[PATCH v7 04/17] kbuild: lto: limit inlining

[Sami Tolvanen]

This change limits function inlining across translation unit boundaries
in order to reduce the binary size with LTO. The -import-instr-limit
flag defines a size limit, as the number of LLVM IR instructions, for
importing functions from other TUs, defaulting to 100.

Based on testing with arm64 defconfig, we found that a limit of 5 is a
reasonable compromise between performance and binary size, reducing the
size of a stripped vmlinux by 11%.

Suggested-by: George Burgess IV <gbiv@xxxxxxxxxx>
Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Makefile b/Makefile
index f27c0da5d05a..bee378f9fd50 100644
--- a/Makefile
+++ b/Makefile
@@ -901,6 +901,9 @@ else
 CC_FLAGS_LTO	+= -flto
 endif
 CC_FLAGS_LTO	+= -fvisibility=default
+
+# Limit inlining across translation units to reduce binary size
+KBUILD_LDFLAGS += -mllvm -import-instr-limit=5
 endif
 
 ifdef CONFIG_LTO
-- 
2.29.2.299.gdc1121823c-goog