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Make lto detection more robust and not resetting cxx flags when lto fails.

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This refines the last commit of parallelizing lto.

Discussion: this is ugly as hell, dispatching 1/ on the availability of
the `-flto` flag, then 2/ on the compiler since GCC allows `-flto=n`
whereas Clang for example does not.

I tried setting the CMake property `INTERPROCEDURAL_OPTIMIZATION`,
without any effect. All I could see was some lto related utilities in
the cmake debug output, but not in the actual compiler or linker
invocation.

This would eliminate the need for our hacks, with 1/ using an option
`WITH_LTO` setting `ON` by default, and based on this value setting the
`INTERPROCEDURAL_OPTIMIZATION` flag with CMake doing the actual work of
selecting the best LTO method on the target platform.

By the way, this also fixes a bug where we reset the `CMAKE_CXX_FLAGS`
to a variable that was never defined, resulting in setting the flags to
an empty string. Yay CMake, as usual.

References:

- http://www.cmake.org/cmake/help/v3.0/prop_tgt/INTERPROCEDURAL_OPTIMIZATION.html
This commit is contained in:
Daniel J. Hofmann 2015-09-04 20:59:53 +02:00
parent 941483c14d
commit 71a00fc01b
1 changed files with 11 additions and 6 deletions
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17
CMakeLists.txt
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@ -124,16 +124,21 @@ endif()
if(CMAKE_BUILD_TYPE MATCHES Release) if(CMAKE_BUILD_TYPE MATCHES Release)
message(STATUS "Configuring OSRM in release mode") message(STATUS "Configuring OSRM in release mode")
# Check if LTO is available # Check if LTO is available
include(ProcessorCount) check_cxx_compiler_flag("-flto" LTO_AVAILABLE)
ProcessorCount(NPROC)
set(LTO_FLAGS "")
check_cxx_compiler_flag("-flto=${NPROC}" LTO_AVAILABLE)
if(LTO_AVAILABLE) if(LTO_AVAILABLE)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto=${NPROC}") set(OLD_CXX_FLAGS ${CMAKE_CXX_FLAGS})
# GCC in addition allows parallelizing LTO
if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
include(ProcessorCount)
ProcessorCount(NPROC)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto=${NPROC}")
else()
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto")
endif()
set(CHECK_LTO_SRC "int main(){return 0;}") set(CHECK_LTO_SRC "int main(){return 0;}")
check_cxx_source_compiles("${CHECK_LTO_SRC}" LTO_WORKS) check_cxx_source_compiles("${CHECK_LTO_SRC}" LTO_WORKS)
if(LTO_WORKS) if(LTO_WORKS)
message(STATUS "LTO working") message(STATUS "LTO working")
else() else()
message(STATUS "LTO broken") message(STATUS "LTO broken")
set(CMAKE_CXX_FLAGS "${OLD_CXX_FLAGS}") set(CMAKE_CXX_FLAGS "${OLD_CXX_FLAGS}")