[libc] Implement the RPC client / server for NVPTX

This patch adds the necessary code to impelement the existing RPC client
/ server interface when targeting NVPTX GPUs. This follows closely to
the implementation in the AMDGPU version. This does not yet enable unit
testing as the `nvlink` linker does not support static libraries. So
that will need to be worked around.

I am ignoring the RPC duplication between the AMDGPU and NVPTX loaders. This
will be changed completely later so there's no point unifying the code at this
stage. The implementation was tested manually with the following file and
compilation flags.

```
namespace __llvm_libc {
void write_to_stderr(const char *msg);
void quick_exit(int);
} // namespace __llvm_libc

using namespace __llvm_libc;

int main(int argc, char **argv, char **envp) {
  for (int i = 0; i < argc; ++i) {
    write_to_stderr(argv[i]);
    write_to_stderr("\n");
  }
  quick_exit(255);
}
```

```
$ clang++ crt1.o rpc_client.o quick_exit.o io.o main.cpp --target=nvptx64-nvidia-cuda -march=sm_70 -o image
$ ./nvptx_loader image 1 2 3
image
1
2
3
$ echo $?
255
```

Depends on D146681

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D146846

2 files changed, 7 insertions, 1 deletions

diff --git a/libc/startup/gpu/nvptx/CMakeLists.txt b/libc/startup/gpu/nvptx/CMakeLists.txt
index 1ee2108..0fe0b2d 100644
--- a/libc/startup/gpu/nvptx/CMakeLists.txt
+++ b/libc/startup/gpu/nvptx/CMakeLists.txt
@@ -2,6 +2,8 @@ add_startup_object(
   crt1
   SRC
     start.cpp
+  DEPENDS
+    libc.src.__support.RPC.rpc_client
   COMPILE_OPTIONS
     -ffreestanding # To avoid compiler warnings about calling the main function.
     -fno-builtin
diff --git a/libc/startup/gpu/nvptx/start.cpp b/libc/startup/gpu/nvptx/start.cpp
index 1e7f4ca..b09d6f6 100644
--- a/libc/startup/gpu/nvptx/start.cpp
+++ b/libc/startup/gpu/nvptx/start.cpp
@@ -1,4 +1,4 @@
-//===-- Implementation of crt for amdgpu ----------------------------------===//
+//===-- Implementation of crt for nvptx -----------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -6,10 +6,14 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "src/__support/RPC/rpc_client.h"
+
 extern "C" int main(int argc, char **argv, char **envp);
 
 extern "C" [[gnu::visibility("protected")]] __attribute__((nvptx_kernel)) void
 _start(int argc, char **argv, char **envp, int *ret, void *in, void *out,
        void *buffer) {
+  __llvm_libc::rpc::client.reset(in, out, buffer);
+
   __atomic_fetch_or(ret, main(argc, argv, envp), __ATOMIC_RELAXED);
 }