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Subject[PATCH 02/22] 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 inline assembly with Clang's integrated assembler,
- and linking with LLD.

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

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

To enable LTO, LLVM tools must be used to handle bitcode files. The
easiest way is to pass the LLVM=1 option to make:

$ make LLVM=1 defconfig
$ scripts/config -e LTO_CLANG
$ make LLVM=1

Alternatively, at least the following LLVM tools must be used:

CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm

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.

Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in
follow-up patches.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
Makefile | 16 ++++++++
arch/Kconfig | 66 +++++++++++++++++++++++++++++++
include/asm-generic/vmlinux.lds.h | 11 ++++--
scripts/Makefile.build | 9 ++++-
scripts/Makefile.modfinal | 9 ++++-
scripts/Makefile.modpost | 24 ++++++++++-
scripts/link-vmlinux.sh | 32 +++++++++++----
7 files changed, 151 insertions(+), 16 deletions(-)

diff --git a/Makefile b/Makefile
index ac2c61c37a73..0c7fe6fb2143 100644
--- a/Makefile
+++ b/Makefile
@@ -886,6 +886,22 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS)
export CC_FLAGS_SCS
endif

+ifdef CONFIG_LTO_CLANG
+ifdef CONFIG_THINLTO
+CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit)
+KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache
+else
+CC_FLAGS_LTO_CLANG := -flto
+endif
+CC_FLAGS_LTO_CLANG += -fvisibility=default
+endif
+
+ifdef CONFIG_LTO
+CC_FLAGS_LTO := $(CC_FLAGS_LTO_CLANG)
+KBUILD_CFLAGS += $(CC_FLAGS_LTO)
+export CC_FLAGS_LTO
+endif
+
# arch Makefile may override CC so keep this after arch Makefile is included
NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)

diff --git a/arch/Kconfig b/arch/Kconfig
index 8cc35dc556c7..e00b122293f8 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -552,6 +552,72 @@ config SHADOW_CALL_STACK
reading and writing arbitrary memory may be able to locate them
and hijack control flow by modifying the stacks.

+config LTO
+ bool
+
+config ARCH_SUPPORTS_LTO_CLANG
+ bool
+ help
+ An architecture should select this option if it supports:
+ - compiling with Clang,
+ - compiling inline assembly with Clang's integrated assembler,
+ - and linking with LLD.
+
+config ARCH_SUPPORTS_THINLTO
+ bool
+ help
+ An architecture should select this option if it supports Clang's
+ ThinLTO.
+
+config THINLTO
+ bool "Clang ThinLTO"
+ depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO
+ default y
+ help
+ This option enables Clang's ThinLTO, which allows for parallel
+ optimization and faster incremental compiles. More information
+ can be found from Clang's documentation:
+
+ https://clang.llvm.org/docs/ThinLTO.html
+
+choice
+ prompt "Link Time Optimization (LTO)"
+ default LTO_NONE
+ help
+ This option enables Link Time Optimization (LTO), which allows the
+ compiler to optimize binaries globally.
+
+ If unsure, select LTO_NONE.
+
+config LTO_NONE
+ bool "None"
+
+config LTO_CLANG
+ bool "Clang's Link Time Optimization (EXPERIMENTAL)"
+ depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD
+ depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)
+ depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
+ depends on ARCH_SUPPORTS_LTO_CLANG
+ depends on !FTRACE_MCOUNT_RECORD
+ depends on !KASAN
+ depends on !MODVERSIONS
+ select LTO
+ help
+ This option enables Clang's Link Time Optimization (LTO), which
+ allows the compiler to optimize the kernel globally. If you enable
+ this option, the compiler generates LLVM bitcode instead of ELF
+ object files, and the actual compilation from bitcode happens at
+ the LTO link step, which may take several minutes depending on the
+ kernel configuration. More information can be found from LLVM's
+ documentation:
+
+ https://llvm.org/docs/LinkTimeOptimization.html
+
+ To select this option, you also need to use LLVM tools to handle
+ the bitcode by passing LLVM=1 to make.
+
+endchoice
+
config HAVE_ARCH_WITHIN_STACK_FRAMES
bool
help
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index db600ef218d7..78079000c05a 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -89,15 +89,18 @@
* .data. We don't want to pull in .data..other sections, which Linux
* has defined. Same for text and bss.
*
+ * With LTO_CLANG, the linker also splits sections by default, so we need
+ * these macros to combine the sections during the final link.
+ *
* RODATA_MAIN is not used because existing code already defines .rodata.x
* sections to be brought in with rodata.
*/
-#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
-#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX*
+#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral*
#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
-#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
-#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
+#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L*
+#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral*
#define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
#else
#define TEXT_MAIN .text
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 2e8810b7e5ed..f307e708a1b7 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -108,7 +108,7 @@ endif
# ---------------------------------------------------------------------------

