How to solve libatomic issue in windows?

Question

I am developing executables from source code of llvm. So I downloaded the llvm source code from github.

I am trying everything from command line on Windows OS

I am following the link for libtooling in clang

http://clang.llvm.org/docs/LibASTMatchersTutorial.html

I tried with 2 options

Option First: I ran the below command
cmake -G Ninja "C:\Users\amith.ks\Desktop\Clang-llvm\llvm-project\llvm" -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -DLLVM_BUILD_TESTS=ON -DCMAKE_BUILD_TYPE=Release

Everything Worked..

Second option: I wan to set cmake_c_compiler and cmake_cxx_compiler from command line.
I dont want to use cmake-gui so I run the below command
cmake -G Ninja "C:\Users\amith.ks\Desktop\Clang-llvm\llvm-project\llvm" -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -DLLVM_BUILD_TESTS=ON -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++

From out of no where error was thrown

CMake Error at cmake/modules/CheckAtomic.cmake:53 (message):
  Host compiler appears to require libatomic, but cannot find it.
Call Stack (most recent call first):
  cmake/config-ix.cmake:343 (include)
  CMakeLists.txt:617 (include)

When I saw the cmake error log It was saying this

LINK : fatal error LNK1104: cannot open file 'atomic.lib'

clang: error: linker command failed with exit code 1104 (use -v to see invocation)

ninja: build stopped: subcommand failed.

I searched my whole pc atomic.lib no where to be found.

How to solve this issue on windows? Please help me with answers.

facetus · Accepted Answer · 2020-09-29 07:57:58Z

4

I know why it fails. It's a bug in the LLVM's cmake files. In order to check if atomic.lib is required CheckAtomic.cmake tries compiling and linking a piece of code with atomic.lib. When it fails (because atomic.lib is not required and thus is not present) it falsely concludes that atomic.lib is needed. LLVM's cmake files is a hot mess with rarely fixed bugs. There are bugs staying there for years. I guess this is because nobody understands anymore how LLVM build system works.

Long story short, I couldn't find why CheckAtomic.cmake inserts atomic.lib while checking that it is not required. As a workaround I just unconditionally set HAVE_CXX_ATOMICS64_WITHOUT_LIB and HAVE_CXX_ATOMICS_WITHOUT_LIB to True in CheckAtomic.cmake:

set(HAVE_CXX_ATOMICS64_WITHOUT_LIB True)
set(HAVE_CXX_ATOMICS_WITHOUT_LIB True)

edited Sep 29, 2020 at 7:57

answered Sep 29, 2020 at 3:19

facetus

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2 Comments

Vlad Over a year ago

These two Cmake settings helped me to solve llvm build on macOS11.3. My issue - github.com/vgorloff/swift-everywhere-toolchain/issues/111

facetus Over a year ago

Thanks for quoting this post in the commit! :)

lulle2007200 · Accepted Answer · 2019-08-20 21:15:29Z

2

Use clang-cl (or MSVC cl), they dont require lib atomic.

answered Aug 20, 2019 at 21:15

lulle2007200

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Comments

nyanpasu64 · Accepted Answer · 2020-10-22 16:42:56Z

I'm trying to use Clang 11 (with MSVC ABI) to compile Clang 11. However, MSVC's headers won't compile in C++11 mode, which CheckAtomic.cmake uses:

set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++11")

results in:

C:\Users\nyanpasu\code\llvm-project>clang  -Werror=unguarded-availability-new -std=c++11 uwu.cpp
In file included from uwu.cpp:1:
In file included from C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.27.29110\include\atomic:19:
In file included from C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.27.29110\include\xatomic.h:13:
In file included from C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.27.29110\include\type_traits:11:
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.27.29110\include\xstddef:283:22: error: 'auto' return
      without trailing return type; deduced return types are a C++14 extension
_NODISCARD constexpr auto _Unfancy(_Ptrty _Ptr) noexcept { // converts from a fancy pointer to a plain pointer
                     ^
1 error generated.

I don't know if this is a MSVC bug where it ships an header incompatible with C++11, or a Clang bug where it's using MSVC headers instead of its own.

In any case, changing c++11 to c++14 fixes this error.

Kuri · Accepted Answer · 2020-07-16 23:03:27Z

0

As lulle mentioned, you need to use the right tools to compile. If you have installed Visual Studio, use the Developer Command prompt for Visual Studio instead of a regular cmd and you'll have the environment variables you need. It is located in your Visual Studio installation directory, under the Tools subdirectory.

For example:

C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\Common7\Tools\VsDevCmd.bat

answered Jul 16, 2020 at 23:03

Kuri

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Tarindu Jayatilaka · Accepted Answer · 2021-07-19 12:17:17Z

0

I encountered the same error in macOS. Adding the CPP Libraries to the path solved it for me. You can try executing the following commands.

export SDKROOT="$(xcrun --sdk macosx --show-sdk-path)"

export CPLUS_INCLUDE_PATH=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1

answered Jul 19, 2021 at 12:17

Tarindu Jayatilaka

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