Question

我尝试将链接时优化与GCC的-flto标志（6.1.1）一起使用。

虽然它可以正常使用我的代码，但它不会链接到静态链接库我还建立并链接到我的项目（Engine，库是{{3} }，仅供参考）。

这是输出：

...
/usr/bin/ranlib: ir_expression_flattening.cpp.o: plugin needed to handle lto object
/usr/bin/ranlib: opt_function_inlining.cpp.o: plugin needed to handle lto object
/usr/bin/ranlib: opt_copy_propagation_elements.cpp.o: plugin needed to handle lto object
...

在那之后，当然，我得到了几个＆＃34;未定义的引用＆＃34;某些功能。

我做了一些研究，发现可能是因为ar，我应该尝试使用gcc-ar，但我不知道我该怎么做。

另外，我使用不支持lto的CMake（在某些平台上除英特尔编译器外，所以我读了......）。即便如此，我尝试使用：

set_property(TARGET glsl_optimizer PROPERTY INTERPROCEDURAL_OPTIMIZATION True)

哪个不起作用。

另外，我尝试了GCC的-fuse-linker-plugin旗帜并没有起作用。

我想我必须使用gcc-ar直接手动执行此操作，或者可能还有其他方法吗？

Answer 1

Here is an MCVE CMake project that reproduces the problem:

$ ls -R hellow
hellow:
CMakeLists.txt  hello.c  libhello.c

$ cat hellow/CMakeLists.txt 
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)


$ cat hellow/hello.c 
extern void hello(void);

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

$ cat hellow/libhello.c 
#include <stdio.h>

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

Configuration is good:

$ mkdir build_hellow
$ cd build_hellow/
$ cmake ../hellow
-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- 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: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- 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: /home/imk/dev/so/build_hellow

Build fails as per problem:

$ make
Scanning dependencies of target hello
[ 25%] Building C object CMakeFiles/hello.dir/libhello.c.o
[ 50%] Linking C static library libhello.a
/usr/bin/ar: CMakeFiles/hello.dir/libhello.c.o: plugin needed to handle lto object
/usr/bin/ranlib: libhello.c.o: plugin needed to handle lto object
[ 50%] Built target hello
Scanning dependencies of target hellow
[ 75%] Building C object CMakeFiles/hellow.dir/hello.c.o
[100%] Linking C executable hellow
/tmp/ccV0lG36.ltrans0.ltrans.o: In function `main':
<artificial>:(.text+0x5): undefined reference to `hello'
collect2: error: ld returned 1 exit status
CMakeFiles/hellow.dir/build.make:95: recipe for target 'hellow' failed
make[2]: *** [hellow] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/hellow.dir/all' failed
make[1]: *** [CMakeFiles/hellow.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

There is more than one solution. One is to uncomment the 3 commented lines in CMakeLists.txt above. Then:

$ cmake ../hellow/
-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- 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: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- 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: /home/imk/dev/so/build_hellow

$ make
Scanning dependencies of target hello
[ 25%] Building C object CMakeFiles/hello.dir/libhello.c.o
[ 50%] Linking C static library libhello.a
[ 50%] Built target hello
Scanning dependencies of target hellow
[ 75%] Building C object CMakeFiles/hellow.dir/hello.c.o
[100%] Linking C executable hellow
[100%] Built target hellow

$ ./hellow 
Hello

This fix makes use of the following facts.

The build-breaking problem:

/usr/bin/ar: CMakeFiles/hello.dir/libhello.c.o: plugin needed to handle lto object
...
/usr/bin/ranlib: libhello.c.o: plugin needed to handle lto object

can solved by giving ar and ranlib the option:

--plugin=$(gcc --print-file-name=liblto_plugin.so)

However, GNU ranlib is merely a synonym for ar -s, and gcc-ar is a wrapper for ar that supplies that plugin.

CMake's build template for a C static library is:

CMAKE_C_ARCHIVE_CREATE ( = <CMAKE_AR> qc <TARGET> <LINK_FLAGS> <OBJECTS>)
CMAKE_C_ARCHIVE_FINISH ( = <CMAKE_RANLIB> <TARGET>)

which for GNU ar is equivalent to:

CMAKE_C_ARCHIVE_CREATE ( = <CMAKE_AR> qcs <TARGET> <LINK_FLAGS> <OBJECTS>)
CMAKE_C_ARCHIVE_FINISH ( = true) # Or any other no-op command

So with these settings plus:

SET(CMAKE_AR  "gcc-ar")

we're good.

For a C++ project, of course, set CMAKE_CXX_ARCHIVE_CREATE and CMAKE_CXX_ARCHIVE_FINISH

使用GCC的链接时优化与静态链接库

1 个答案: