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[v2,12/28] kbuild: lto: limit inlining

Message ID 20200903203053.3411268-13-samitolvanen@google.com
State New, archived
Headers show
Series Add support for Clang LTO | expand

Commit Message

Sami Tolvanen Sept. 3, 2020, 8:30 p.m. UTC
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@google.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 Makefile | 4 ++++
 1 file changed, 4 insertions(+)

Comments

Kees Cook Sept. 3, 2020, 10:20 p.m. UTC | #1
On Thu, Sep 03, 2020 at 01:30:37PM -0700, Sami Tolvanen wrote:
> 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@google.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>
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Patch

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