[libc] Partially implement 'quick_exit' for the GPU

[jhuber6]

Summary

This patch adds a partial implementation of quick_exit for the GPU
target. This is mainly done to test object libraries for the GPU and
will be expanded later. This will simply cause the threads to terminate
on the GPU without returning an error code. This functionality will be
added later to facilitate unit tests.

[jhuber6]

Right now this has two issues I'd like to ask how to resolve.

1. The libc/src/__support/OSUtil/gpu/quick_exit.cpp file currently cannot include the architectures header. It's not seen as in the path. I figured it just needed to depend on the common target to use it.
2. The object library is created, but not included in the final libcgpu.a. How does the logic for including object libraries in with the entrypoints work?

[jhuber6]

1. The headers weren't being added correctly and I fixed that.
2. I was accidentally using the libc.__support.OSUtil.osutil target which is just a header library.

So this works now.

libc/src/__support/OSUtil/gpu/CMakeLists.txt

+add_object_library(
+  gpu_util
+  SRCS
+    quick_exit.cpp
+  HDRS
+    quick_exit.h
+  DEPENDS
+    libc.src.__support.common
+)

libc/src/__support/OSUtil/gpu/quick_exit.h

+//===---------- GPU implementation of a quick exit function -----*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_OSUTIL_GPU_QUICK_EXIT_H
+#define LLVM_LIBC_SRC_SUPPORT_OSUTIL_GPU_QUICK_EXIT_H
+
+namespace __llvm_libc {
+
+void quick_exit(int status);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_SUPPORT_OSUTIL_GPU_QUICK_EXIT_H

libc/src/__support/OSUtil/gpu/quick_exit.cpp

+//===---------- GPU implementation of a quick exit function -----*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_OSUTIL_GPU_QUICK_EXIT_H
+#define LLVM_LIBC_SRC_SUPPORT_OSUTIL_GPU_QUICK_EXIT_H
+
+#include "quick_exit.h"
+
+#include "src/__support/macros/properties/architectures.h"
+
+namespace __llvm_libc {
+
+void quick_exit(int status) {
+#if defined(LIBC_TARGET_ARCH_IS_NVPTX)
+  asm("exit" ::: "memory");
+#elif defined(LIBC_TARGET_ARCH_IS_AMDGPU)
+  // This will terminate the entire wavefront, may not be valid with divergent
+  // work items.
+  asm("s_endpgm" ::: "memory");
+#endif
+  __builtin_unreachable();
+}
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_SUPPORT_OSUTIL_GPU_QUICK_EXIT_H

libc/src/__support/OSUtil/quick_exit.h

 //===---------- Implementation of a quick exit function ---------*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_SRC_SUPPORT_OSUTIL_QUICK_EXIT_H
 #define LLVM_LIBC_SRC_SUPPORT_OSUTIL_QUICK_EXIT_H
 
-#ifdef __unix__
+#include "src/__support/macros/properties/architectures.h"
+
+#if defined(LIBC_TARGET_ARCH_IS_GPU)
+#include "gpu/quick_exit.h"
+#elif defined(__unix__)
 #include "linux/quick_exit.h"
 #endif
 
 #endif // LLVM_LIBC_SRC_SUPPORT_OSUTIL_QUICK_EXIT_H