Link-Time Optimization (LTO) is a compiler toolchain feature that enables additional program optimizations by deferring optimization passes to link time.

Build and link a kernel with:

• WITHOUT_CTF=yes
• WITHOUT_DTRACE=yes
• sys/conf/kern.mk -flto=thin (for all clang invocations)

LTO is enabled on LLVM 3.7, but it needs the Gold binutils ld(1) plugin.

To build with LTO, change your make.conf this way:

CC=/usr/local/bin/clang37
CXX=/usr/local/bin/clang++37
CPP=/usr/local/bin/clang-cpp37
CFLAGS.clang+=-flto

NM=/usr/local/bin/nm
NMFLAGS+=--plugin /usr/local/llvm37/lib/LLVMgold.so

LD=/usr/local/bin/ld.gold
LDFLAGS+=-fuse-ld=gold -Wl,--plugin /usr/local/llvm37/lib/LLVMgold.so