LinkTimeOptimization [FreeBSD WIKI]

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===== Introduction =====

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

===== Kernel =====

[[https://docs.freebsd.org/en/books/handbook/kernelconfig/#kernelconfig-building|Build and link a kernel]] with:

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

===== Ports =====

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:

<code>
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

</code>

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{{tag>Stale Performance}}