Undefined References with Link-Time Optimization

Question

I am trying to link a static library with an executable file, following this example, but on MinGW-w64.

My CMakeLists file: (note that this is identical to the one in that answer)

cmake_minimum_required (VERSION 2.6)
project (hellow)
SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -flto")
SET(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -flto")
SET(CMAKE_AR  "gcc-ar")
SET(CMAKE_C_ARCHIVE_CREATE "<CMAKE_AR> qcs <TARGET> <LINK_FLAGS> <OBJECTS>")
SET(CMAKE_C_ARCHIVE_FINISH   true)
add_library(hello STATIC libhello.c)
add_executable(hellow hello.c)
target_link_libraries(hellow hello)
add_dependencies(hellow hello)

hello.c:

extern void hello(void);

int main(void) {
    hello();
    return 0;
}

libhello.c:

#include <stdio.h>

void hello(void) {
    puts("Hello");
}

Configuration works as expected:

-- The C compiler identification is GNU 7.1.0
-- The CXX compiler identification is GNU 7.1.0
-- Check for working C compiler: C:/Program Files (x86)/mingw-w64/i686-7.1.0-win32-dwarf-rt_v5-rev2/mingw32/bin/gcc.exe
-- Check for working C compiler: C:/Program Files (x86)/mingw-w64/i686-7.1.0-win32-dwarf-rt_v5-rev2/mingw32/bin/gcc.exe -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: C:/Program Files (x86)/mingw-w64/i686-7.1.0-win32-dwarf-rt_v5-rev2/mingw32/bin/g++.exe
-- Check for working CXX compiler: C:/Program Files (x86)/mingw-w64/i686-7.1.0-win32-dwarf-rt_v5-rev2/mingw32/bin/g++.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: C:/.../project

When compiling as-is, however, the following error is generated:

Scanning dependencies of target hello
[ 25%] Building C object CMakeFiles/hello.dir/libhello.c.obj
[ 50%] Linking C static library libhello.a
Error running link command: The system cannot find the file specified
mingw32-make.exe[3]: *** [CMakeFiles\hello.dir\build.make:95: libhello.a] Error 2
mingw32-make.exe[3]: *** Deleting file 'libhello.a'
mingw32-make.exe[2]: *** [CMakeFiles\Makefile2:67: CMakeFiles/hello.dir/all] Error 2
mingw32-make.exe[1]: *** [CMakeFiles\Makefile2:116: CMakeFiles/hellow.dir/rule] Error 2
mingw32-make.exe: *** [Makefile:130: hellow] Error 2

Removing the SET(CMAKE_C_ARCHIVE_FINISH true) line in CMakeLists.txt file and recompiling results in this "undefined reference" error:

[ 25%] Linking C static library libhello.a
[ 50%] Built target hello
Scanning dependencies of target hellow
[ 75%] Building C object CMakeFiles/hellow.dir/hello.c.obj
[100%] Linking C executable hellow.exe
C:\...\Local\Temp\ccPctpZp.ltrans0.ltrans.o: In function `main':
E:/Documents/MONAD/projects/ltotest/hello.c:4: undefined reference to `hello'
collect2.exe: error: ld returned 1 exit status
mingw32-make.exe[3]: *** [CMakeFiles\hellow.dir\build.make:97: hellow.exe] Error 1
mingw32-make.exe[2]: *** [CMakeFiles\Makefile2:104: CMakeFiles/hellow.dir/all] Error 2
mingw32-make.exe[1]: *** [CMakeFiles\Makefile2:116: CMakeFiles/hellow.dir/rule] Error 2
mingw32-make.exe: *** [Makefile:130: hellow] Error 2

Running nm libhello.a shows that libhello.c is being compiled with LTO:

libhello.c.obj:
00000000 b .bss
00000000 d .data
00000000 r .gnu.lto_.decls.d7da3e90
00000000 r .gnu.lto_.inline.d7da3e90
00000000 r .gnu.lto_.opts
00000000 r .gnu.lto_.refs.d7da3e90
00000000 r .gnu.lto_.symbol_nodes.d7da3e90
00000000 r .gnu.lto_.symtab.d7da3e90
00000000 r .gnu.lto_hello.d7da3e90
00000000 r .rdata$zzz
00000000 t .text
00000001 C ___gnu_lto_slim
00000001 C ___gnu_lto_v1

This seems to be an issue only on MinGW, as the code seemed to work fine in the answer linked previously (although I haven't been able to test it myself). Any ideas on what's going wrong/how to work around it?

@HansPassant gcc-ar.exe is a wrapper for ar.exe that provides the gcc LTO plugin (see http://www.tin.org/bin/man.cgi?section=1&topic=gcc-ar-5) — tomb, Oct 01 '17 at 12:58
@HansPassant Yep, it can be found (I have checked, and besides, I'd expect to get an error mentioning an unknown exe if it couldn't). — tomb, Oct 01 '17 at 13:04

score 4 · Accepted Answer · answered Oct 01 '17 at 13:04

Did some more research, and stumbled upon this page, which mentions that it's also necessary to use a wrapper for ranlib.

Sure enough, changing CMakeLists.txt to the following contents allows the project to build successfully:

cmake_minimum_required (VERSION 2.6)
project (hellow)
SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -flto")
SET(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -flto")
SET(CMAKE_AR  "gcc-ar")
SET(CMAKE_RANLIB "gcc-ranlib")
SET(CMAKE_C_ARCHIVE_CREATE "<CMAKE_AR> qcs <TARGET> <LINK_FLAGS> <OBJECTS>")
SET(CMAKE_C_ARCHIVE_FINISH "<CMAKE_RANLIB> <TARGET>")
add_library(hello STATIC libhello.c)
add_executable(hellow hello.c)
target_link_libraries(hellow hello)
add_dependencies(hellow hello)

(notice the new SET(CMAKE_RANLIB "gcc-ranlib") line)

Additionally it seems like the SET(CMAKE_C_ARCHIVE_FINISH "<CMAKE_RANLIB> <TARGET>") line is not necessary, as this is the default value for CMAKE_C_ARCHIVE_FINISH.

The gcc docu also mentions `The next version of binutils will support automatic loading of the liblto_plugin`, so with a recent binutils the wrapper shouldn't be necessary. — ssbssa, Oct 03 '17 at 10:18
I would upvote twice if I could. Exactly what is needed on older ubuntus/binutils (16.04). — oarfish, Oct 12 '20 at 11:52

Undefined References with Link-Time Optimization

1 Answers1