[OpenMP] Change build of OpenMP device runtime to be a separate runtime

Summary:
Currently we build the OpenMP device runtime as part of the `offload/`
project. This is problematic because it has several restrictions when
compared to the normal offloading runtime. It can only be built with an
up-to-date clang and we need to set the target appropriately. Currently
we hack around this by creating the compiler invocation manually, but
this patch moves it into a separate runtimes build.

This follows the same build we use for libc, libc++, compiler-rt, and
flang-rt. This also moves it from `offload/` into `openmp/` because it
is still the `openmp/` runtime and I feel it is more appropriate. We do
want a generic `offload/` library at some point, but it would be trivial
to then add that as a separate library now that we have the
infrastructure that makes adding these new libraries trivial.

This most importantly will require that users update their build
configs, mostly adding the following lines at a minimum. I was debating
whether or not I should 'auto-upgrade' this, but I just went with a
warning.

```
    -DLLVM_RUNTIME_TARGETS='default;amdgcn-amd-amdhsa;nvptx64-nvidia-cuda'     \
    -DRUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES=openmp \
    -DRUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES=openmp \
```

This also changed where the `.bc` version of the library lives, but it's
still created.

Author: jhuber6

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -2794,6 +2794,11 @@ void tools::addOpenMPDeviceRTL(const Driver &D,
   for (const auto &LibPath : HostTC.getFilePaths())
     LibraryPaths.emplace_back(LibPath);
 
+  // Check the target specific library path for the triple as well.
+  SmallString<128> P(D.Dir);
+  llvm::sys::path::append(P, "..", "lib", Triple.getTriple());
+  LibraryPaths.emplace_back(P);
+
   OptSpecifier LibomptargetBCPathOpt =
       Triple.isAMDGCN()  ? options::OPT_libomptarget_amdgpu_bc_path_EQ
       : Triple.isNVPTX() ? options::OPT_libomptarget_nvptx_bc_path_EQ

offload/CMakeLists.txt

@@ -113,6 +113,13 @@ else()
   set(CMAKE_CXX_EXTENSIONS NO)
 endif()
 
+# Emit a warning for people who haven't updated their build.
+if(NOT "openmp" IN_LIST RUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES AND
+   NOT "openmp" IN_LIST RUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES)
+  message(WARNING "Building the offloading runtime with no device library. See "
+                  "https://openmp.llvm.org//SupportAndFAQ.html for help.")
+endif()
+
 # Set the path of all resulting libraries to a unified location so that it can
 # be used for testing.
 set(LIBOMPTARGET_LIBRARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
@@ -373,7 +380,6 @@ set(LIBOMPTARGET_LLVM_LIBRARY_INTDIR "${LIBOMPTARGET_INTDIR}" CACHE STRING
 
 # Build offloading plugins and device RTLs if they are available.
 add_subdirectory(plugins-nextgen)
-add_subdirectory(DeviceRTL)
 add_subdirectory(tools)
 
 # Build target agnostic offloading library.

offload/DeviceRTL/CMakeLists.txt

@@ -5,5 +5,5 @@ set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR ON CACHE BOOL "")
 set(LLVM_RUNTIME_TARGETS default;amdgcn-amd-amdhsa;nvptx64-nvidia-cuda CACHE STRING "") 
 set(RUNTIMES_nvptx64-nvidia-cuda_CACHE_FILES "${CMAKE_SOURCE_DIR}/../libcxx/cmake/caches/NVPTX.cmake" CACHE STRING "")
 set(RUNTIMES_amdgcn-amd-amdhsa_CACHE_FILES "${CMAKE_SOURCE_DIR}/../libcxx/cmake/caches/AMDGPU.cmake" CACHE STRING "")
-set(RUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi" CACHE STRING "")
-set(RUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi" CACHE STRING "")
+set(RUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES "compiler-rt;libc;openmp;libcxx;libcxxabi" CACHE STRING "")
+set(RUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES "compiler-rt;libc;openmp;libcxx;libcxxabi" CACHE STRING "")

offload/cmake/caches/Offload.cmake

@@ -88,6 +88,14 @@ else()
   set(CMAKE_CXX_EXTENSIONS NO)
 endif()
 
