FROMLIST: BACKPORT: kbuild: add support for clang LTO
This change adds the configuration option CONFIG_LTO_CLANG, and
build system support for clang's Link Time Optimization (LTO). In
preparation for LTO support for other compilers, potentially common
parts of the changes are gated behind CONFIG_LTO instead.
With -flto, instead of object files, clang produces LLVM bitcode,
which is compiled into a native object at link time, allowing the
final binary to be optimized globally. For more details, see:
https://llvm.org/docs/LinkTimeOptimization.html
While the kernel normally uses GNU ld for linking, LLVM supports LTO
only with lld or GNU gold linkers. This patch set assumes gold will
be used with the LLVMgold plug-in to perform the LTO link step. Due
to potential incompatibilities with GNU ld, this change also adds
LDFINAL_vmlinux for using a different linker for the vmlinux_link
step, and defaults to using GNU ld.
Assuming LLVMgold.so is in LD_LIBRARY_PATH and CONFIG_LTO_CLANG has
been selected, an LTO kernel can be built simply by running make
CC=clang. LTO requires clang >= 5.0 and gold from binutils >= 2.27.
Bug: 62093296
Bug: 67506682
Change-Id: Ibcd9fc7ec501b4f30b43b4877897615645f8655f
(am from https://patchwork.kernel.org/patch/10060329/)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index f742c65..d16dc7c 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -53,9 +53,40 @@
${AR} rcsT${KBUILD_ARFLAGS} built-in.o \
${KBUILD_VMLINUX_INIT} \
${KBUILD_VMLINUX_MAIN}
+
+ if [ -n "${CONFIG_LTO_CLANG}" ]; then
+ mv -f built-in.o built-in.o.tmp
+ ${LLVM_AR} rcsT${KBUILD_ARFLAGS} built-in.o $(${AR} t built-in.o.tmp)
+ rm -f built-in.o.tmp
+ fi
fi
}
+# If CONFIG_LTO_CLANG is selected, collect generated symbol versions into
+# .tmp_symversions
+modversions()
+{
+ if [ -z "${CONFIG_LTO_CLANG}" ]; then
+ return
+ fi
+
+ if [ -z "${CONFIG_MODVERSIONS}" ]; then
+ return
+ fi
+
+ rm -f .tmp_symversions
+
+ for a in built-in.o ${KBUILD_VMLINUX_LIBS}; do
+ for o in $(${AR} t $a); do
+ if [ -f ${o}.symversions ]; then
+ cat ${o}.symversions >> .tmp_symversions
+ fi
+ done
+ done
+
+ echo "-T .tmp_symversions"
+}
+
# Link of vmlinux.o used for section mismatch analysis
# ${1} output file
modpost_link()
@@ -70,7 +101,16 @@
${KBUILD_VMLINUX_MAIN} \
--end-group"
fi
- ${LD} ${LDFLAGS} -r -o ${1} ${objects}
+
+ if [ -n "${CONFIG_LTO_CLANG}" ]; then
+ # This might take a while, so indicate that we're doing
+ # an LTO link
+ info LTO vmlinux.o
+ else
+ info LD vmlinux.o
+ fi
+
+ ${LD} ${LDFLAGS} -r -o ${1} $(modversions) ${objects}
}
# Link of vmlinux
@@ -82,7 +122,15 @@
local objects
if [ "${SRCARCH}" != "um" ]; then
- if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
+ local ld=${LD}
+ local ldflags="${LDFLAGS} ${LDFLAGS_vmlinux}"
+
+ if [ -n "${LDFINAL_vmlinux}" ]; then
+ ld=${LDFINAL_vmlinux}
+ ldflags="${LDFLAGS_FINAL_vmlinux} ${LDFLAGS_vmlinux}"
+ fi
+
+ if [[ -n "${CONFIG_THIN_ARCHIVES}" && -z "${CONFIG_LTO_CLANG}" ]]; then
objects="--whole-archive built-in.o ${1}"
else
objects="${KBUILD_VMLINUX_INIT} \
@@ -92,8 +140,7 @@
${1}"
fi
- ${LD} ${LDFLAGS} ${LDFLAGS_vmlinux} -o ${2} \
- -T ${lds} ${objects}
+ ${ld} ${ldflags} -o ${2} -T ${lds} ${objects}
else
if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
objects="-Wl,--whole-archive built-in.o ${1}"
@@ -113,7 +160,6 @@
fi
}
-
# Create ${2} .o file with all symbols from the ${1} object file
kallsyms()
{
@@ -164,6 +210,7 @@
rm -f .tmp_System.map
rm -f .tmp_kallsyms*
rm -f .tmp_version
+ rm -f .tmp_symversions
rm -f .tmp_vmlinux*
rm -f built-in.o
rm -f System.map
@@ -209,15 +256,6 @@
. "./${KCONFIG_CONFIG}"
esac
-archive_builtin
-
-#link vmlinux.o
-info LD vmlinux.o
-modpost_link vmlinux.o
-
-# modpost vmlinux.o to check for section mismatches
-${MAKE} -f "${srctree}/scripts/Makefile.modpost" vmlinux.o
-
# Update version
info GEN .version
if [ ! -r .version ]; then
@@ -228,9 +266,24 @@
expr 0$(cat .old_version) + 1 >.version;
fi;
+archive_builtin
+
+#link vmlinux.o
+modpost_link vmlinux.o
+
+# modpost vmlinux.o to check for section mismatches
+${MAKE} -f "${srctree}/scripts/Makefile.modpost" vmlinux.o
+
# final build of init/
${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init GCC_PLUGINS_CFLAGS="${GCC_PLUGINS_CFLAGS}"
+if [ -n "${CONFIG_LTO_CLANG}" ]; then
+ # Re-use vmlinux.o, so we can avoid the slow LTO link step in
+ # vmlinux_link
+ KBUILD_VMLINUX_INIT=
+ KBUILD_VMLINUX_MAIN=vmlinux.o
+fi
+
kallsymso=""
kallsyms_vmlinux=""
if [ -n "${CONFIG_KALLSYMS}" ]; then