[libc] Mutex implementation for single-threaded baremetal

libc/cmake/modules/LLVMLibCCompileOptionRules.cmake

@@ -110,6 +110,10 @@ function(_get_compile_options_from_config output_var)
     list(APPEND config_options "-DLIBC_ERRNO_MODE=${LIBC_CONF_ERRNO_MODE}")
   endif()
 
+  if(LIBC_CONF_THREAD_MODE)
+    list(APPEND config_options "-DLIBC_THREAD_MODE=${LIBC_CONF_THREAD_MODE}")
+  endif()
+
   set(${output_var} ${config_options} PARENT_SCOPE)
 endfunction(_get_compile_options_from_config)
 

libc/config/baremetal/config.json

@@ -4,6 +4,11 @@
       "value": "LIBC_ERRNO_MODE_EXTERNAL"
     }
   },
+  "threads": {
+    "LIBC_CONF_THREAD_MODE": {
+      "value": "LIBC_THREAD_MODE_SINGLE"
+    }
+  },
   "printf": {
     "LIBC_CONF_PRINTF_DISABLE_FIXED_POINT": {
       "value": true

libc/config/config.json

@@ -5,6 +5,12 @@
       "doc": "The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_DEFAULT, LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, LIBC_ERRNO_MODE_SYSTEM, and LIBC_ERRNO_MODE_SYSTEM_INLINE."
     }
   },
+  "threads": {
+    "LIBC_CONF_THREAD_MODE": {
+      "value": "LIBC_THREAD_MODE_PLATFORM",
+      "doc": "The implementation used for Mutex, acceptable values are LIBC_THREAD_MODE_PLATFORM, LIBC_THREAD_MODE_SINGLE, and LIBC_THREAD_MODE_EXTERNAL."
+    }
+  },
   "printf": {
     "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
       "value": false,

libc/config/gpu/amdgpu/config.json

@@ -4,6 +4,11 @@
       "value": "LIBC_ERRNO_MODE_SHARED"
     }
   },
+  "threads": {
+    "LIBC_CONF_THREAD_MODE": {
+      "value": "LIBC_THREAD_MODE_SINGLE"
+    }
+  },
   "printf": {
     "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
       "value": true

libc/config/gpu/nvptx/config.json

@@ -4,6 +4,11 @@
       "value": "LIBC_ERRNO_MODE_SHARED"
     }
   },
+  "threads": {
+    "LIBC_CONF_THREAD_MODE": {
+      "value": "LIBC_THREAD_MODE_SINGLE"
+    }
+  },
   "printf": {
     "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
       "value": true

libc/docs/configure.rst

@@ -60,5 +60,7 @@ to learn about the defaults for your platform and target.
 * **"string" options**
     - ``LIBC_CONF_MEMSET_X86_USE_SOFTWARE_PREFETCHING``: Inserts prefetch for write instructions (PREFETCHW) for memset on x86 to recover performance when hardware prefetcher is disabled.
     - ``LIBC_CONF_STRING_UNSAFE_WIDE_READ``: Read more than a byte at a time to perform byte-string operations like strlen.
+* **"threads" options**
+    - ``LIBC_CONF_THREAD_MODE``: The implementation used for Mutex, acceptable values are LIBC_THREAD_MODE_PLATFORM, LIBC_THREAD_MODE_SINGLE, and LIBC_THREAD_MODE_EXTERNAL.
 * **"time" options**
     - ``LIBC_CONF_TIME_64BIT``: Force the size of time_t to 64 bits, even on platforms where compatibility considerations would otherwise make it 32-bit.

libc/src/__support/macros/attributes.h

@@ -18,6 +18,7 @@
 #define LLVM_LIBC_SRC___SUPPORT_MACROS_ATTRIBUTES_H
 
 #include "properties/architectures.h"
+#include "properties/os.h"
 
 #ifndef __has_attribute
 #define __has_attribute(x) 0
@@ -28,7 +29,9 @@
 #define LIBC_INLINE_ASM __asm__ __volatile__
 #define LIBC_UNUSED __attribute__((unused))
 
