[OpenMP] Implicitly include the 'cgpu' and 'mgpu' libraries for OpenMP (#67557)

Summary:
[The LLVM C library for GPUs](https://libc.llvm.org/gpu/) supports
standard function calls on the GPU that users are familiar with.
Currently, this requires that users include these manually. The support
for this library is dependent upon whether or not associated LLVM build
was built with the GPU C library support. This patch implicitly adds
these for an OpenMP offloading compilation if we find that the toolchain
contains the GPU declarations that allow us to use this.

I do not know how to test this, given that it requires information from
the resource directory in the install. That means it won't be present
for any internal tests. It works when I test it and the idea is simple
enough so it should be simple enough.

Author: jhuber6

clang/include/clang/Driver/Options.td

@@ -5055,6 +5055,9 @@ def nogpulib : Flag<["-"], "nogpulib">, MarshallingInfoFlag<LangOpts<"NoGPULib">
   Visibility<[ClangOption, CC1Option]>,
   HelpText<"Do not link device library for CUDA/HIP device compilation">;
 def : Flag<["-"], "nocudalib">, Alias<nogpulib>;
+def gpulibc : Flag<["-"], "gpulibc">, Visibility<[ClangOption, CC1Option]>,
+  HelpText<"Link the LLVM C Library for GPUs">;
+def nogpulibc : Flag<["-"], "nogpulibc">, Visibility<[ClangOption, CC1Option]>;
 def nodefaultlibs : Flag<["-"], "nodefaultlibs">;
 def nodriverkitlib : Flag<["-"], "nodriverkitlib">;
 def nofixprebinding : Flag<["-"], "nofixprebinding">;

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -869,6 +869,26 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
                          /*IsLTO=*/true, PluginOptPrefix);
 }
 
+/// Adds the '-lcgpu' and '-lmgpu' libraries to the compilation to include the
+/// LLVM C library for GPUs.
+static void addOpenMPDeviceLibC(const ToolChain &TC, const ArgList &Args,
+                                ArgStringList &CmdArgs) {
+  if (Args.hasArg(options::OPT_nogpulib) || Args.hasArg(options::OPT_nolibc))
+    return;
+
+  // Check the resource directory for the LLVM libc GPU declarations. If it's
+  // found we can assume that LLVM was built with support for the GPU libc.
+  SmallString<256> LibCDecls(TC.getDriver().ResourceDir);
+  llvm::sys::path::append(LibCDecls, "include", "llvm_libc_wrappers",
+                          "llvm-libc-decls");
+  bool HasLibC = llvm::sys::fs::exists(LibCDecls) &&
+                 llvm::sys::fs::is_directory(LibCDecls);
+  if (Args.hasFlag(options::OPT_gpulibc, options::OPT_nogpulibc, HasLibC)) {
+    CmdArgs.push_back("-lcgpu");
+    CmdArgs.push_back("-lmgpu");
+  }
+}
+
 void tools::addOpenMPRuntimeLibraryPath(const ToolChain &TC,
                                         const ArgList &Args,
                                         ArgStringList &CmdArgs) {
@@ -939,6 +959,9 @@ bool tools::addOpenMPRuntime(ArgStringList &CmdArgs, const ToolChain &TC,
   if (IsOffloadingHost && !Args.hasArg(options::OPT_nogpulib))
     CmdArgs.push_back("-lomptarget.devicertl");
 
+  if (IsOffloadingHost)
+    addOpenMPDeviceLibC(TC, Args, CmdArgs);
+
   addArchSpecificRPath(TC, Args, CmdArgs);
   addOpenMPRuntimeLibraryPath(TC, Args, CmdArgs);
 

clang/test/Driver/openmp-offload-gpu.c

@@ -387,3 +387,20 @@
 // RUN:   | FileCheck --check-prefix=XARCH-DEVICE %s
 // XARCH-DEVICE: "-cc1" "-triple" "nvptx64-nvidia-cuda"{{.*}}"-O3"
 // XARCH-DEVICE-NOT: "-cc1" "-triple" "x86_64-unknown-linux-gnu"{{.*}}"-O3"
+
+//
+// Check that `-gpulibc` includes the LLVM C libraries for the GPU.
+//
+// RUN:   %clang -### --target=x86_64-unknown-linux-gnu -fopenmp=libomp \
+// RUN:      --libomptarget-nvptx-bc-path=%S/Inputs/libomptarget/libomptarget-nvptx-test.bc \
+// RUN:      --cuda-path=%S/Inputs/CUDA_102/usr/local/cuda \
+// RUN:      --offload-arch=sm_52 -gpulibc -nogpuinc %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=LIBC-GPU %s
+// LIBC-GPU: "-lcgpu"{{.*}}"-lmgpu"
+
+// RUN:   %clang -### --target=x86_64-unknown-linux-gnu -fopenmp=libomp \
+// RUN:      --libomptarget-nvptx-bc-path=%S/Inputs/libomptarget/libomptarget-nvptx-test.bc \
+// RUN:      --cuda-path=%S/Inputs/CUDA_102/usr/local/cuda \
+// RUN:      --offload-arch=sm_52 -nogpulibc -nogpuinc %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=NO-LIBC-GPU %s
+// NO-LIBC-GPU-NOT: "-lcgpu"{{.*}}"-lmgpu"