+# Targeting the GPU directly requires a few flags to make CMake happy.
+if("${CMAKE_CXX_COMPILER_TARGET}" MATCHES "^amdgcn")
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nogpulib")
+elseif("${CMAKE_CXX_COMPILER_TARGET}" MATCHES "^nvptx")
+  set(CMAKE_REQUIRED_FLAGS
+      "${CMAKE_REQUIRED_FLAGS} -flto -c -Wno-unused-command-line-argument")
+endif()
+
 # Check and set up common compiler flags.
 include(config-ix)
 include(HandleOpenMPOptions)
@@ -122,35 +130,41 @@ else()
   get_clang_resource_dir(LIBOMP_HEADERS_INSTALL_PATH SUBDIR include)
 endif()
 
-# Build host runtime library, after LIBOMPTARGET variables are set since they are needed
-# to enable time profiling support in the OpenMP runtime.
-add_subdirectory(runtime)
-
-set(ENABLE_OMPT_TOOLS ON)
-# Currently tools are not tested well on Windows or MacOS X.
-if (APPLE OR WIN32)
-  set(ENABLE_OMPT_TOOLS OFF)
-endif()
+# Use the current compiler target to determine the appropriate runtime to build.
+if("${LLVM_DEFAULT_TARGET_TRIPLE}" MATCHES "^amdgcn|^nvptx" OR
+   "${CMAKE_CXX_COMPILER_TARGET}" MATCHES "^amdgcn|^nvptx")
+  add_subdirectory(device)
+else()
+  # Build host runtime library, after LIBOMPTARGET variables are set since they
+  # are needed to enable time profiling support in the OpenMP runtime.
+  add_subdirectory(runtime)
+
+  set(ENABLE_OMPT_TOOLS ON)
+  # Currently tools are not tested well on Windows or MacOS X.
+  if (APPLE OR WIN32)
+    set(ENABLE_OMPT_TOOLS OFF)
+  endif()
 
-option(OPENMP_ENABLE_OMPT_TOOLS "Enable building ompt based tools for OpenMP."
-       ${ENABLE_OMPT_TOOLS})
-if (OPENMP_ENABLE_OMPT_TOOLS)
-  add_subdirectory(tools)
-endif()
+  option(OPENMP_ENABLE_OMPT_TOOLS "Enable building ompt based tools for OpenMP."
+         ${ENABLE_OMPT_TOOLS})
+  if (OPENMP_ENABLE_OMPT_TOOLS)
+    add_subdirectory(tools)
+  endif()
 
-# Propagate OMPT support to offload
-if(NOT ${OPENMP_STANDALONE_BUILD})
-  set(LIBOMP_HAVE_OMPT_SUPPORT ${LIBOMP_HAVE_OMPT_SUPPORT} PARENT_SCOPE)
-  set(LIBOMP_OMP_TOOLS_INCLUDE_DIR ${LIBOMP_OMP_TOOLS_INCLUDE_DIR} PARENT_SCOPE)
-endif()
+  # Propagate OMPT support to offload
+  if(NOT ${OPENMP_STANDALONE_BUILD})
+    set(LIBOMP_HAVE_OMPT_SUPPORT ${LIBOMP_HAVE_OMPT_SUPPORT} PARENT_SCOPE)
+    set(LIBOMP_OMP_TOOLS_INCLUDE_DIR ${LIBOMP_OMP_TOOLS_INCLUDE_DIR} PARENT_SCOPE)
+  endif()
 
-option(OPENMP_MSVC_NAME_SCHEME "Build dll with MSVC naming scheme." OFF)
+  option(OPENMP_MSVC_NAME_SCHEME "Build dll with MSVC naming scheme." OFF)
 
-# Build libompd.so
-add_subdirectory(libompd)
+  # Build libompd.so
+  add_subdirectory(libompd)
 
-# Build documentation
-add_subdirectory(docs)
+  # Build documentation
+  add_subdirectory(docs)
 
-# Now that we have seen all testsuites, create the check-openmp target.
-construct_check_openmp_target()
+  # Now that we have seen all testsuites, create the check-openmp target.
+  construct_check_openmp_target()
+endif()