gollvm: -fPIC is not supported, if using ld or lld, with Clang++ #41527
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ianlancetaylor
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CC @thanm @cherrymui |
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For some reason it looks as though some of the cmake config checks that LLVM runs (this is outside of the gollvm cmake rules) are using -Werror (but only for C++ compiles, not for C compiles). So what's happening is that the cmake rules run "clang++ -c foo.cpp -fuse-ld=..." and clang warns because the linker option is not used ... and then the warning is turned into an error. I guess I would tack on "-Qunused-arguments" in this situation in addition to "-fuse-ld=xxx". Also, not sure what the intended effect of "-fuse-ld=ld" is. The argument of this option should be one of {bfd,gold,ldd}. |
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@lattner , @topperc, @RKSimon, @arsenm, @echristo , @tobiasgrosser , @EricWF , @ppc : could you clarify an actual summary, for specific linkers, without CMake's auto-detection? |
It seems possible to invoke the C compiler in a way that `-fuse-linker` is considered unused, and this is often the way that flags are probed. This was keeping the LLVM `-fPIC` check from working properly. The go folks reported a similar issue: golang/go#41527
It seems possible to invoke the C compiler in a way that `-fuse-linker` is considered unused, and this is often the way that flags are probed. This was keeping the LLVM `-fPIC` check from working properly. The go folks reported a similar issue: golang/go#41527
Discovered an issue working in StableHLO when attempting to build with `-Wall -Werror`: openxla/stablehlo#137 Currently, if `LLVM_USE_LLD` and `-DCMAKE_CXX_FLAGS="${CMAKE_CXX_FLAGS} -Wall -Werror"` are both specified for build, the build will error with: ``` -- Performing Test CXX_SUPPORTS_CUSTOM_LINKER - Failed CMake Error at /usr/local/google/home/gleasonk/Coding/llvm-build/lib/cmake/llvm/HandleLLVMOptions.cmake:309 (message): Host compiler does not support '-fuse-ld=lld' ... $ cat <build_dir>/CMakeFiles/CMakeError.log ... clang: error: argument unused during compilation: '-fuse-ld=lld' [-Werror,-Wunused-command-line-argument] ``` It looks like other repos have hit this same issue: - golang/go#41527 (mentioned in comment) - iree-org/iree#7450 This can be reproduced in llvm-project with the following build command: ``` # Compile command taken from https://mlir.llvm.org/getting_started/ # and modified for use case cmake -G Ninja ../llvm \ -DLLVM_ENABLE_PROJECTS=mlir \ -DLLVM_BUILD_EXAMPLES=ON \ -DLLVM_TARGETS_TO_BUILD="X86;NVPTX;AMDGPU" \ -DCMAKE_BUILD_TYPE=Release \ -DLLVM_ENABLE_ASSERTIONS=ON \ -DCMAKE_CXX_COMPILER=clang++ \ -DCMAKE_C_COMPILER=clang \ -DLLVM_ENABLE_LLD=ON \ -DCMAKE_CXX_FLAGS="${CMAKE_CXX_FLAGS} -Wall -Werror" ``` Reviewed By: mehdi_amini Differential Revision: https://reviews.llvm.org/D134206
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Can this be closed? |
I was figuring out linker's options.
So if I am properly dealing with CMake's flags
should be what I tried.
And lld is invoked via
If I do like this:
There is no clear/current summary on -fPIC flag support, in some practical manner. I am proposing that it should be documented.
Ivan
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