[libc] Add basic support for calling host functions from the GPU

This patch adds the `rpc_host_call` function as a GPU extension. This is
exported from the `libc` project to use the RPC interface to call a
function pointer via RPC any copying the arguments by-value. The
interface can only support a single void pointer argument much like
pthreads. The function call here is the bare-bones version of what's
required for OpenMP reverse offloading. Full support will require
interfacing with the mapping table, nowait support, etc.

I decided to test this interface in `libomptarget` as that will be the
primary consumer and it would be more difficult to make a test in `libc`
due to the testing infrastructure not really having a concept of the
"host" as it runs directly on the GPU as if it were a CPU target.

Reviewed By: jplehr

Differential Revision: https://reviews.llvm.org/D155003

1 files changed, 30 insertions, 0 deletions

diff --git a/libc/src/gpu/rpc_host_call.cpp b/libc/src/gpu/rpc_host_call.cpp
new file mode 100644
index 0000000..67b839d
--- /dev/null
+++ b/libc/src/gpu/rpc_host_call.cpp
@@ -0,0 +1,30 @@
+//===---------- GPU implementation of the external RPC call function ------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/gpu/rpc_host_call.h"
+
+#include "llvm-libc-types/rpc_opcodes_t.h"
+#include "src/__support/GPU/utils.h"
+#include "src/__support/RPC/rpc_client.h"
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+// This calls the associated function pointer on the RPC server with the given
+// arguments. We expect that the pointer here is a valid pointer on the server.
+LLVM_LIBC_FUNCTION(void, rpc_host_call, (void *fn, void *data, size_t size)) {
+  rpc::Client::Port port = rpc::client.open<RPC_HOST_CALL>();
+  port.send_n(data, size);
+  port.send([=](rpc::Buffer *buffer) {
+    buffer->data[0] = reinterpret_cast<uintptr_t>(fn);
+  });
+  port.recv([](rpc::Buffer *) {});
+  port.close();
+}
+
+} // namespace __llvm_libc