[PATCH v2 12/28] 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 at google.com>
Signed-off-by: Sami Tolvanen <samitolvanen at google.com>
---
 Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Makefile b/Makefile
index 2752be67b460..c69e07bd506a 100644
--- a/Makefile
+++ b/Makefile
@@ -917,6 +917,10 @@ else
 CC_FLAGS_LTO_CLANG := -flto
 endif
 CC_FLAGS_LTO_CLANG += -fvisibility=default
+
+# Limit inlining across translation units to reduce binary size
+LD_FLAGS_LTO_CLANG := -mllvm -import-instr-limit=5
+KBUILD_LDFLAGS += $(LD_FLAGS_LTO_CLANG)
 endif
 
 ifdef CONFIG_LTO
-- 
2.28.0.402.g5ffc5be6b7-goog