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 the string -lto, so or the functionality is still not present, or it requires manual intervertion (in that case a text search for LTO or Link time optimization) over the documentation gives no result, so I see nowhere in official documentation a way to enable LTO.

Is there any way to enable LTO "globally" (for all compilers) without specifying manually flags that could be at worst supported only by GCC?

1

5 Answers

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

8

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:

---

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

Link :

To enable it on a target MyLib :

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

4

Yep, according to source code there are no support for LTO for gcc/clang compilers. They have some support for Intel compiler only.

I have some grep output for cmake 3.4.3:

grep -HiR 'INTERPROCEDURAL_OPTIMIZATION' *
Help/manual/cmake-properties.7.rst:
/prop_dir/INTERPROCEDURAL_OPTIMIZATION_CONFIG
Help/manual/cmake-properties.7.rst:
/prop_dir/INTERPROCEDURAL_OPTIMIZATION
Help/manual/cmake-properties.7.rst:
/prop_tgt/INTERPROCEDURAL_OPTIMIZATION_CONFIG
Help/manual/cmake-properties.7.rst:
/prop_tgt/INTERPROCEDURAL_OPTIMIZATION
Help/prop_dir/INTERPROCEDURAL_OPTIMIZATION.rst:INTERPROCEDURAL_OPTIMIZATION
Help/prop_dir/INTERPROCEDURAL_OPTIMIZATION_CONFIG.rst:INTERPROCEDURAL_OPTIMIZATION_<CONFIG
Help/prop_dir/INTERPROCEDURAL_OPTIMIZATION_CONFIG.rst:This is a
per-configuration version of INTERPROCEDURAL_OPTIMIZATION.
Help/prop_tgt/INTERPROCEDURAL_OPTIMIZATION.rst:INTERPROCEDURAL_OPTIMIZATION
Help/prop_tgt/INTERPROCEDURAL_OPTIMIZATION_CONFIG.rst:INTERPROCEDURAL_OPTIMIZATION_<CONFIG
Help/prop_tgt/INTERPROCEDURAL_OPTIMIZATION_CONFIG.rst:This is a
per-configuration version of INTERPROCEDURAL_OPTIMIZATION.
Modules/Platform/Linux-Intel.cmake: # INTERPROCEDURAL_OPTIMIZATION
Source/cmCommonTargetGenerator.cxx:
if(this->GetFeatureAsBool("INTERPROCEDURAL_OPTIMIZATION"))
Source/cmGeneratorTarget.cxx:
"INTERPROCEDURAL_OPTIMIZATION", config))
Source/cmMakefileLibraryTargetGenerator.cxx:
if(this->GetFeatureAsBool("INTERPROCEDURAL_OPTIMIZATION") &&
Tests/IPO/CMakeLists.txt:set_property(DIRECTORY PROPERTY
INTERPROCEDURAL_OPTIMIZATION 1)

As you can see we have only Intel platform module.

So I have created feature request for this extremely useful feature:

Revisiting this problem again in a project where I had to include the cmake configuration of an immutable subproject, it seems the most forceful way to enable LTO is adding this before defining any targets or including anything:

set(CMAKE_POLICY_DEFAULT_CMP0069 NEW)
set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)

This enables LTO and shuts up CMAKE even if later included projects did not handle LTO via CheckIPOSupported themselves.

At the moment both gcc and clang support lto. (See: ) You can enable it in via cmake: set (CMAKE_INTERPROCEDURAL_OPTIMIZATION True) However you need to have version cmake >= 3.9, and I think may be set some policy and check whether LTO is supported by the compiler / toolchain ( check CMAKE_IPO_AVAILABLE and throw error otherwise)

And set flags for gcc/clang ( -flto and -ffat-lto-files for fcc >4.7 or -flto=full/thin for clang)

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