[libc] Ignore 'errno' on the GPU and support 'atoi'

The 'errno' value is most likely not useful on the GPU and it prevents
us from providing certain functions on the GPU that depend on it, like
`atoi`. This patch makes the necessary changes to support `errno` by
simple replacing it with a consumer class.

Supporting `errno` on the GPU is possible in some aspects. The first
approach would be to use a buffer of shared memory that has enough space
for all threads. Another option would be to change code generation to
support `thread_local` using `address_space(5)` memory allocated at
kernel launch. The former could look like the following, which could be
implemented in a later patch:

```
template <typename T>
using SharedBuffer = T[gpu::MAX_THREADS] [[clang::address_space(3)]];
template <typename T> struct ErrnoSetter {
  constexpr ErrnoSetter(SharedBuffer<T> &storage) : storage(storage) {}
  SharedBuffer<T> &storage;
  void operator=(const T &val) { storage[gpu::get_thread_id()] = val; }
};

static SharedBuffer<int> thread_local_buffer [[clang::loader_uninitialized]];
ErrnoSetter<int> __llvmlibc_internal_errno(thread_local_buffer);
```

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149107

libc/config/gpu/entrypoints.txt

@@ -53,6 +53,9 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strstr
     libc.src.string.strtok
     libc.src.string.strtok_r
+
+    # stdlib.h entrypoints
+    libc.src.stdlib.atoi
 )
 
 set(TARGET_LLVMLIBC_ENTRYPOINTS

libc/config/gpu/headers.txt

@@ -1,4 +1,5 @@
 set(TARGET_PUBLIC_HEADERS
     libc.include.ctype
     libc.include.string
+    libc.include.stdlib
 )

libc/include/errno.h.def

@@ -43,7 +43,9 @@
 #include <llvm-libc-macros/generic-error-number-macros.h>
 #endif
 
+#if !defined(__AMDGPU__) && !defined(__NVPTX__)
 extern _Thread_local int __llvmlibc_errno;
 #define errno __llvmlibc_errno
+#endif
 
 #endif // LLVM_LIBC_ERRNO_H

libc/src/errno/libc_errno.cpp

@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "src/__support/macros/properties/architectures.h"
+
 namespace __llvm_libc {
 
 extern "C" {
@@ -18,9 +20,17 @@ extern "C" {
 // macro defined in LLVM libc's public errno.h header file.
 // TODO: Use a macro to distinguish full build and overlay build which can be
 //       used to exclude __llvmlibc_errno under overlay build.
+#ifdef LIBC_TARGET_ARCH_IS_GPU
+ErrnoConsumer __llvmlibc_errno;
+#else
 thread_local int __llvmlibc_errno;
+#endif // LIBC_TARGET_ARCH_IS_GPU
+#else
+#ifdef LIBC_TARGET_ARCH_IS_GPU
+ErrnoConsumer __llvmlibc_internal_errno;
 #else
 thread_local int __llvmlibc_internal_errno;
+#endif // LIBC_TARGET_ARCH_IS_GPU
 #endif
 } // extern "C"
 

libc/src/errno/libc_errno.h

@@ -9,21 +9,42 @@
 #ifndef LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H
 #define LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H
 
+#include "src/__support/macros/properties/architectures.h"
+
 #include <errno.h>
 
+// If we are targeting the GPU we currently don't support 'errno'. We simply
+// consume it.
+#ifdef LIBC_TARGET_ARCH_IS_GPU
+namespace __llvm_libc {
+struct ErrnoConsumer {
+  void operator=(int) {}
+};
+} // namespace __llvm_libc
+#endif
+
 // All of the libc runtime and test code should use the "libc_errno" macro. They
 // should not refer to the "errno" macro directly.
 #ifdef LIBC_COPT_PUBLIC_PACKAGING
+#ifdef LIBC_TARGET_ARCH_IS_GPU
+extern "C" __llvm_libc::ErrnoConsumer __llvmlibc_errno;
+#define libc_errno __llvmlibc_errno
+#else
 // This macro will resolve to errno from the errno.h file included above. Under
 // full build, this will be LLVM libc's errno. In overlay build, it will be
 // system libc's errno.
 #define libc_errno errno
+#endif
 #else
 namespace __llvm_libc {
 
+#ifdef LIBC_TARGET_ARCH_IS_GPU
+extern "C" ErrnoConsumer __llvmlibc_internal_errno;
+#else // LIBC_TARGET_ARCH_IS_GPU
 extern "C" {
 extern thread_local int __llvmlibc_internal_errno;
 } // extern "C"
+#endif
 
 // TODO: After all of libc/src and libc/test are switched over to use
 // libc_errno, this header file will be "shipped" via an add_entrypoint_object