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cmakeg++flags

CMake, g++ : set CXX Flags -02 but -03 is still there?

i think want to add specific compiler cxx flags to the release-mode. I read here in another thread that -O2 are good flags for release configuration

set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -Wall -O2")

but if i check now the CXX Flags

message(${CMAKE_CXX_FLAGS_RELEASE})

he write me

-O3 -DNDEBUG -Wall -O2
  1. Did it make sense using -02 instead of -03 ?
  2. How can i delete -03 from the Flags?
  3. What is DNDEBUG using for?

best regards

Solution

  • Use compiler documentation to see difference between O2 and O3 and make your choice (: for example - gcc. Here you can found recommendation to use O2 for stability.

    You can use this macro for removing flags:

    macro(remove_cxx_flag flag)
  string(REPLACE "${flag}" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
endmacro()

    [usage]

      macro(remove_cxx_flag flag)
    string(REPLACE "${flag}" "" CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE}")
  endmacro()

  message(${CMAKE_CXX_FLAGS_RELEASE}) # print "-O3 -DNDEBUG"
  remove_cxx_flag("-O3")
  message(${CMAKE_CXX_FLAGS_RELEASE}) # print "-DNDEBUG"

    Here is used macro because you need to update variable from parent scope, read this - http://www.cmake.org/cmake/help/v2.8.10/cmake.html#command:macro (or you can use function with PARENT_SCOPE modifier)

    NDEBUG used for disabling assert, see What is the NDEBUG preprocessor macro used for (on different platforms)? for more info.