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kbuild: add support for Clang LTO

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This change adds build system support for Clang's Link Time
Optimization (LTO). With -flto, instead of ELF object files, Clang
produces LLVM bitcode, which is compiled into native code at link
time, allowing the final binary to be optimized globally. For more
details, see:

  https://llvm.org/docs/LinkTimeOptimization.html

The Kconfig option CONFIG_LTO_CLANG is implemented as a choice,
which defaults to LTO being disabled. To use LTO, the architecture
must select ARCH_SUPPORTS_LTO_CLANG and support:

  - compiling with Clang,
  - compiling all assembly code with Clang's integrated assembler,
  - and linking with LLD.

While using CONFIG_LTO_CLANG_FULL results in the best runtime
performance, the compilation is not scalable in time or
memory. CONFIG_LTO_CLANG_THIN enables ThinLTO, which allows
parallel optimization and faster incremental builds. ThinLTO is
used by default if the architecture also selects
ARCH_SUPPORTS_LTO_CLANG_THIN:

  https://clang.llvm.org/docs/ThinLTO.html

To enable LTO, LLVM tools must be used to handle bitcode files, by
passing LLVM=1 and LLVM_IAS=1 options to make:

  $ make LLVM=1 LLVM_IAS=1 defconfig
  $ scripts/config -e LTO_CLANG_THIN
  $ make LLVM=1 LLVM_IAS=1

To prepare for LTO support with other compilers, common parts are
gated behind the CONFIG_LTO option, and LTO can be disabled for
specific files by filtering out CC_FLAGS_LTO.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201211184633.3213045-3-samitolvanen@google.com
This commit is contained in:
Sami Tolvanen 2020-12-11 10:46:19 -08:00 • committed by Kees Cook
parent 3b15cdc159
commit dc5723b02e
7 changed files with 174 additions and 18 deletions
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32
scripts/link-vmlinux.sh
View file

@ -56,6 +56,14 @@ modpost_link()
${KBUILD_VMLINUX_LIBS} \
--end-group"
if [ -n "${CONFIG_LTO_CLANG}" ]; then
# This might take a while, so indicate that we're doing
# an LTO link
info LTO ${1}
else
info LD ${1}
fi
${LD} ${KBUILD_LDFLAGS} -r -o ${1} ${objects}
}
@ -103,13 +111,22 @@ vmlinux_link()
fi
if [ "${SRCARCH}" != "um" ]; then
objects="--whole-archive \
${KBUILD_VMLINUX_OBJS} \
--no-whole-archive \
--start-group \
${KBUILD_VMLINUX_LIBS} \
--end-group \
${@}"
if [ -n "${CONFIG_LTO_CLANG}" ]; then
# Use vmlinux.o instead of performing the slow LTO
# link again.
objects="--whole-archive \
vmlinux.o \
--no-whole-archive \
${@}"
else
objects="--whole-archive \
${KBUILD_VMLINUX_OBJS} \
--no-whole-archive \
--start-group \
${KBUILD_VMLINUX_LIBS} \
--end-group \
${@}"
fi
${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux} \
${strip_debug#-Wl,} \
@ -274,7 +291,6 @@ fi;
${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init need-builtin=1
#link vmlinux.o
info LD vmlinux.o
modpost_link vmlinux.o
objtool_link vmlinux.o