containing lists
      GenBasicBlock(..), blockId,
@@ -48,8 +48,10 @@ import Data.Word        ( Word8 )
 -- A CmmProgram is a list of CmmGroups  
 -- A CmmGroup is a list of top-level declarations  
 
--- When object-splitting is on,each group is compiled into a separate
+-- When object-splitting is on, each group is compiled into a separate
 -- .o file. So typically we put closely related stuff in a CmmGroup.
+-- Section-splitting follows suit and makes one .text subsection for each
+-- CmmGroup.
 
 type CmmProgram = [CmmGroup]
 
@@ -163,7 +165,7 @@ needsSRT (C_SRT _ _ _) = True
 --              Static Data
 -----------------------------------------------------------------------------
 
-data Section
+data SectionType
   = Text
   | Data
   | ReadOnlyData
@@ -171,6 +173,9 @@ data Section
   | UninitialisedData
   | ReadOnlyData16      -- .rodata.cst16 on x86_64, 16-byte aligned
   | OtherSection String
+  deriving (Show)
+
+data Section = Section SectionType CLabel
 
 data CmmStatic
   = CmmStaticLit CmmLit