How do I enable link time optimization (LTO) with CMake?

Edit: as of 28 October, 2015 on CMake IRC

jcelerier | I have a question about INTERPROCEDURAL_OPTIMIZATION
jcelerier | it sounds like it should enable -flto on gcc
+ngladitz | jcelerier: its only implemented for the intel compiler
jcelerier | ngladitz: ah, okay
jcelerier | are there other switches for pgo / lto ?
jcelerier | or must it be done by hand ?
+ngladitz | there currently is no first class support otherwise

Related CMake issue: https://gitlab.kitware.com/cmake/cmake/issues/15245

CMake has the INTERPROCEDURAL_OPTIMIZATION property on targets, which sounds like it may enable LTO on some platforms.

Link : http://www.cmake.org/cmake/help/v3.0/prop_tgt/INTERPROCEDURAL_OPTIMIZATION.html

To enable it on a target MyLib :

add_library(MyLib ...)
...
set_property(TARGET MyLib PROPERTY INTERPROCEDURAL_OPTIMIZATION True)

Good news! CMake v3.9 finally supports LTO.

Example

Here's an example code to show how it works:

cmake_minimum_required(VERSION 3.9.4)

include(CheckIPOSupported)
check_ipo_supported(RESULT supported OUTPUT error)

add_executable(example Example.cpp)

if( supported )
    message(STATUS "IPO / LTO enabled")
    set_property(TARGET example PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
else()
    message(STATUS "IPO / LTO not supported: <${error}>")
endif()

For GCC this adds -flto -fno-fat-lto-objects to the targets compile commands.

Checking Compiler Support

The Module CheckIPOSupported provides checking whether interprocedural optimization (IPO/LTO) is supported by the compiler or not:

check_ipo_supported([RESULT <result>] [OUTPUT <output>]
                    [LANGUAGES <lang>...])

If no arguments is passed (= check_ipo_supported()) an error is raised to indicate it's not supported, otherwise the result variable is set to either YES or NO. More details are described in the documentation of the module.

Enabling LTO

LTO is enabled either for a single target or as default for all targets.

LTO for a single target

To enable LTO for a target set INTERPROCEDURAL_OPTIMIZATION to TRUE. This is done by the set_property() command:

set_property(TARGET name-target-here
             PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)

LTO as default

It's possible to enable LTO per default by setting CMAKE_INTERPROCEDURAL_OPTIMIZATION to TRUE:

set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)

This will enable INTERPROCEDURAL_OPTIMIZATION for all targets created after this line. Those created before are not affected.

See also

  • CMake issue
  • Related Pull Requests

Tags:

Cmake

Lto

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