[lld] Ensure we link to the threading library used by std::thread (e.g. pthreads).

[dsanders]

Summary

This fixes the BUILD_SHARED_LIBS=ON build. We must link to the threading
library even when LLVM_ENABLE_THREADS is OFF.

[ruiu]

Currently LLD does not spawn threads using std::thread if LLVM_ENABLE_THREADS=OFF, but it surely includes <thread> even if the flag is OFF. Is anyone who thinks that's wrong in the first place?

I'm OK with this patch, but I'd like to get experts opinions.

[dsanders]

FWIW, it seems odd that we'd need pthreads when we don't use threads. I've just had a look at the preprocessed source for COFF/SymbolTable.cpp and the pthread_create reference comes from this line:

__asm ("" : : "r" (&pthread_create));

which was in the <thread> that was included by <future>.

[vkalintiris]

In D14165#277751, @ruiu wrote:

I'm OK with this patch, but I'd like to get experts opinions.

FWIW, this solves pr25064 (https://llvm.org/bugs/show_bug.cgi?id=25064).

[• rafael]

Which C++ library is that? On my system (gcc 4.9) running

grep -r '&pthread_create' /usr/include/c++/

finds nothing.

And in the preprocessed file:

$ grep pthread_create SymbolTable.ii
extern int pthread_create (pthread_t *restrict newthread,
static typeof(pthread_create) gthrw_pthread_create attribute
((weakref("pthread_create")));

return __gthrw_pthread_create (__threadid, __null, __func, __args);

Cheers,
Rafael

[dsanders]

It's the libstdc++-4.9-dev in Debian Jessie (amd64). The package version is 4.9.2-10.

CMakeLists.txt

if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
   install(DIRECTORY include/
     DESTINATION include
     FILES_MATCHING
     PATTERN "*.h"
     PATTERN ".svn" EXCLUDE
     )
 endif()
 
+# Detect libraries needed to use std::thread. This seems to be necessary for
+# LLVM_ENABLE_THREADS=OFF too.
+find_package(Threads)
+
 add_subdirectory(lib)
 add_subdirectory(tools)
 
 if (LLVM_INCLUDE_TESTS)
   add_subdirectory(test)
   add_subdirectory(unittests)
 endif()
 
 add_subdirectory(docs)
 add_subdirectory(COFF)
 add_subdirectory(ELF)

COFF/CMakeLists.txt

add_llvm_library(lldCOFF
   LTO
   LibDriver
   Object
   MC
   MCDisassembler
   Target
   Option
   Support
+  LINK_LIBS
+    ${CMAKE_THREAD_LIBS_INIT}
   )
 
 add_dependencies(lldCOFF COFFOptionsTableGen)

lib/Driver/CMakeLists.txt

LINK_LIBS
     lldExampleSubTarget
     lldX86_64ELFTarget
     lldCore
     lldReaderWriter
     lldYAML
     LLVMObject
     LLVMOption
     LLVMSupport
+    ${CMAKE_THREAD_LIBS_INIT}
   )
 
 add_dependencies(lldDriver DriverOptionsTableGen)
 

lib/ReaderWriter/CMakeLists.txt

add_llvm_library(lldReaderWriter
   CoreLinkingContext.cpp
   FileArchive.cpp
   LinkerScript.cpp
   LINK_LIBS
     lldCore
     lldYAML
     LLVMObject
     LLVMSupport
+    ${CMAKE_THREAD_LIBS_INIT}
   )

lib/ReaderWriter/ELF/CMakeLists.txt

add_llvm_library(lldELF
   TargetLayout.cpp
   Writer.cpp
   LINK_LIBS
     lldReaderWriter
     lldCore
     lldYAML
     LLVMObject
     LLVMSupport
+    ${CMAKE_THREAD_LIBS_INIT}
   )
 
 include_directories(.)
 
 add_subdirectory(X86)
 add_subdirectory(X86_64)
 add_subdirectory(Mips)
 add_subdirectory(Hexagon)
 add_subdirectory(AArch64)
 add_subdirectory(ARM)
 add_subdirectory(AMDGPU)

lib/ReaderWriter/MachO/CMakeLists.txt

add_llvm_library(lldMachO
   StubsPass.cpp
   TLVPass.cpp
   WriterMachO.cpp
   LINK_LIBS
     lldCore
     lldYAML
     LLVMObject
     LLVMSupport
+    ${CMAKE_THREAD_LIBS_INIT}
   )
 
 include_directories(.)

unittests/CMakeLists.txt

 add_custom_target(LLDUnitTests)
 set_target_properties(LLDUnitTests PROPERTIES FOLDER "lld tests")
 
 # add_lld_unittest(test_dirname file1.cpp file2.cpp)
 #
 # Will compile the list of files together and link against lld
 # Produces a binary named 'basename(test_dirname)'.
 function(add_lld_unittest test_dirname)
   add_unittest(LLDUnitTests ${test_dirname} ${ARGN})
-  target_link_libraries(${test_dirname} ${LLVM_COMMON_LIBS})
+  target_link_libraries(${test_dirname}
+                        ${LLVM_COMMON_LIBS}
+                        ${CMAKE_THREAD_LIBS_INIT})
 endfunction()
 
 add_subdirectory(CoreTests)
 add_subdirectory(DriverTests)
 add_subdirectory(MachOTests)