[libc] Partially implement 'errno' on the GPU

libc/docs/gpu/motivation.rst

@@ -44,7 +44,7 @@ Limitations
 
 We only implement a subset of the standard C library. The GPU does not
 currently support thread local variables in all cases, so variables like
-``errno`` are not provided. Furthermore, the GPU under the OpenCL execution
+``errno`` are atomic and global. Furthermore, the GPU under the OpenCL execution
 model cannot safely provide a mutex interface. This means that features like
 file buffering are not implemented on the GPU. We can also not easily provide
 threading features on the GPU due to the execution model so these will be

libc/include/errno.h.def

@@ -25,17 +25,18 @@
 #include "llvm-libc-macros/generic-error-number-macros.h"
 #endif
 
-#if !defined(__AMDGPU__) && !defined(__NVPTX__)
-
+#if defined(__AMDGPU__) || defined(__NVPTX__)
+extern int __llvmlibc_errno; // Not thread_local!
+#else
 #ifdef __cplusplus
 extern "C" {
 extern thread_local int __llvmlibc_errno;
 }
 #else
 extern _Thread_local int __llvmlibc_errno;
 #endif // __cplusplus
+#endif
 
 #define errno __llvmlibc_errno
-#endif
 
 #endif // LLVM_LIBC_ERRNO_H

libc/src/errno/libc_errno.cpp

@@ -7,16 +7,21 @@
 //===----------------------------------------------------------------------===//
 
 #include "libc_errno.h"
+#include "src/__support/CPP/atomic.h"
 
 #ifdef LIBC_TARGET_ARCH_IS_GPU
-// LIBC_THREAD_LOCAL on GPU currently does nothing.  So essentially this is just
+// LIBC_THREAD_LOCAL on GPU currently does nothing. So essentially this is just
 // a global errno for gpu to use for now.
 extern "C" {
-LIBC_THREAD_LOCAL int __llvmlibc_gpu_errno;
+LIBC_THREAD_LOCAL LIBC_NAMESPACE::cpp::Atomic<int> __llvmlibc_errno;
 }
 
-void LIBC_NAMESPACE::Errno::operator=(int a) { __llvmlibc_gpu_errno = a; }
-LIBC_NAMESPACE::Errno::operator int() { return __llvmlibc_gpu_errno; }
+void LIBC_NAMESPACE::Errno::operator=(int a) {
+  __llvmlibc_errno.store(a, cpp::MemoryOrder::RELAXED);
+}
+LIBC_NAMESPACE::Errno::operator int() {
+  return __llvmlibc_errno.load(cpp::MemoryOrder::RELAXED);
+}
 
 #elif !defined(LIBC_COPT_PUBLIC_PACKAGING)
 // This mode is for unit testing.  We just use our internal errno.