build: support building with Link Time Optimizations
Add plumbing for building U-Boot with Link Time Optimizations.
When building with LTO, $(PLATFORM_LIBS) has to be in --whole-archive /
--no-whole-archive group, otherwise some functions declared in assembly
may not be resolved and linking may fail.
Note: clang may throw away linker list symbols it thinks are unused when
compiling with LTO. To force these symbols to be included, we refer to
them via the __ADDRESSABLE macro in a C file generated from compiled
built-in.o files before linking.
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/Kconfig b/Kconfig
index 86f0a39..f8c1a77 100644
--- a/Kconfig
+++ b/Kconfig
@@ -85,6 +85,30 @@
do what it thinks is best, which is desirable in some cases for size
reasons.
+config ARCH_SUPPORTS_LTO
+ bool
+
+config LTO
+ bool "Enable Link Time Optimizations"
+ depends on ARCH_SUPPORTS_LTO
+ default n
+ help
+ This option enables Link Time Optimization (LTO), a mechanism which
+ allows the compiler to optimize between different compilation units.
+
+ This can optimize away dead code paths, resulting in smaller binary
+ size (if CC_OPTIMIZE_FOR_SIZE is enabled).
+
+ This option is not available for every architecture and may
+ introduce bugs.
+
+ Currently, when compiling with GCC, due to a weird bug regarding
+ jobserver, the final linking will not respect make's --jobs argument.
+ Instead all available processors will be used (as reported by the
+ nproc command).
+
+ If unsure, say n.
+
config TPL_OPTIMIZE_INLINING
bool "Allow compiler to uninline functions marked 'inline' in TPL"
depends on TPL