ANDROID: kbuild: limit LTO inlining

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.
The default value is 100, and decreasing it to 5 reduces the size
of a stripped arm64 defconfig vmlinux by 11%.

Bug: 145297228
Change-Id: Iaf366f843582972a5dfadc4695abb8f9c59882af
Suggested-by: George Burgess IV <gbiv@google.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Yahya Wessam <yahyawessam2002@gmail.com>

Author: samitolvanen

Makefile

@@ -968,6 +968,12 @@ endif
 KBUILD_CFLAGS	+= -mllvm -unroll-threshold=1200
 lto-clang-flags += -fvisibility=hidden
 
+# Limit inlining across translation units to reduce binary size
+LD_FLAGS_LTO_CLANG := -mllvm -import-instr-limit=5
+
+KBUILD_LDFLAGS += $(LD_FLAGS_LTO_CLANG)
+KBUILD_LDFLAGS_MODULE += $(LD_FLAGS_LTO_CLANG)
+
 KBUILD_LDFLAGS_MODULE += -T scripts/module-lto.lds
 
 # allow disabling only clang LTO where needed