[libc] Implement 'getenv' on the GPU target (#102376)

Summary:
This patch implements 'getenv'. I was torn on how to implement this,
since realistically we only have access to this environment pointer in
the "loader" interface. An alternative would be to use an RPC call every
time, but I think that's overkill for what this will be used for. A
better solution is just to emit a common `DataEnvironment` that contains
all of the host visible resources to initialize. Right now this is the
`env_ptr`, `clock_freq`, and `rpc_client`.

I did this by making the `app.h` interface that Linux uses more general,
could possibly move that into a separate patch, but I figured it's
easier to see with the usage.

Author: jhuber6

libc/config/CMakeLists.txt

@@ -1,3 +1,7 @@
-#TODO: Properly select the correct subdirectory.
-
-add_subdirectory(linux)
+add_header_library(
+  app_h
+  HDRS
+    app.h
+  DEPENDS
+    libc.src.__support.common
+)

libc/config/app.h

@@ -0,0 +1,20 @@
+//===-- Classes to capture properites of applications -----------*- 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_CONFIG_APP_H
+#define LLVM_LIBC_CONFIG_APP_H
+
+#include "src/__support/macros/properties/architectures.h"
+
+#if defined(LIBC_TARGET_ARCH_IS_GPU)
+#include "gpu/app.h"
+#elif defined(__linux__)
+#include "linux/app.h"
+#endif
+
+#endif // LLVM_LIBC_CONFIG_APP_H

libc/config/gpu/app.h

@@ -0,0 +1,28 @@
+//===-- Classes to capture properites of GPU applications -------*- 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_CONFIG_GPU_APP_H
+#define LLVM_LIBC_CONFIG_GPU_APP_H
+
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/architectures.h"
+
+#include <stdint.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+// TODO: Move other global values here and export them to the host.
+struct DataEnvironment {
+  uintptr_t *env_ptr;
+};
+
+extern DataEnvironment app;
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_CONFIG_GPU_APP_H

libc/config/gpu/entrypoints.txt

@@ -167,6 +167,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdlib.strtoull
     libc.src.stdlib.at_quick_exit
     libc.src.stdlib.quick_exit
+    libc.src.stdlib.getenv
 
     # TODO: Implement these correctly
     libc.src.stdlib.aligned_alloc

libc/config/linux/CMakeLists.txt

@@ -77,7 +77,7 @@ add_object_library(
     thread.cpp
   DEPENDS
     .futex_utils
-    libc.config.linux.app_h
+    libc.config.app_h
     libc.include.sys_syscall
     libc.include.fcntl
     libc.src.errno.errno

libc/src/__support/threads/linux/CMakeLists.txt

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/__support/threads/thread.h"
-#include "config/linux/app.h"
+#include "config/app.h"
 #include "src/__support/CPP/atomic.h"
 #include "src/__support/CPP/string_view.h"
 #include "src/__support/CPP/stringstream.h"

libc/src/__support/threads/linux/thread.cpp

@@ -62,7 +62,7 @@ add_entrypoint_object(
   HDRS
     getenv.h
   DEPENDS
-    libc.config.linux.app_h
+  libc.config.app_h
 )
 
 add_entrypoint_object(

libc/src/stdlib/CMakeLists.txt

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/stdlib/getenv.h"
-#include "config/linux/app.h"
+#include "config/app.h"
 #include "src/__support/CPP/string_view.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"

libc/src/stdlib/getenv.cpp

@@ -11,7 +11,7 @@ add_entrypoint_object(
     libc.src.__support.threads.callonce
     libc.src.__support.common
     libc.src.errno.errno
-    libc.config.linux.app_h
+    libc.config.app_h
     libc.src.fcntl.open
     libc.src.unistd.read
     libc.src.unistd.close