Add XNN_HAS_PTHREADS to common.h, as a way to indicate that a given p…

…latform doesn't support pthreads() at all.

Note that it's initially (effectively) unused, since it's only set to 0 for Windows, which already has its own custom code path for threading; however, this flag will be used for at least one upcoming target and landing it separately from introduction of that target seems easier to review.

PiperOrigin-RevId: 660518641

src/xnnpack/common.h

@@ -149,6 +149,12 @@
   #define XNN_HAS_MMAP 1
 #endif
 
+#if XNN_PLATFORM_WINDOWS
+  #define XNN_HAS_PTHREADS 0
+#else
+  #define XNN_HAS_PTHREADS 1
+#endif
+
 // Define compile identification macros
 
 #if defined(__clang__)

src/xnnpack/init-once.h

@@ -1,4 +1,4 @@
-// Copyright 2019 Google LLC
+// Copyright 2024 Google LLC
 //
 // This source code is licensed under the BSD-style license found in the
 // LICENSE file in the root directory of this source tree.
@@ -9,6 +9,7 @@
   #include <windows.h>
 #else
   #include <pthread.h>
+  #include <stdbool.h>
 #endif
 
 #include "xnnpack.h"
@@ -27,7 +28,7 @@
   #define XNN_INIT_ONCE(name) \
     InitOnceExecuteOnce(&name##_guard, &name##_windows_wrapper, NULL, NULL) /* no semicolon */
 
-#else
+#elif XNN_HAS_PTHREADS
 
   #define XNN_INIT_ONCE_GUARD(name) \
     static void init_##name##_config(void); \
@@ -36,4 +37,21 @@
   #define XNN_INIT_ONCE(name) \
     pthread_once(&name##_guard, &init_##name##_config) /* no semicolon */
 
+#else
+
+  // If we don't have pthreads available (and there isn't any other platform
+  // specialization), assume we can just get by without special synchronization.
+
+  #define XNN_INIT_ONCE_GUARD(name) \
+    static void init_##name##_config(void); \
+    static bool name##_guard = 0 /* no semicolon */
+
+  #define XNN_INIT_ONCE(name) \
+    do { \
+      if (!(name##_guard)) { \
+        init_##name##_config(); \
+        name##_guard = 1; \
+      } \
+    } while (0)  /* no semicolon */
+
 #endif