[OpenMP] Respect LLVM per-target install directories (#83282)

Summary:
One recurring problem we have with the OpenMP libraries is that they are
potentially conflicting with ones found on the system, this occurs when
there are two copies and one is used for linking that it not attached to
the correspoding clang compiler. LLVM already uses target specific
directories for this, like with libc++, which are always searched first.
This patch changes the install directory to be
`lib/x86_64-unknown-linux-gnu` for example.

Notable changes would be that users will need to change their
LD_LIBRARY_PATH settings optionally, or use default rt-rpath options.
This should fix problems were users are linking the wrong versions of
static libraries

Author: jhuber6

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -2763,13 +2763,13 @@ void tools::addOpenMPDeviceRTL(const Driver &D,
                                const llvm::opt::ArgList &DriverArgs,
                                llvm::opt::ArgStringList &CC1Args,
                                StringRef BitcodeSuffix,
-                               const llvm::Triple &Triple) {
+                               const llvm::Triple &Triple,
+                               const ToolChain &HostTC) {
   SmallVector<StringRef, 8> LibraryPaths;
 
-  // Add path to clang lib / lib64 folder.
-  SmallString<256> DefaultLibPath = llvm::sys::path::parent_path(D.Dir);
-  llvm::sys::path::append(DefaultLibPath, CLANG_INSTALL_LIBDIR_BASENAME);
-  LibraryPaths.emplace_back(DefaultLibPath.c_str());
+  // Check all of the standard library search paths used by the compiler.
+  for (const auto &LibPath : HostTC.getFilePaths())
+    LibraryPaths.emplace_back(LibPath);
 
   // Add user defined library paths from LIBRARY_PATH.
   std::optional<std::string> LibPath =

clang/lib/Driver/ToolChains/CommonArgs.h

@@ -214,7 +214,8 @@ void addMachineOutlinerArgs(const Driver &D, const llvm::opt::ArgList &Args,
 
 void addOpenMPDeviceRTL(const Driver &D, const llvm::opt::ArgList &DriverArgs,
                         llvm::opt::ArgStringList &CC1Args,
-                        StringRef BitcodeSuffix, const llvm::Triple &Triple);
+                        StringRef BitcodeSuffix, const llvm::Triple &Triple,
+                        const ToolChain &HostTC);
 
 void addOutlineAtomicsArgs(const Driver &D, const ToolChain &TC,
                            const llvm::opt::ArgList &Args,

clang/lib/Driver/ToolChains/Cuda.cpp

@@ -903,7 +903,7 @@ void CudaToolChain::addClangTargetOptions(
       return;
 
     addOpenMPDeviceRTL(getDriver(), DriverArgs, CC1Args, GpuArch.str(),
-                       getTriple());
+                       getTriple(), HostTC);
   }
 }
 

openmp/CMakeLists.txt

@@ -46,9 +46,15 @@ if (OPENMP_STANDALONE_BUILD)
   set(CMAKE_CXX_EXTENSIONS NO)
 else()
   set(OPENMP_ENABLE_WERROR ${LLVM_ENABLE_WERROR})
-  # If building in tree, we honor the same install suffix LLVM uses.
-  set(OPENMP_INSTALL_LIBDIR "lib${LLVM_LIBDIR_SUFFIX}" CACHE STRING
-      "Path where built OpenMP libraries should be installed.")
+
+  # When building in tree we install the runtime according to the LLVM settings.
+  if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
+    set(OPENMP_INSTALL_LIBDIR lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE} CACHE STRING
+        "Path where built openmp libraries should be installed.")
+  else()
+    set(OPENMP_INSTALL_LIBDIR "lib${LLVM_LIBDIR_SUFFIX}" CACHE STRING
+        "Path where built OpenMP libraries should be installed.")
+  endif()
 
   if (NOT MSVC)
     set(OPENMP_TEST_C_COMPILER ${LLVM_RUNTIME_OUTPUT_INTDIR}/clang)