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libpayload: Add support for link time optimization 

Link time optimization is a technique for whole-program optimization.
Instead of doing code generation during compilation, the compiler saves
its intermediate representation to the object files. During the final
linking step, it will then merge all the object files together and
perform optimizations on the entire program. This can often reduce the
final binary size, but also may increase the total compilation time.

This patch introduces a Kconfig option for enabling link time
optimization in libpayload. Since libpayload does no linking of its own,
its LTO archive files will contain only IR and no generated code.
Downstream projects will need to use LTO-aware tools when manipulating
the archives (eg. gcc-ar and gcc-nm), but otherwise do not need to use
LTO themselves -- the compiler will recognize which files are LTO and
which are not, so enabling this option should mostly be "drop in".

For example, when building coreinfo.elf using tinycurses libpayload:

		binary size	compilation time
default		114 KiB		11.49s
LTO		95 KiB		10.36s

In this case the total compilation time was actually shorter -- despite
the final linking step taking longer, this was offset by the shorter
compilation times for each individual file (since there is no code gen
until the very end).

Change-Id: I048f2ff6298ed0d891098942e1e8b29d35487b91
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38291
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
This commit is contained in:
Jacob Garber 2019-10-02 18:55:07 -06:00 committed by Stefan Reinauer
parent b524d2bb1f
commit 04540f1549
2 changed files with 12 additions and 0 deletions
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8
payloads/libpayload/Kconfig
View File

@ -79,6 +79,14 @@ config COMPILER_LLVM_CLANG
endchoice endchoice
config LTO
bool "Use link time optimization (LTO)"
default n
depends on COMPILER_GCC
help
Compile with link time optimization. This can often decrease the
final binary size, but may increase compilation time.
config REMOTEGDB config REMOTEGDB
bool "Remote GDB stub" bool "Remote GDB stub"
default n default n

4
payloads/libpayload/Makefile.inc
View File

@ -64,6 +64,10 @@ CFLAGS += -Wall -Wundef -Wstrict-prototypes -Wmissing-prototypes -Wvla
CFLAGS += -Wwrite-strings -Wredundant-decls -Wno-trigraphs -Wimplicit-fallthrough CFLAGS += -Wwrite-strings -Wredundant-decls -Wno-trigraphs -Wimplicit-fallthrough
CFLAGS += -Wstrict-aliasing -Wshadow -Werror CFLAGS += -Wstrict-aliasing -Wshadow -Werror
ifeq ($(CONFIG_LP_LTO),y)
CFLAGS += -flto
endif
$(obj)/libpayload-config.h: $(KCONFIG_AUTOHEADER) $(obj)/libpayload-config.h: $(KCONFIG_AUTOHEADER)
cmp $@ $< 2>/dev/null || cp $< $@ cmp $@ $< 2>/dev/null || cp $< $@