From: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
ld -r is an incremental link used to create built-in.o files in build
subdirectories. It produces relocatable object files containing all
its input files, and these are are then pulled together and relocated
in the final link. Aside from the bloat, this constrains the final
link relocations, which has bitten large powerpc builds with
unresolvable relocations in the final link.
Alan Modra has recommended the kernel use thin archives for linking.
This is an alternative and means that the linker has more information
available to it when it links the kernel.
This patch enables a config option architectures can select, which
causes all built-in.o files to be built as thin archives. built-in.o
files in subdirectories do not get symbol table or index attached,
which improves speed and size. The final link pass creates a
built-in.o archive in the root output directory which includes the
symbol table and index. The linker then uses takes this file to link.
The --whole-archive linker option is required, because the linker now
has visibility to every individual object file, and it will otherwise
just completely avoid including those without external references
(consider a file with EXPORT_SYMBOL or initcall or hardware exceptions
as its only entry points). The traditional built works "by luck" as
built-in.o files are large enough that they're going to get external
references. However this optimisation is unpredictable for the kernel
(due to above external references), ineffective at culling unused, and
costly because the .o files have to be searched for references.
Superior alternatives for link-time culling should be used instead.
Build characteristics for inclink vs thinarc, on a small powerpc64le
pseries VM with a modest .config:
inclink thinarc
sizes
vmlinux 15 618 680 15 625 028
sum of all built-in.o 56 091 808 1 054 334
sum excluding root built-in.o 151 430
find -name built-in.o | xargs rm ; time make vmlinux
real 22.772s 21.143s
user 13.280s 13.430s
sys 4.310s 2.750s
- Final kernel pulled in only about 6K more, which shows how
ineffective the object file culling is.
- Build performance looks improved due to less pagecache activity.
On IO constrained systems it could be a bigger win.
- Build size saving is significant.
Side note, the toochain understands archives, so there's some tricks,
$ ar t built-in.o # list all files you linked with
$ size built-in.o # and their sizes
$ objdump -d built-in.o # disassembly (unrelocated) with filenames
Implementation by sfr, minor tweaks by npiggin.
Signed-off-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
---
Changes since v1
- Fixed um build breakage
- Fixed linking against lib.a archives (add symbol table)
- Tested x86 builds
- Tested arm64 defconfig thin archives cross compile
inclinc - 385s 2.7GB; thinarc - 377s 1.8G
Changes since v2
- Add symbol index for lib.a archives, same as incremental link build
arch/Kconfig | 6 +++++
scripts/Makefile.build | 23 +++++++++++++---
scripts/link-vmlinux.sh | 71 +++++++++++++++++++++++++++++++++++++++++--------
3 files changed, 85 insertions(+), 15 deletions(-)
diff --git a/arch/Kconfig b/arch/Kconfig
index d794384..1330bf4 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -424,6 +424,12 @@ 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 HAVE_CONTEXT_TRACKING
bool
help
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 0d1ca5b..578ded1 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -358,12 +358,22 @@ $(sort $(subdir-obj-y)): $(subdir-ym) ;
# Rule to compile a set of .o files into one .o file
#
ifdef builtin-target
-quiet_cmd_link_o_target = LD $@
+
+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
+
# If the list of objects to link is empty, just create an empty built-in.o
cmd_link_o_target = $(if $(strip $(obj-y)),\
- $(LD) $(ld_flags) -r -o $@ $(filter $(obj-y), $^) \
+ $(cmd_make_builtin) $@ $(filter $(obj-y), $^) \
$(cmd_secanalysis),\
- rm -f $@; $(AR) rcs$(KBUILD_ARFLAGS) $@)
+ $(cmd_make_empty_builtin) $@)
$(builtin-target): $(obj-y) FORCE
$(call if_changed,link_o_target)
@@ -389,7 +399,12 @@ $(modorder-target): $(subdir-ym) FORCE
#
ifdef lib-target
quiet_cmd_link_l_target = AR $@
-cmd_link_l_target = rm -f $@; $(AR) rcs$(KBUILD_ARFLAGS) $@ $(lib-y)
+
+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
$(lib-target): $(lib-y) FORCE
$(call if_changed,link_l_target)
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index f0f6d9d..2f8a615 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -37,12 +37,40 @@ info()
fi
}
+# Thin archive build here makes a final archive with
+# symbol table and indexes from vmlinux objects, which can be
+# used as input to linker.
+#
+# Traditional incremental style of link does not require this step
+#
+# built-in.o output file
+#
+archive_builtin()
+{
+ if [ -n "${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
+}
+
# Link of vmlinux.o used for section mismatch analysis
# ${1} output file
modpost_link()
{
- ${LD} ${LDFLAGS} -r -o ${1} ${KBUILD_VMLINUX_INIT} \
- --start-group ${KBUILD_VMLINUX_MAIN} --end-group
+ local objects
+
+ if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
+ objects="--whole-archive built-in.o"
+ else
+ objects="${KBUILD_VMLINUX_INIT} \
+ --start-group \
+ ${KBUILD_VMLINUX_MAIN} \
+ --end-group"
+ fi
+ ${LD} ${LDFLAGS} -r -o ${1} ${objects}
}
# Link of vmlinux
@@ -51,18 +79,36 @@ modpost_link()
vmlinux_link()
{
local lds="${objtree}/${KBUILD_LDS}"
+ local objects
if [ "${SRCARCH}" != "um" ]; then
- ${LD} ${LDFLAGS} ${LDFLAGS_vmlinux} -o ${2} \
- -T ${lds} ${KBUILD_VMLINUX_INIT} \
- --start-group ${KBUILD_VMLINUX_MAIN} --end-group ${1}
+ if [ -n "${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
+
+ ${LD} ${LDFLAGS} ${LDFLAGS_vmlinux} -o ${2} \
+ -T ${lds} ${objects}
else
- ${CC} ${CFLAGS_vmlinux} -o ${2} \
- -Wl,-T,${lds} ${KBUILD_VMLINUX_INIT} \
- -Wl,--start-group \
- ${KBUILD_VMLINUX_MAIN} \
- -Wl,--end-group \
- -lutil -lrt -lpthread ${1}
+ if [ -n "${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
+
+ ${CC} ${CFLAGS_vmlinux} -o ${2} \
+ -Wl,-T,${lds} \
+ ${objects} \
+ -lutil -lrt -lpthread
rm -f linux
fi
}
@@ -119,6 +165,7 @@ cleanup()
rm -f .tmp_kallsyms*
rm -f .tmp_version
rm -f .tmp_vmlinux*
+ rm -f built-in.o
rm -f System.map
rm -f vmlinux
rm -f vmlinux.o
@@ -162,6 +209,8 @@ case "${KCONFIG_CONFIG}" in
. "./${KCONFIG_CONFIG}"
esac
+archive_builtin
+
#link vmlinux.o
info LD vmlinux.o
modpost_link vmlinux.o
--
2.8.1
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