[libc][NFC] Add the platform independent file target only if mutex is available.

[sivachandra]

Summary

The platform independent file implementation is not an entrypoint so it
cannot be excluded via the entrypoints.txt file. Hence, we need a
special treatment to exclude it from the build.

[michaelrj]

LGTM

libc/src/__support/CMakeLists.txt

 )
 
 add_header_library(
   integer_operations
   HDRS
     integer_operations.h
 )
 
+# Thread support is used by other support libraries. So, we add the "threads"
+# before other directories.
+add_subdirectory(threads)
+
 add_subdirectory(File)
 add_subdirectory(FPUtil)
 add_subdirectory(OSUtil)
-add_subdirectory(threads)

libc/src/__support/File/CMakeLists.txt

+if(NOT (TARGET libc.src.__support.threads.mutex))
+  # Not all platforms have a mutex implementation. If mutex is unvailable,
+  # we just skip everything about files.
+  return()
+endif()
+
 add_object_library(
   file
   SRCS
     file.cpp
   HDRS
     file.h
   DEPENDS
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
     libc.include.errno
     libc.src.errno.errno
 )

libc/src/__support/threads/CMakeLists.txt

+add_header_library(
+  mutex_common
+  HDRS
+    mutex_common.h
+)
+
 if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
   add_subdirectory(${LIBC_TARGET_OS})
 endif()
 
+if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.mutex)
   add_header_library(
-  thread
+    mutex
     HDRS
       mutex.h
     DEPENDS
-    .${LIBC_TARGET_OS}.thread
+      .${LIBC_TARGET_OS}.mutex
   )
+endif()

libc/src/__support/threads/linux/CMakeLists.txt

 add_header_library(
-  thread
+  mutex
   HDRS
     mutex.h
   DEPENDS
     libc.include.sys_syscall
     libc.src.__support.CPP.atomic
     libc.src.__support.OSUtil.osutil
+    libc.src.__support.threads.mutex_common
 )

libc/src/__support/threads/linux/mutex.h

 //===--- Implementation of a Linux mutex class ------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_SRC_SUPPORT_THREAD_LINUX_MUTEX_H
 #define LLVM_LIBC_SRC_SUPPORT_THREAD_LINUX_MUTEX_H
 
 #include "src/__support/CPP/atomic.h"
 #include "src/__support/OSUtil/syscall.h" // For syscall functions.
-#include "src/__support/threads/mutex.h"
+#include "src/__support/threads/mutex_common.h"
 
 #include <linux/futex.h>
 #include <stdint.h>
 #include <sys/syscall.h> // For syscall numbers.
 
 namespace __llvm_libc {
 
 struct Mutex {

libc/src/__support/threads/mutex.h

 //===--- A platform independent abstraction layer for mutexes ---*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_SRC_SUPPORT_THREAD_MUTEX_H
 #define LLVM_LIBC_SRC_SUPPORT_THREAD_MUTEX_H
 
-namespace __llvm_libc {
-
-enum class MutexError : int {
-  NONE,
-  BUSY,
-  TIMEOUT,
-  UNLOCK_WITHOUT_LOCK,
-  BAD_LOCK_STATE,
-};
-
-} // namespace __llvm_libc
-
 // Platform independent code will include this header file which pulls
 // the platfrom specific specializations using platform macros.
 //
 // The platform specific specializations should define a class by name
 // Mutex with non-static methods having the following signature:
 //
 // MutexError lock();
 // MutexError trylock();

libc/src/__support/threads/mutex_common.h

+//===--- Common definitions useful for mutex implementations ----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_THREAD_MUTEX_COMMON_H
+#define LLVM_LIBC_SRC_SUPPORT_THREAD_MUTEX_COMMON_H
+
+namespace __llvm_libc {
+
+enum class MutexError : int {
+  NONE,
+  BUSY,
+  TIMEOUT,
+  UNLOCK_WITHOUT_LOCK,
+  BAD_LOCK_STATE,
+};
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_SUPPORT_THREAD_MUTEX_COMMON_H

libc/src/stdlib/CMakeLists.txt

add_entrypoint_object(
   SRCS
     atexit.cpp
   HDRS
     atexit.h
   CXX_STANDARD
     20 # For constinit of the atexit callback list.
   DEPENDS
     libc.src.__support.CPP.blockstore
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
   exit
   SRCS
     exit.cpp
   HDRS
     exit.h

libc/src/threads/CMakeLists.txt

 add_entrypoint_object(
   mtx_init
   SRCS
     mtx_init.cpp
   HDRS
     mtx_init.h
   DEPENDS
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
   mtx_destroy
   SRCS
     mtx_destroy.cpp
   HDRS
     mtx_destroy.h
   DEPENDS
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
   mtx_lock
   SRCS
     mtx_lock.cpp
   HDRS
     mtx_lock.h
   DEPENDS
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
   mtx_unlock
   SRCS
     mtx_unlock.cpp
   HDRS
     mtx_unlock.h
   DEPENDS
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
   cnd_init
   ALIAS
   DEPENDS
     .${LIBC_TARGET_OS}.cnd_init
 )

libc/src/threads/linux/CMakeLists.txt

HDRS
     Futex.h
     Thread.h
   DEPENDS
     .thread_start_args_h
     libc.include.sys_syscall
     libc.include.threads
     libc.src.__support.CPP.atomic
     libc.src.__support.OSUtil.osutil
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
   thrd_create
   SRCS
     thrd_create.cpp
   HDRS
     ../thrd_create.h
add_entrypoint_object(
   cnd_wait
   SRCS
     cnd_wait.cpp
   HDRS
     ../cnd_wait.h
   DEPENDS
     .threads_utils
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
   cnd_signal
   SRCS
     cnd_signal.cpp
   HDRS
     ../cnd_signal.h