[OpenMP] Don't pass empty files to nvlink

[jhuber6]

Summary

This patch adds and exception to the nvlink wrapper tool to not pass
empty cubin files to the nvlink job. If an empty file is passed to
nvlink it will cause an error indicating that the file could not be
opened. This would occur if the user tried to link object files that
contained offloading code with a file that didnt. This will act as a
workaround until the new OpenMP offloading driver becomes the default.

[jdoerfert]

test?

[jhuber6]

Adding test

[jdoerfert]

LG

clang/test/Driver/fat_archive_nvptx.cpp

 // REQUIRES: clang-driver
 // REQUIRES: x86-registered-target
 // REQUIRES: nvptx-registered-target
 
 // See the steps to create a fat archive are given at the end of the file.
 
 // Given a FatArchive, clang-offload-bundler should be called to create a
 // device specific archive, which should be passed to clang-nvlink-wrapper.
 // RUN: %clang -O2 -### -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda %s -L%S/Inputs/openmp_static_device_link -lFatArchive 2>&1 | FileCheck %s
 // CHECK: clang{{.*}}"-cc1"{{.*}}"-triple" "nvptx64-nvidia-cuda"{{.*}}"-target-cpu" "[[GPU:sm_[0-9]+]]"{{.*}}"-o" "[[HOSTBC:.*.s]]" "-x" "c++"{{.*}}.cpp
 // CHECK: clang-offload-bundler" "-unbundle" "-type=a" "-inputs={{.*}}/Inputs/openmp_static_device_link/libFatArchive.a" "-targets=openmp-nvptx64-nvidia-cuda-[[GPU]]" "-outputs=[[DEVICESPECIFICARCHIVE:.*.a]]" "-allow-missing-bundles"
 // CHECK: clang-nvlink-wrapper{{.*}}"-o" "{{.*}}.out" "-arch" "[[GPU]]" "{{.*}}[[DEVICESPECIFICARCHIVE]]"
-// expected-no-diagnostics
+// RUN: not %clang -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda %s %S/Inputs/openmp_static_device_link/empty.o --libomptarget-nvptx-bc-path=%S/Inputs/openmp_static_device_link/lib.bc 2>&1 | FileCheck %s --check-prefix=EMPTY
+// EMPTY-NOT: Could not open input file
 
 #ifndef HEADER
 #define HEADER
 
 #define N 10
 
 #pragma omp declare target
 // Functions defined in Fat Archive.

clang/tools/clang-nvlink-wrapper/ClangNvlinkWrapper.cpp

 static cl::opt<std::string> NvlinkUserPath("nvlink-path",
                                            cl::desc("Path of nvlink binary"),
                                            cl::cat(ClangNvlinkWrapperCategory));
 
 // Do not parse nvlink options
 static cl::list<std::string>
     NVArgs(cl::Sink, cl::desc("<options to be passed to nvlink>..."));
 
+static bool isEmptyFile(StringRef Filename) {
+  ErrorOr<std::unique_ptr<MemoryBuffer>> BufOrErr =
+      MemoryBuffer::getFileOrSTDIN(Filename, false, false);
+  if (std::error_code EC = BufOrErr.getError())
+    return false;
+  return (*BufOrErr)->getBuffer().empty();
+}
+
 static Error runNVLink(std::string NVLinkPath,
                        SmallVectorImpl<std::string> &Args) {
   std::vector<StringRef> NVLArgs;
   NVLArgs.push_back(NVLinkPath);
+  StringRef Output = *(llvm::find(Args, "-o") + 1);
   for (auto &Arg : Args) {
+    if (!(sys::fs::exists(Arg) && Arg != Output && isEmptyFile(Arg)))
       NVLArgs.push_back(Arg);
   }
 
   if (sys::ExecuteAndWait(NVLinkPath, NVLArgs))
     return createStringError(inconvertibleErrorCode(), "'nvlink' failed");
   return Error::success();
 }
 
 static Error extractArchiveFiles(StringRef Filename,