[openmp/macOS] Fails to find pthread even though it's found by llvm

[Phatcat]

When compiling llvm from source with openmp:

(…) 
-- Looking for pthread_create in pthread 
-- Looking for pthread_create in pthread - found 
-- Looking for pthread_rwlock_init in pthread 
-- Looking for pthread_rwlock_init in pthread - found 
-- Looking for pthread_mutex_lock in pthread 
-- Looking for pthread_mutex_lock in pthread - found 
(…) 
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD 
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success 
-- Found Threads: TRUE 
(…) 
-- Looking for pthread_getname_np 
-- Looking for pthread_getname_np - found 
-- Looking for pthread_setname_np 
-- Looking for pthread_setname_np - found 
(…)
-- Check if compiler accepts -pthread 
-- Check if compiler accepts -pthread - no 
CMake Error at /Applications/CMake.app/Contents/share/cmake-4.0/Modules/FindPackageHandleStandardArgs.cmake:227 (message): Could NOT find Threads (missing: Threads_FOUND) Call Stack (most recent call first): /Applications/CMake.app/Contents/share/cmake-4.0/Modules/FindPackageHandleStandardArgs.cmake:591 (_FPHSA_FAILURE_MESSAGE) /Applications/CMake.app/Contents/share/cmake-4.0/Modules/FindThreads.cmake:226 (find_package_handle_standard_args) /Users/holsthein/llvm-project/openmp/runtime/cmake/config-ix.cmake:162 (find_package) /Users/holsthein/llvm-project/openmp/runtime/CMakeLists.txt:283 (include)

I then edited the FindThreads in Cmake to return true on apple without the flag and... I got this instead:

-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - not found
-- Found Threads: TRUE
CMake Error at /Users/holsthein/llvm-project/openmp/runtime/cmake/LibompUtils.cmake:26 (message):
  LIBOMP: Need pthread interface on Unix-like systems.
Call Stack (most recent call first):
  /Users/holsthein/llvm-project/openmp/runtime/cmake/config-ix.cmake:169 (libomp_error_say)
  /Users/holsthein/llvm-project/openmp/runtime/CMakeLists.txt:283 (include)

The way openmp is set up to look for and utilize threads on mac seems to be faulty?

[llvmbot]

@llvm/issue-subscribers-openmp

Author: Thorbjørn H.H. Sindrup (Phatcat)

When compiling llvm from source with openmp: ``` (…)  -- Looking for pthread_create in pthread  -- Looking for pthread_create in pthread - found  -- Looking for pthread_rwlock_init in pthread  -- Looking for pthread_rwlock_init in pthread - found  -- Looking for pthread_mutex_lock in pthread  -- Looking for pthread_mutex_lock in pthread - found  (…)  -- Performing Test CMAKE_HAVE_LIBC_PTHREAD  -- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success  -- Found Threads: TRUE  (…)  -- Looking for pthread_getname_np  -- Looking for pthread_getname_np - found  -- Looking for pthread_setname_np  -- Looking for pthread_setname_np - found  (…) -- Check if compiler accepts -pthread  -- Check if compiler accepts -pthread - no  CMake Error at /Applications/CMake.app/Contents/share/cmake-4.0/Modules/FindPackageHandleStandardArgs.cmake:227 (message): Could NOT find Threads (missing: Threads_FOUND) Call Stack (most recent call first): /Applications/CMake.app/Contents/share/cmake-4.0/Modules/FindPackageHandleStandardArgs.cmake:591 (_FPHSA_FAILURE_MESSAGE) /Applications/CMake.app/Contents/share/cmake-4.0/Modules/FindThreads.cmake:226 (find_package_handle_standard_args) /Users/holsthein/llvm-project/openmp/runtime/cmake/config-ix.cmake:162 (find_package) /Users/holsthein/llvm-project/openmp/runtime/CMakeLists.txt:283 (include) ```

I then edited the FindThreads in Cmake to return true on apple without the flag and... I got this instead:

-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - not found
-- Found Threads: TRUE
CMake Error at /Users/holsthein/llvm-project/openmp/runtime/cmake/LibompUtils.cmake:26 (message):
  LIBOMP: Need pthread interface on Unix-like systems.
Call Stack (most recent call first):
  /Users/holsthein/llvm-project/openmp/runtime/cmake/config-ix.cmake:169 (libomp_error_say)
  /Users/holsthein/llvm-project/openmp/runtime/CMakeLists.txt:283 (include)

The way openmp is set up to look for and utilize threads on mac seems to be faulty?

[shiltian]

We completely rely one CMake for finding pthread and nothing else.

llvm-project/openmp/cmake/DetectTestCompiler/CMakeLists.txt

Line 28 in cbaa359

find_package(Threads REQUIRED)

This doesn't look like an issue on libomp side.

[Phatcat]

It's nothing to do with libomp, that's probably just the first module which tries to utilize threads, it's for sure a linking issue. I thought the issue initially may be with cmake but clearly llvm finds it before compiling openmp, but then openmp later assumes because it's not taking the -pthread flag that it doesn't have threads? It has to be a linking issue somewhere.