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

3 files changed, 14 insertions, 2 deletions

diff --git a/rules/build-package-way.mk b/rules/build-package-way.mk
index a10e53833e..d048f74b95 100644
--- a/rules/build-package-way.mk
+++ b/rules/build-package-way.mk
@@ -136,6 +136,8 @@ BINDIST_LIBS += $$($1_$2_$3_LIB)
 BINDIST_LIBS += $$($1_$2_$3_LIB0)
 endif
 
+$1_$2_LD_SCRIPT = driver/utils/merge_sections.ld
+
 # Build the GHCi library
 ifeq "$$(DYNAMIC_GHC_PROGRAMS)" "YES"
 $1_$2_GHCI_LIB = $$($1_$2_dyn_LIB)
@@ -148,8 +150,8 @@ ifneq "$4" "0"
 BINDIST_LIBS += $$($1_$2_GHCI_LIB)
 endif
 endif
-$$($1_$2_GHCI_LIB) : $$($1_$2_$3_HS_OBJS) $$($1_$2_$3_CMM_OBJS) $$($1_$2_$3_C_OBJS) $$($1_$2_$3_S_OBJS) $$($1_$2_EXTRA_OBJS)
-	$$(call cmd,LD) $$(CONF_LD_LINKER_OPTS_STAGE$4) -r -o $$@ $$(EXTRA_LD_OPTS) $$($1_$2_$3_HS_OBJS) $$($1_$2_$3_CMM_OBJS) $$($1_$2_$3_C_OBJS) $$($1_$2_$3_S_OBJS) $$($1_$2_EXTRA_OBJS)
+$$($1_$2_GHCI_LIB) : $$($1_$2_$3_HS_OBJS) $$($1_$2_$3_CMM_OBJS) $$($1_$2_$3_C_OBJS) $$($1_$2_$3_S_OBJS) $$($1_$2_EXTRA_OBJS) $$($1_$2_LD_SCRIPT)
+	$$(call cmd,LD) $$(CONF_LD_LINKER_OPTS_STAGE$4) -r $(if $(filter YES,$(LdIsGNULd)),-T $$($1_$2_LD_SCRIPT)) -o $$@ $$(EXTRA_LD_OPTS) $$($1_$2_$3_HS_OBJS) $$($1_$2_$3_CMM_OBJS) $$($1_$2_$3_C_OBJS) $$($1_$2_$3_S_OBJS) $$($1_$2_EXTRA_OBJS)
 
 ifeq "$$($1_$2_BUILD_GHCI_LIB)" "YES"
 # Don't bother making ghci libs for bootstrapping packages
diff --git a/rules/build-package.mk b/rules/build-package.mk
index 688e1d2ca5..9599c57eb3 100644
--- a/rules/build-package.mk
+++ b/rules/build-package.mk
@@ -110,6 +110,15 @@ else
 $1_$2_SplitObjs = NO
 endif
 endif
+# Disable split sections when building with stage0, it won't be supported yet
+# and it's probably not very relevant anyway (smaller stage1 ghc?).
+ifeq "$$($1_$2_SplitSections)" ""
+ifeq "$3" "1"
+$1_$2_SplitSections = $(SplitSections)
+else
+$1_$2_SplitSections = NO
+endif
+endif
 
 $(call hs-sources,$1,$2)
 $(call c-sources,$1,$2)
diff --git a/rules/distdir-way-opts.mk b/rules/distdir-way-opts.mk
index 920ff07f20..47f6f90237 100644
--- a/rules/distdir-way-opts.mk
+++ b/rules/distdir-way-opts.mk
@@ -136,6 +136,7 @@ $1_$2_$3_ALL_HC_OPTS = \
  -hisuf $$($3_hisuf) -osuf  $$($3_osuf) -hcsuf $$($3_hcsuf) \
  $$($1_$2_$3_MOST_DIR_HC_OPTS) \
  $$(if $$(findstring YES,$$($1_$2_SplitObjs)),$$(if $$(findstring dyn,$3),,-split-objs),) \
+ $$(if $$(findstring YES,$$($1_$2_SplitSections)),$$(if $$(findstring dyn,$3),,-split-sections),) \
  $$(if $$(findstring YES,$$($1_$2_DYNAMIC_TOO)),$$(if $$(findstring v,$3),-dynamic-too))
 
 ifeq "$3" "dyn"