quiet_cmd_cc_s_c = CC $(quiet_modtag) $@
- cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS), $(c_flags)) $(DISABLE_LTO) -fverbose-asm -S -o $@ $<
+ cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS) $(CC_FLAGS_LTO), $(c_flags)) -fverbose-asm -S -o $@ $<

$(obj)/%.s: $(src)/%.c FORCE
$(call if_changed_dep,cc_s_c)
@@ -424,8 +424,15 @@ $(obj)/lib.a: $(lib-y) FORCE
# Do not replace $(filter %.o,^) with $(real-prereqs). When a single object
# module is turned into a multi object module, $^ will contain header file
# dependencies recorded in the .*.cmd file.
+ifdef CONFIG_LTO_CLANG
+quiet_cmd_link_multi-m = AR [M] $@
+cmd_link_multi-m = \
+ rm -f $@; \
+ $(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(filter %.o,$^)
+else
quiet_cmd_link_multi-m = LD [M] $@
cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ $(filter %.o,$^)
+endif

$(multi-used-m): FORCE
$(call if_changed,link_multi-m)
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 411c1e600e7d..1005b147abd0 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -6,6 +6,7 @@
PHONY := __modfinal
__modfinal:

+include $(objtree)/include/config/auto.conf
include $(srctree)/scripts/Kbuild.include

# for c_flags
@@ -29,6 +30,12 @@ quiet_cmd_cc_o_c = CC [M] $@

ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)

+ifdef CONFIG_LTO_CLANG
+# With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to
+# avoid a second slow LTO link
+prelink-ext := .lto
+endif
+
quiet_cmd_ld_ko_o = LD [M] $@
cmd_ld_ko_o = \
$(LD) -r $(KBUILD_LDFLAGS) \
@@ -37,7 +44,7 @@ quiet_cmd_ld_ko_o = LD [M] $@
-o $@ $(filter %.o, $^); \
$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)

-$(modules): %.ko: %.o %.mod.o $(KBUILD_LDS_MODULE) FORCE
+$(modules): %.ko: %$(prelink-ext).o %.mod.o $(KBUILD_LDS_MODULE) FORCE
+$(call if_changed,ld_ko_o)

targets += $(modules) $(modules:.ko=.mod.o)
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 3651cbf6ad49..9ced8aecd579 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -102,12 +102,32 @@ $(input-symdump):
@echo >&2 'WARNING: Symbol version dump "$@" is missing.'
@echo >&2 ' Modules may not have dependencies or modversions.'

+ifdef CONFIG_LTO_CLANG
+# With CONFIG_LTO_CLANG, .o files might be LLVM bitcode, so we need to run
+# LTO to compile them into native code before running modpost
+prelink-ext = .lto
+
+quiet_cmd_cc_lto_link_modules = LTO [M] $@
+cmd_cc_lto_link_modules = \
+ $(LD) $(ld_flags) -r -o $@ \
+ --whole-archive $(filter-out FORCE,$^)
+
+%.lto.o: %.o FORCE
+ $(call if_changed,cc_lto_link_modules)
+
+PHONY += FORCE
+FORCE:
+
+endif
+
+modules := $(sort $(shell cat $(MODORDER)))
+
# Read out modules.order to pass in modpost.
# Otherwise, allmodconfig would fail with "Argument list too long".
quiet_cmd_modpost = MODPOST $@
- cmd_modpost = sed 's/ko$$/o/' $< | $(MODPOST) -T -
+ cmd_modpost = sed 's/\.ko$$/$(prelink-ext)\.o/' $< | $(MODPOST) -T -

-$(output-symdump): $(MODORDER) $(input-symdump) FORCE
+$(output-symdump): $(MODORDER) $(input-symdump) $(modules:.ko=$(prelink-ext).o) FORCE
$(call if_changed,modpost)

targets += $(output-symdump)
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 92dd745906f4..a681b3b6722e 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -52,6 +52,14 @@ 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}
}

@@ -99,13 +107,22 @@ 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,} \
@@ -270,7 +287,6 @@ 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

--
2.27.0.212.ge8ba1cc988-goog
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