[PATCH v6 12/25] kbuild: lto: limit inlining

From: Sami Tolvanen
Date: Mon Oct 12 2020 - 22:41:00 EST


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 91cd6caefa6e..41b01248d400 100644
--- a/Makefile
+++ b/Makefile
@@ -894,6 +894,9 @@ 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
--
2.28.0.1011.ga647a8990f-goog