[PATCH] kbuild: switch to thin archives

[Nicholas Piggin <npiggin@gmail.com>]

Switch from incremental build to thin archives for packaging built-in.o.
binutils version must be bumped to 2.20. Proposed patch for 4.13.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---

 Documentation/process/changes.rst |  9 ++++-----
 Makefile                          |  3 +++
 arch/Kconfig                      |  6 ------
 arch/powerpc/Kconfig              |  8 --------
 scripts/Makefile.build            | 27 ++++++-------------------
 scripts/link-vmlinux.sh           | 42 +++++++++------------------------------
 6 files changed, 22 insertions(+), 73 deletions(-)

diff --git a/Documentation/process/changes.rst b/Documentation/process/changes.rst
index e25d63f8c0da..56fd0687bd4a 100644
--- a/Documentation/process/changes.rst
+++ b/Documentation/process/changes.rst
@@ -31,7 +31,7 @@ you probably needn't concern yourself with isdn4k-utils.
 ====================== ===============  ========================================
 GNU C                  3.2              gcc --version
 GNU make               3.81             make --version
-binutils               2.12             ld -v
+binutils               2.20             ld -v
 util-linux             2.10o            fdformat --version
 module-init-tools      0.9.10           depmod -V
 e2fsprogs              1.41.4           e2fsck -V
@@ -75,10 +75,9 @@ You will need GNU make 3.81 or later to build the kernel.
 Binutils
 --------
 
-Linux on IA-32 has recently switched from using ``as86`` to using ``gas`` for
-assembling the 16-bit boot code, removing the need for ``as86`` to compile
-your kernel.  This change does, however, mean that you need a recent
-release of binutils.
+The build system has recently switched to using thin archives (`ar T`) rather
+than incremental linking (`ld -r`) for built-in.o intermediate steps. This
+requires binutils 2.20 or newer.
 
 Perl
 ----
diff --git a/Makefile b/Makefile
index 470bd4d9513a..58658d7a04a0 100644
--- a/Makefile
+++ b/Makefile
@@ -818,6 +818,9 @@ KBUILD_CFLAGS   += $(call cc-option,-Werror=designated-init)
 # use the deterministic mode of AR if available
 KBUILD_ARFLAGS := $(call ar-option,D)
 
+# Use thin archives.
+KBUILD_ARFLAGS :=$(KBUILD_ARFLAGS)T
+
 include scripts/Makefile.kasan
 include scripts/Makefile.extrawarn
 include scripts/Makefile.ubsan
diff --git a/arch/Kconfig b/arch/Kconfig
index 6c00e5b00f8b..0a306916968a 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -510,12 +510,6 @@ config CC_STACKPROTECTOR_STRONG
 
 endchoice
 
-config THIN_ARCHIVES
-	bool
-	help
-	  Select this if the architecture wants to use thin archives
-	  instead of ld -r to create the built-in.o files.
-
 config LD_DEAD_CODE_DATA_ELIMINATION
 	bool
 	help
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index f7c8f9972f61..80d882a78426 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -472,14 +472,6 @@ config MPROFILE_KERNEL
 	depends on PPC64 && CPU_LITTLE_ENDIAN
 	def_bool !DISABLE_MPROFILE_KERNEL
 
-config USE_THIN_ARCHIVES
-	bool "Build the kernel using thin archives"
-	default n
-	select THIN_ARCHIVES
-	help
-	  Build the kernel using thin archives.
-	  If you're unsure say N.
-
 config IOMMU_HELPER
 	def_bool PPC64
 
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 733e044fff8b..6cc07d09fce3 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -436,15 +436,9 @@ $(sort $(subdir-obj-y)): $(subdir-ym) ;
 #
 ifdef builtin-target
 
-ifdef CONFIG_THIN_ARCHIVES
-  cmd_make_builtin = rm -f $@; $(AR) rcST$(KBUILD_ARFLAGS)
-  cmd_make_empty_builtin = rm -f $@; $(AR) rcST$(KBUILD_ARFLAGS)
-  quiet_cmd_link_o_target = AR      $@
-else
-  cmd_make_builtin = $(LD) $(ld_flags) -r -o
-  cmd_make_empty_builtin = rm -f $@; $(AR) rcs$(KBUILD_ARFLAGS)
-  quiet_cmd_link_o_target = LD      $@
-endif
+cmd_make_builtin = rm -f $@; $(AR) rcS$(KBUILD_ARFLAGS)
+cmd_make_empty_builtin = rm -f $@; $(AR) rcS$(KBUILD_ARFLAGS)
+quiet_cmd_link_o_target = AR      $@
 
