[libc] Support configurable errno modes

libc/config/baremetal/config.json

@@ -1,4 +1,9 @@
 {
+  "errno": {
+    "LIBC_CONF_ERRNO_MODE": {
+      "value": "LIBC_ERRNO_MODE_EXTERNAL"
+    }
+  },
   "printf": {
     "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
       "value": true

libc/config/config.json

@@ -1,4 +1,10 @@
 {
+  "errno": {
+    "LIBC_CONF_ERRNO_MODE": {
+      "value": "",
+      "doc": "The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_NONE, LIBC_ERRNO_MODE_INTERNAL, LIBC_ERRNO_MODE_EXTERNAL, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_GLOBAL, and LIBC_ERRNO_MODE_SYSTEM."
+    }
+  },
   "printf": {
     "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
       "value": false,

libc/config/gpu/config.json

@@ -1,4 +1,9 @@
 {
+  "errno": {
+    "LIBC_CONF_ERRNO_MODE": {
+      "value": "LIBC_ERRNO_MODE_GLOBAL"
+    }
+  },
   "printf": {
     "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
       "value": true

libc/docs/configure.rst

@@ -28,6 +28,8 @@ to learn about the defaults for your platform and target.
 * **"codegen" options**
     - ``LIBC_CONF_ENABLE_STRONG_STACK_PROTECTOR``: Enable -fstack-protector-strong to defend against stack smashing attack.
     - ``LIBC_CONF_KEEP_FRAME_POINTER``: Keep frame pointer in functions for better debugging experience.
+* **"errno" options**
+    - ``LIBC_CONF_ERRNO_MODE``: The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_NONE, LIBC_ERRNO_MODE_INTERNAL, LIBC_ERRNO_MODE_EXTERNAL, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_GLOBAL, and LIBC_ERRNO_MODE_SYSTEM.
 * **"malloc" options**
     - ``LIBC_CONF_FREELIST_MALLOC_BUFFER_SIZE``: Default size for the constinit freelist buffer used for the freelist malloc implementation (default 1o 1GB).
 * **"math" options**

libc/include/errno.h.def

@@ -25,18 +25,12 @@
 #include "llvm-libc-macros/generic-error-number-macros.h"
 #endif
 
-#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
+__BEGIN_C_DECLS
+
+int *__llvm_libc_errno(void) __NOEXCEPT;
+
+__END_C_DECLS
 
-#define errno __llvmlibc_errno
+#define errno (*__llvm_libc_errno())
 
 #endif // LLVM_LIBC_ERRNO_H

libc/src/errno/CMakeLists.txt

@@ -9,6 +9,10 @@ if(LLVM_LIBC_FULL_BUILD)
   set(full_build_flag "-DLIBC_FULL_BUILD")
 endif()
 
+if(LIBC_CONF_ERRNO_MODE)
+  set(errno_config_copts "-DLIBC_ERRNO_MODE=${LIBC_CONF_ERRNO_MODE}")
+endif()
+
 add_entrypoint_object(
   errno
   SRCS
@@ -17,6 +21,7 @@ add_entrypoint_object(
     libc_errno.h     # Include this
   COMPILE_OPTIONS
     ${full_build_flag}
+    ${errno_config_copts}
   DEPENDS
     libc.hdr.errno_macros
     libc.src.__support.common