-#ifdef LIBC_TARGET_ARCH_IS_GPU
+// TODO: we need a proper way to only turn this on for single-threaded baremetal
+// We cannot use LIBC_THREAD_MODE because libcxx doesn't define that
+#if defined(LIBC_TARGET_ARCH_IS_GPU) || defined(LIBC_TARGET_OS_IS_BAREMETAL)
 #define LIBC_THREAD_LOCAL
 #else
 #define LIBC_THREAD_LOCAL thread_local

libc/src/__support/macros/properties/os.h

@@ -29,4 +29,12 @@
 #define LIBC_TARGET_OS_IS_FUCHSIA
 #endif
 
+#if !defined(LIBC_TARGET_OS_IS_FREEBSD) &&                                     \
+    !defined(LIBC_TARGET_OS_IS_ANDROID) &&                                     \
+    !defined(LIBC_TARGET_OS_IS_LINUX) &&                                       \
+    !defined(LIBC_TARGET_OS_IS_WINDOWS) &&                                     \
+    !defined(LIBC_TARGET_OS_IS_FUCHSIA) && defined(__ELF__)
+#define LIBC_TARGET_OS_IS_BAREMETAL
+#endif
+
 #endif // LLVM_LIBC_SRC___SUPPORT_MACROS_PROPERTIES_OS_H

libc/src/__support/threads/CMakeLists.txt

@@ -42,6 +42,14 @@ if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.mutex)
       .mutex
       libc.src.__support.CPP.mutex
   )
+elseif(NOT (LIBC_CONF_THREAD_MODE STREQUAL LIBC_THREAD_MODE_PLATFORM))
+  add_header_library(
+    mutex
+  HDRS
+    mutex.h
+  DEPENDS
+    .mutex_common
+  )
 endif()
 
 add_header_library(

libc/src/__support/threads/mutex.h

@@ -9,10 +9,35 @@
 #ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_MUTEX_H
 #define LLVM_LIBC_SRC___SUPPORT_THREADS_MUTEX_H
 
-#include "src/__support/macros/properties/architectures.h"
+#include "src/__support/macros/attributes.h"
+#include "src/__support/macros/config.h"
+
+// Uses the platform specific specialization
+#define LIBC_THREAD_MODE_PLATFORM 0
+
+// Mutex guards nothing, used in single-threaded implementations
+#define LIBC_THREAD_MODE_SINGLE 1
+
+// Vendor provides implementation
+#define LIBC_THREAD_MODE_EXTERNAL 2
+
+#if !defined(LIBC_THREAD_MODE)
+#error LIBC_THREAD_MODE is undefined
+#endif // LIBC_THREAD_MODE
+
+#if LIBC_THREAD_MODE != LIBC_THREAD_MODE_PLATFORM &&                           \
+    LIBC_THREAD_MODE != LIBC_THREAD_MODE_SINGLE &&                             \
+    LIBC_THREAD_MODE != LIBC_THREAD_MODE_EXTERNAL
+#error LIBC_THREAD_MODE must be one of the following values: \
+LIBC_THREAD_MODE_PLATFORM, \
+LIBC_THREAD_MODE_SINGLE, \
+LIBC_THREAD_MODE_EXTERNAL.
+#endif
+
+#if LIBC_THREAD_MODE == LIBC_THREAD_MODE_PLATFORM
 
 // Platform independent code will include this header file which pulls
-// the platfrom specific specializations using platform macros.
+// the platform specific specializations using platform macros.
 //
 // The platform specific specializations should define a class by name
 // Mutex with non-static methods having the following signature:
@@ -39,8 +64,32 @@
 
 #if defined(__linux__)
 #include "src/__support/threads/linux/mutex.h"
-#elif defined(LIBC_TARGET_ARCH_IS_GPU)
-#include "src/__support/threads/gpu/mutex.h"
 #endif // __linux__
 
+#elif LIBC_THREAD_MODE == LIBC_THREAD_MODE_SINGLE
+
+#include "src/__support/threads/mutex_common.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+/// Implementation of a simple passthrough mutex which guards nothing. A
+/// complete Mutex locks in general cannot be implemented on the GPU, or on some
+/// baremetal platforms. We simply define the Mutex interface and require that
+/// only a single thread executes code requiring a mutex lock.
+struct Mutex {
+  LIBC_INLINE constexpr Mutex(bool, bool, bool, bool) {}
+
+  LIBC_INLINE MutexError lock() { return MutexError::NONE; }
+  LIBC_INLINE MutexError unlock() { return MutexError::NONE; }
+  LIBC_INLINE MutexError reset() { return MutexError::NONE; }
+};
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#elif LIBC_THREAD_MODE == LIBC_THREAD_MODE_EXTERNAL
+
+// TODO: Implement the interfacing, if necessary, e.g. "extern struct Mutex;"
+
+#endif // LIBC_THREAD_MODE == LIBC_THREAD_MODE_PLATFORM
+
 #endif // LLVM_LIBC_SRC___SUPPORT_THREADS_MUTEX_H