 # If the list of objects to link is empty, just create an empty built-in.o
 cmd_link_o_target = $(if $(strip $(obj-y)),\
@@ -477,11 +471,7 @@ $(modorder-target): $(subdir-ym) FORCE
 ifdef lib-target
 quiet_cmd_link_l_target = AR      $@
 
-ifdef CONFIG_THIN_ARCHIVES
-  cmd_link_l_target = rm -f $@; $(AR) rcsT$(KBUILD_ARFLAGS) $@ $(lib-y)
-else
-  cmd_link_l_target = rm -f $@; $(AR) rcs$(KBUILD_ARFLAGS) $@ $(lib-y)
-endif
+cmd_link_l_target = rm -f $@; $(AR) rcs$(KBUILD_ARFLAGS) $@ $(lib-y)
 
 $(lib-target): $(lib-y) FORCE
 	$(call if_changed,link_l_target)
@@ -529,13 +519,8 @@ $($(subst $(obj)/,,$(@:.o=-m)))), $^)
 
 cmd_link_multi-link = $(LD) $(ld_flags) -r -o $@ $(link_multi_deps) $(cmd_secanalysis)
 
-ifdef CONFIG_THIN_ARCHIVES
-  quiet_cmd_link_multi-y = AR      $@
-  cmd_link_multi-y = rm -f $@; $(AR) rcST$(KBUILD_ARFLAGS) $@ $(link_multi_deps)
-else
-  quiet_cmd_link_multi-y = LD      $@
-  cmd_link_multi-y = $(cmd_link_multi-link)
-endif
+quiet_cmd_link_multi-y = AR      $@
+cmd_link_multi-y = rm -f $@; $(AR) rcS$(KBUILD_ARFLAGS) $@ $(link_multi_deps)
 
 quiet_cmd_link_multi-m = LD [M]  $@
 cmd_link_multi-m = $(cmd_link_multi-link)
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index c80291319cb2..ac0d3b2faa09 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -47,13 +47,11 @@ info()
 #
 archive_builtin()
 {
-	if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
-		info AR built-in.o
-		rm -f built-in.o;
-		${AR} rcsT${KBUILD_ARFLAGS} built-in.o			\
-					${KBUILD_VMLINUX_INIT}		\
-					${KBUILD_VMLINUX_MAIN}
-	fi
+	info AR built-in.o
+	rm -f built-in.o;
+	${AR} rcs${KBUILD_ARFLAGS} built-in.o			\
+				${KBUILD_VMLINUX_INIT}		\
+				${KBUILD_VMLINUX_MAIN}
 }
 
 # Link of vmlinux.o used for section mismatch analysis
@@ -62,14 +60,8 @@ modpost_link()
 {
 	local objects
 
-	if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
-		objects="--whole-archive built-in.o"
-	else
-		objects="${KBUILD_VMLINUX_INIT}				\
-			--start-group					\
-			${KBUILD_VMLINUX_MAIN}				\
-			--end-group"
-	fi
+	objects="--whole-archive built-in.o"
+
 	${LD} ${LDFLAGS} -r -o ${1} ${objects}
 }
 
@@ -82,28 +74,12 @@ vmlinux_link()
 	local objects
 
 	if [ "${SRCARCH}" != "um" ]; then
-		if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
-			objects="--whole-archive built-in.o ${1}"
-		else
-			objects="${KBUILD_VMLINUX_INIT}			\
-				--start-group				\
-				${KBUILD_VMLINUX_MAIN}			\
-				--end-group				\
-				${1}"
-		fi
+		objects="--whole-archive built-in.o ${1}"
 
 		${LD} ${LDFLAGS} ${LDFLAGS_vmlinux} -o ${2}		\
 			-T ${lds} ${objects}
 	else
-		if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
-			objects="-Wl,--whole-archive built-in.o ${1}"
-		else
-			objects="${KBUILD_VMLINUX_INIT}			\
-				-Wl,--start-group			\
-				${KBUILD_VMLINUX_MAIN}			\
-				-Wl,--end-group				\
-				${1}"
-		fi
+		objects="-Wl,--whole-archive built-in.o ${1}"
 
 		${CC} ${CFLAGS_vmlinux} -o ${2}				\
 			-Wl,-T,${lds}					\
-- 
2.11.0