libc/src/errno/libc_errno.cpp

@@ -9,44 +9,91 @@
 #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
-// a global errno for gpu to use for now.
+#define LIBC_ERRNO_MODE_UNDEFINED 0x01
+#define LIBC_ERRNO_MODE_THREAD_LOCAL 0x02
+#define LIBC_ERRNO_MODE_GLOBAL 0x04
+#define LIBC_ERRNO_MODE_EXTERNAL 0x08
+#define LIBC_ERRNO_MODE_SYSTEM 0x10
+
+#ifndef LIBC_ERRNO_MODE
+#ifndef LIBC_COPT_PUBLIC_PACKAGING
+// This mode is for unit testing. We just use our internal errno.
+#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_INTERNAL
+#elif defined(LIBC_FULL_BUILD)
+// In full build mode, we provide the errno storage ourselves.
+#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_THREAD_LOCAL
+#else
+#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_SYSTEM
+#endif
+#endif // LIBC_ERRNO_MODE
+
+#if LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_UNDEFINED &&                            \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_THREAD_LOCAL &&                         \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_GLOBAL &&                               \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_EXTERNAL &&                             \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SYSTEM
+#error LIBC_ERRNO_MODE must be one of the following values: \
+LIBC_ERRNO_MODE_NONE, \
+LIBC_ERRNO_MODE_THREAD_LOCAL, \
+LIBC_ERRNO_MODE_GLOBAL, \
+LIBC_ERRNO_MODE_EXTERNAL, \
+LIBC_ERRNO_MODE_SYSTEM
+#endif
+
+namespace LIBC_NAMESPACE {
+
+// Define the global `libc_errno` instance.
+Errno libc_errno;
+
+#if LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_UNDEFINED
+
+void Errno::operator=(int) {}
+Errno::operator int() { return 0; }
+
+#elif LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_THREAD_LOCAL
+
+namespace {
+LIBC_THREAD_LOCAL int __libc_errno;
+}
+
 extern "C" {
-LIBC_THREAD_LOCAL LIBC_NAMESPACE::cpp::Atomic<int> __llvmlibc_errno;
+int *__llvm_libc_errno(void) { return &__libc_errno; }
 }
 
-void LIBC_NAMESPACE::Errno::operator=(int a) {
-  __llvmlibc_errno.store(a, cpp::MemoryOrder::RELAXED);
+void Errno::operator=(int a) { __libc_errno = a; }
+Errno::operator int() { return __libc_errno; }
+
+#elif LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_GLOBAL
+
+namespace {
+LIBC_NAMESPACE::cpp::Atomic<int> __libc_errno;
 }
-LIBC_NAMESPACE::Errno::operator int() {
-  return __llvmlibc_errno.load(cpp::MemoryOrder::RELAXED);
+
+extern "C" {
+int *__llvm_libc_errno(void) { return &__libc_errno; }
 }
 
-#elif !defined(LIBC_COPT_PUBLIC_PACKAGING)
-// This mode is for unit testing.  We just use our internal errno.
-LIBC_THREAD_LOCAL int __llvmlibc_internal_errno;
+void Errno::operator=(int a) {
+  __libc_errno.store(a, cpp::MemoryOrder::RELAXED);
+}
+Errno::operator int() {
+  return __libc_errno.load(cpp::MemoryOrder::RELAXED);
+}
 
-void LIBC_NAMESPACE::Errno::operator=(int a) { __llvmlibc_internal_errno = a; }
-LIBC_NAMESPACE::Errno::operator int() { return __llvmlibc_internal_errno; }
+#elif LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_EXTERNAL
 
-#elif defined(LIBC_FULL_BUILD)
-// This mode is for public libc archive, hermetic, and integration tests.
-// In full build mode, we provide the errno storage ourselves.
 extern "C" {
-LIBC_THREAD_LOCAL int __llvmlibc_errno;
+int *__llvm_libc_errno(void);
 }
 
-void LIBC_NAMESPACE::Errno::operator=(int a) { __llvmlibc_errno = a; }
-LIBC_NAMESPACE::Errno::operator int() { return __llvmlibc_errno; }
+void Errno::operator=(int a) { *__llvm_libc_errno() = a; }
+Errno::operator int() { return *__llvm_libc_errno(); }
 
-#else
-void LIBC_NAMESPACE::Errno::operator=(int a) { errno = a; }
-LIBC_NAMESPACE::Errno::operator int() { return errno; }
+#elif LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_SYSTEM
 
-#endif // LIBC_FULL_BUILD
+void Errno::operator=(int a) { errno = a; }
+Errno::operator int() { return errno; }
+
+#endif
 
-namespace LIBC_NAMESPACE {
-// Define the global `libc_errno` instance.
-Errno libc_errno;
 } // namespace LIBC_NAMESPACE

libc/src/errno/libc_errno.h

@@ -31,6 +31,7 @@
 // - Still depend on libc.src.errno.errno
 
 namespace LIBC_NAMESPACE {
+
 struct Errno {
   void operator=(int);
   operator int();