qt5base-lts/mkspecs/features/ltcg.prf

CONFIG(release, debug|release) {
    # We need fat object files when creating static libraries on some platforms
    # so the linker will know to load a particular object from the library
    # in the first place. On others, we have special ar and nm to create the symbol
    # tables so the linker will know  better. For other compilers, we disable LTCG
    # for static libraries.
    msvc {
        # Nothing to do
    } else {
        !isEmpty(QMAKE_AR_LTCG): QMAKE_AR = $$QMAKE_AR_LTCG
        !isEmpty(QMAKE_NM_LTCG): QMAKE_NM = $$QMAKE_NM_LTCG

        equals(QMAKE_RANLIB_LTCG, true): QMAKE_RANLIB =
        else: !isEmpty(QMAKE_RANLIB_LTCG): QMAKE_RANLIB = $$QMAKE_RANLIB_LTCG

        static:isEmpty(QMAKE_AR_LTCG):isEmpty(QMAKE_RANLIB_LTCG) {
            # We don't know how to create symbol tables for static libs with this
            # compiler, so disable LTCG
            QMAKE_CFLAGS_LTCG =
            QMAKE_CXXFLAGS_LTCG =
            QMAKE_LFLAGS_LTCG =
        }
    }

    fat-lto {
        QMAKE_CFLAGS_LTCG += $$QMAKE_CFLAGS_LTCG_FATOBJECTS
        QMAKE_CXXFLAGS_LTCG += $$QMAKE_CXXFLAGS_LTCG_FATOBJECTS
    }

    load(link_ltcg)
    QMAKE_CFLAGS *= $$QMAKE_CFLAGS_LTCG
    QMAKE_CXXFLAGS *= $$QMAKE_CXXFLAGS_LTCG
    QMAKE_LFLAGS *= $$QMAKE_LFLAGS_LTCG
}