Do not set an RPATH on statically-linked LLVM executables.

[rsmith]

Summary

This RPATH breaks the ability to run executables from the build area if
the host compiler provides a libc++ and we also build an
ABI-incompatible libc++ as part of the same monorepo compilation.

(This only works around the problem, which still exists for the
LLVM_ENABLE_SHARED configuration.)

[chandlerc]

(Will likely need more eyes than just mine -- RPATH is mostly a mystery to me...)

llvm/cmake/modules/AddLLVM.cmake

758

Just to make sure I understand correctly, *any* time we build a shared library of some form, we'll get here? Even if its libclang.so or some such?

Provided I've understood correctly, then this make sense. However, I wonder, won't this still fail for things like c-index-test?

[mgorny]

llvm/cmake/modules/AddLLVM.cmake

756

I think this is also needed when building with BUILD_SHARED_LIBS. Possibly also when the target explicitly links to one of the shared-only libraries.

[rnk]

This RPATH breaks the ability to run executables from the build area if
the host compiler provides a libc++ and we also build an
ABI-incompatible libc++ as part of the same monorepo compilation.

This is the RPATH we set:

set(_install_rpath "\$ORIGIN/../lib${LLVM_LIBDIR_SUFFIX}" ${extra_libdir})

I believe LLVM_LIBDIR_SUFFIX is typically empty.

This is a pretty normal convention, and it should work from the build directory and the install directory. It just says, if there are shared libraries, go to ${root}/bin/../lib/ and search for any shared libs there first.

I think maybe we shouldn't build libc++, and any other shared libraries that might conflict with system shared libraries, into ${builddir}/lib. Don't we install libcxx into lib/clang/9.0.0/lib/... in the end anyway, or is libcxx not treated as a compiler runtime library?

llvm/cmake/modules/AddLLVM.cmake

756

Yep.

758

I think we only come here for executables, but yes, your concern about c-index-test seems warranted. In the standard, static configuration, libclang seems to be a shared library, and c-index-test links to it. Without the rpath, it probably won't find it. But, this code sets the "install rpath", and I think cmake actually goes back and tweaks the rpath when it does the install step, so maybe this won't affect that.

[rsmith]

In D63637#1553781, @rnk wrote:

I think maybe we shouldn't build libc++, and any other shared libraries that might conflict with system shared libraries, into ${builddir}/lib.

Yeah, that'd be my preference, but Chandler pointed out that that would be surprising for people who build libc++ in order to use it with GCC. Maybe we can solve that problem with better build logic, though.

Don't we install libcxx into lib/clang/9.0.0/lib/... in the end anyway, or is libcxx not treated as a compiler runtime library?

We don't appear to. I think that would be reasonable for Clang's uses, but I don't think we have any meaningful idea of whether we're building libc++ standalone or as a Clang runtime.

Perhaps (for a monorepo checkout):

• Default to building runtime libraries into the clang runtimes directory
• Provide a flag to build standalone runtimes
• If the flag is specified, then runtimes instead go into <build area>/lib, and we implicitly disable the build of llvm, clang, lldb, lld, and so on in that checkout (eg, llvm_add_executable and llvm_add_library could add build rules that fail with a suitable error).

If we run cmake directly on (eg) the libc++ directory (not as part of a llvm + clang build), it could still default to building standalone runtimes.

Would that make sense?

llvm/cmake/modules/AddLLVM.cmake

macro(add_llvm_executable name)
   if( EXCLUDE_FROM_ALL )
     add_executable(${name} EXCLUDE_FROM_ALL ${ALL_FILES})
   else()
     add_executable(${name} ${ALL_FILES})
   endif()
 
   setup_dependency_debugging(${name} ${LLVM_COMMON_DEPENDS})
 
+  if (LLVM_LINK_LLVM_DYLIB AND NOT ARG_DISABLE_LLVM_LINK_LLVM_DYLIB)
+    set(USE_SHARED USE_SHARED)
     if(NOT ARG_NO_INSTALL_RPATH)
       llvm_setup_rpath(${name})
     endif()
+  endif()
 
   if(DEFINED windows_resource_file)
     set_windows_version_resource_properties(${name} ${windows_resource_file})
   endif()
 
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT LLVM_ENABLE_OBJLIB)
     llvm_update_compile_flags(${name})
   endif()
   add_link_opts( ${name} )
 
   # Do not add -Dname_EXPORTS to the command-line when building files in this
   # target. Doing so is actively harmful for the modules build because it
   # creates extra module variants, and not useful because we don't use these
   # macros.
   set_target_properties( ${name} PROPERTIES DEFINE_SYMBOL "" )
 
   if (LLVM_EXPORTED_SYMBOL_FILE)
     add_llvm_symbol_exports( ${name} ${LLVM_EXPORTED_SYMBOL_FILE} )
   endif(LLVM_EXPORTED_SYMBOL_FILE)
 
-  if (LLVM_LINK_LLVM_DYLIB AND NOT ARG_DISABLE_LLVM_LINK_LLVM_DYLIB)
-    set(USE_SHARED USE_SHARED)
-  endif()
-
   set(EXCLUDE_FROM_ALL OFF)
   set_output_directory(${name} BINARY_DIR ${LLVM_RUNTIME_OUTPUT_INTDIR} LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
   llvm_config( ${name} ${USE_SHARED} ${LLVM_LINK_COMPONENTS} )
   if( LLVM_COMMON_DEPENDS )
     add_dependencies( ${name} ${LLVM_COMMON_DEPENDS} )
   endif( LLVM_COMMON_DEPENDS )
 
   if(NOT ARG_IGNORE_EXTERNALIZE_DEBUGINFO)