[libc] Partially implement 'errno' on the GPU

[jhuber6]

Summary:
The errno variable is expected to be thread_local by the standard.
However, the GPU targets do not support thread_local and implementing
that would be a large endeavor. Because of that, we previously didn't
provide the errno symbol at all. However, to build some programs we at
least need to be able to link against errno. Many things that would
normally set errno completely ignore it currently (i.e. stdio) but
some programs still need to be able to link against correct C programs.

For this purpose this patch exports the errno symbol as a simple
global. Internally, this will be updated atomically so it's at least not
racy. Externally, this will be on the user. I've updated the
documentation to state as such. This is required to get libc++ to
build.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Joseph Huber (jhuber6)

Changes

Summary:
The errno variable is expected to be thread_local by the standard.
However, the GPU targets do not support thread_local and implementing
that would be a large endeavor. Because of that, we previously didn't
provide the errno symbol at all. However, to build some programs we at
least need to be able to link against errno. Many things that would
normally set errno completely ignore it currently (i.e. stdio) but
some programs still need to be able to link against correct C programs.

For this purpose this patch exports the errno symbol as a simple
global. Internally, this will be updated atomically so it's at least not
racy. Externally, this will be on the user. I've updated the
documentation to state as such. This is required to get libc++ to
build.

---

Full diff: https://github.com/llvm/llvm-project/pull/97107.diff

2 Files Affected:

• (modified) libc/include/errno.h.def (+4-3)
• (modified) libc/src/errno/libc_errno.cpp (+9-4)

diff --git a/libc/include/errno.h.def b/libc/include/errno.h.def
index 3ffcd3fe4c721..1f7120e63bfc9 100644
--- a/libc/include/errno.h.def
+++ b/libc/include/errno.h.def
@@ -25,8 +25,9 @@
 #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;
@@ -34,8 +35,8 @@ 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
diff --git a/libc/src/errno/libc_errno.cpp b/libc/src/errno/libc_errno.cpp
index 64f9f522ca296..bd1438c226143 100644
--- a/libc/src/errno/libc_errno.cpp
+++ b/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.

[lntue]

LGTM on the libc side.