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

Question

I have the most recent cmake build and trying all build configurations (Debug, MinSizeRel, RelWithDebugInfo, Release, General) I see nowhere (text search) in generated makefiles

Answer 1

Good news! CMake v3.9 finally supports LTO.

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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.

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Checking Compiler Support

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

check_ipo_supported([RESULT ] [OUTPUT 
]
                    [LANGUAGES ...])

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.

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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.

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See also

• CMake issue
• Related Pull Requests