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Hello,

One of my package builds with cmake.

All is fine except that the built static library gets the no-code-sections error complaint from lintian.
It appears [1,2] that /usr/share/cmake-3.22/Modules/Compiler/GNU.cmake enforces
-fno-fat-lto-objects unconditionally for GCC + IPO

What is the easiest way to revert the -fno-fat-lto-objects flags ?

Thanks in advance,
Jerome



[1] https://gitlab.kitware.com/cmake/cmake/-/issues/21696
[2] https://stackoverflow.com/questions/70806677/why-does-cmake-set-no-fat-lto-objects-when-i-enable-lto-ipo
--
Jerome BENOIT | calculus+at-rezozer^dot*net https://qa.debian.org/developer.php?login=calculus@rezozer.net
AE28 AE15 710D FF1D 87E5 A762 3F92 19A6 7F36 C68B


--o2OSMtTBFAAQcBkQ1wEySPEcOs8TA3DTa--

-----BEGIN PGP SIGNATURE-----

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Hi Jerome,

* Jerome BENOIT <calculus@rezozer.net> [2022-03-16 22:37]:

Hello,

One of my package builds with cmake.

All is fine except that the built static library gets the no-code-sections error complaint from lintian.
It appears [1,2] that /usr/share/cmake-3.22/Modules/Compiler/GNU.cmake enforces
-fno-fat-lto-objects unconditionally for GCC + IPO

What is the easiest way to revert the -fno-fat-lto-objects flags ?

I recommend you explicitly disable LTO for the static library with:

set_target_properties(staticlib PROPERTIES
INTERPROCEDURAL_OPTIMIZATION OFF)

LTO does not really achieve anything for static libraries: They are
merely an archive of object files, so the linker is never invoked on
them. You might think that LTO is useful when the static library is
linked with an executable eventually. Alas, the LTO intermediate
code is very compiler-specific, so it will not work with a different
compiler and most likely not even with a different version of the
same compiler.

Alternatively, you could add a snippet such as

if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
target_compile_options(staticlib PRIVATE -ffat-lto-objects)
endif()

This will work because the target-specific compile options happen to
end up after the LTO compile options on the command line right now.
However, dh_strip will remove the intermediate code anyway, for the
reasons outlined above.


Cheers
Timo

--
⢀⣴⠾⠻⢶⣦⠀ ╭────────────────────────────────────────────────────╮
⣾⠁⢠⠒⠀⣿⡁ │ Timo Röhling │
⢿⡄⠘⠷⠚⠋⠀ │ 9B03 EBB9 8300 DF97 C2B1 23BF CC8C 6BDD 1403 F4CA │
⠈⠳⣄⠀⠀⠀⠀ ╰────────────────────────────────────────────────────╯

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This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --wMMZe9Mp86ub5ljSqnh4GKdUCTElgVtCq
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: quoted-printable

Hi Timo, thanks a lot for the hint.
Cheers,
Jerome

On 16/03/2022 23:21, Timo Röhling wrote:

Hi Jerome,

* Jerome BENOIT <calculus@rezozer.net> [2022-03-16 22:37]:

Hello,

One of my package builds with cmake.

All is fine except that the built static library gets the no-code-sections error complaint from lintian.
It appears [1,2] that /usr/share/cmake-3.22/Modules/Compiler/GNU.cmake enforces
-fno-fat-lto-objects unconditionally for GCC + IPO

What is the easiest way to revert the  -fno-fat-lto-objects flags ?

I recommend you explicitly disable LTO for the static library with:

   set_target_properties(staticlib PROPERTIES
                         INTERPROCEDURAL_OPTIMIZATION OFF)

LTO does not really achieve anything for static libraries: They are
merely an archive of object files, so the linker is never invoked on
them. You might think that LTO is useful when the static library is
linked with an executable eventually. Alas, the LTO intermediate
code is very compiler-specific, so it will not work with a different
compiler and most likely not even with a different version of the
same compiler.

Alternatively, you could add a snippet such as

    if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
        target_compile_options(staticlib PRIVATE -ffat-lto-objects)
    endif()

This will work because the target-specific compile options happen to
end up after the LTO compile options on the command line right now.
However, dh_strip will remove the intermediate code anyway, for the
reasons outlined above.

Cheers
Timo

--
Jerome BENOIT | calculus+at-rezozer^dot*net https://qa.debian.org/developer.php?login=calculus@rezozer.net
AE28 AE15 710D FF1D 87E5 A762 3F92 19A6 7F36 C68B


--wMMZe9Mp86ub5ljSqnh4GKdUCTElgVtCq--

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