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

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.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D121947

libc/src/__support/CMakeLists.txt

@@ -54,7 +54,10 @@ add_header_library(
     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

@@ -1,3 +1,9 @@
+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
@@ -5,7 +11,7 @@ add_object_library(
   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

@@ -1,11 +1,19 @@
+add_header_library(
+  mutex_common
+  HDRS
+    mutex_common.h
+)
+
 if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
   add_subdirectory(${LIBC_TARGET_OS})
 endif()
 
-add_header_library(
-  thread
-  HDRS
-    mutex.h
-  DEPENDS
-    .${LIBC_TARGET_OS}.thread
-)
+if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.mutex)
+  add_header_library(
+    mutex
+    HDRS
+      mutex.h
+    DEPENDS
+      .${LIBC_TARGET_OS}.mutex
+  )
+endif()

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

@@ -1,9 +1,10 @@
 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

@@ -11,7 +11,7 @@
 
 #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>

libc/src/__support/threads/mutex.h

@@ -9,18 +9,6 @@
 #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.
 //

libc/src/__support/threads/mutex_common.h

@@ -0,0 +1,24 @@
+//===--- 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

@@ -280,7 +280,7 @@ add_entrypoint_object(
     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(

libc/src/threads/CMakeLists.txt

@@ -31,7 +31,7 @@ add_entrypoint_object(
     mtx_init.h
   DEPENDS
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
@@ -42,7 +42,7 @@ add_entrypoint_object(
     mtx_destroy.h
   DEPENDS
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
@@ -53,7 +53,7 @@ add_entrypoint_object(
     mtx_lock.h
   DEPENDS
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(
@@ -64,7 +64,7 @@ add_entrypoint_object(
     mtx_unlock.h
   DEPENDS
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(

libc/src/threads/linux/CMakeLists.txt

@@ -34,7 +34,7 @@ add_header_library(
     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(
@@ -105,7 +105,7 @@ add_entrypoint_object(
   DEPENDS
     .threads_utils
     libc.include.threads
-    libc.src.__support.threads.thread
+    libc.src.__support.threads.mutex
 )
 
 add_entrypoint_object(