[libcxx-commits] [PATCH] D112112: [libunwind] Link with -unwindlib=none

[Petr Hosek via Phabricator via libcxx-commits]

phosek added a comment.

In D112112#3074743 <https://reviews.llvm.org/D112112#3074743>, @mstorsjo wrote:

> In D112112#3074741 <https://reviews.llvm.org/D112112#3074741>, @mstorsjo wrote:
>
>> If inspecting `CMakeFiles/CMakeError.log` to see what happened when testing the `-unwindlib=none` flag, I find this:
>>
>>   Performing C SOURCE FILE Test LIBUNWIND_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG failed with the following output:
>>   Change Dir: /build/llvm-project/libunwind/build-i686-shared/CMakeFiles/CMakeTmp
>>   
>>   Run Build Command(s):/usr/bin/ninja cmTC_94016 && [1/2] Building C object CMakeFiles/cmTC_94016.dir/src.c.obj
>>   [2/2] Linking C executable cmTC_94016.exe
>>   FAILED: cmTC_94016.exe
>>   : && /opt/llvm-mingw/bin/i686-w64-mingw32-clang   CMakeFiles/cmTC_94016.dir/src.c.obj -o cmTC_94016.exe -Wl,--out-implib,libcmTC_94016.dll.a -Wl,--major-image-version,0,--minor-image-version,0  -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 && :
>>   lld: error: unable to find library -lunwind
>>   lld: error: unable to find library -lunwind
>>   clang-13: error: linker command failed with exit code 1 (use -v to see invocation)
>>   ninja: build stopped: subcommand failed.
>>
>> During this particular test, CMake does apply the tested flag, `-unwindlib=none`, when compiling the test object (`CMakeFiles/cmTC_94016.dir/src.c.obj`) although that's not visible in the log. But it's not applying that flag when it does linking of the test executable. See e.g. https://gitlab.kitware.com/cmake/cmake/-/issues/15934 for discussion on that issue.
>>
>> I'm curious to know how it succeeds in your case, if it's linking in `-lunwind` explicitly when linking the final libunwind library, but not while trying the `check_c_compiler_flag()`. Is it a case where `-lunwind` is added implicitly only when linking as C++ but not C? (As far as I've read the clang driver, if it adds it implicitly for either of them, it should add it for both.)
>
> FWIW, it seems like there's more useful info in `CMakeFiles/CMakeError.log` about what compile command it did use, when testing `check_c_compiler_flag(-unwindlib=none)`, if using the makefile generator instead of the ninja one - as ninja hides the commands that are run except for the commands that actually resulted in an error.

@mstorsjo CMake introduced `CMAKE_REQUIRED_LINK_OPTIONS` to address specifically this issue (see https://cmake.org/cmake/help/latest/module/CheckCSourceCompiles.html), but unfortunately it was only introduced in 3.14 and LLVM is still on 3.13.

In my testing, in CMake versions <3.14 `CMAKE_REQUIRED_FLAGS` apply to link command as well but in >=3.14 you need to use `CMAKE_REQUIRED_LINK_OPTIONS` so I think we'll need to have conditional logic based on the CMake version.


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