Bug 1784018 - Remove deprecated OSSpinLocks r=glandium

On macOS versions prior to 10.15 os_unfair_locks cannot spin in kernel-space
which degrades performance significantly. To obviate for this we spin in
user-space like OSSpinLock does, for the same number of times and invoking
x86-specific pause instructions in-between the locking attempts to avoid
starving a thread that might be running on the same physical core.

Differential Revision: https://phabricator.services.mozilla.com/D154205

memory/build/Mutex.cpp

@@ -7,7 +7,7 @@
 #if defined(XP_DARWIN)
 
 // static
-bool Mutex::UseUnfairLocks() {
+bool Mutex::SpinInKernelSpace() {
   if (__builtin_available(macOS 10.15, *)) {
     return true;
   }
@@ -16,6 +16,6 @@ bool Mutex::UseUnfairLocks() {
 }
 
 // static
-bool Mutex::gFallbackToOSSpinLock = !UseUnfairLocks();
+bool Mutex::gSpinInKernelSpace = SpinInKernelSpace();
 
 #endif  // defined(XP_DARWIN)

memory/build/Mutex.h

@@ -10,7 +10,7 @@
 #if defined(XP_WIN)
 #  include <windows.h>
 #elif defined(XP_DARWIN)
-#  include <libkern/OSAtomic.h>
+#  include "mozilla/Assertions.h"
 #  include <os/lock.h>
 #else
 #  include <pthread.h>
@@ -31,12 +31,6 @@ OS_UNFAIR_LOCK_AVAILABILITY
 OS_EXPORT OS_NOTHROW OS_NONNULL_ALL void os_unfair_lock_lock_with_options(
     os_unfair_lock_t lock, os_unfair_lock_options_t options);
 }
-
-static_assert(OS_UNFAIR_LOCK_INIT._os_unfair_lock_opaque == OS_SPINLOCK_INIT,
-              "OS_UNFAIR_LOCK_INIT and OS_SPINLOCK_INIT have the same "
-              "value");
-static_assert(sizeof(os_unfair_lock) == sizeof(OSSpinLock),
-              "os_unfair_lock and OSSpinLock are the same size");
 #endif  // defined(XP_DARWIN)
 
 // Mutexes based on spinlocks.  We can't use normal pthread spinlocks in all
@@ -48,10 +42,7 @@ struct Mutex {
 #if defined(XP_WIN)
   CRITICAL_SECTION mMutex;
 #elif defined(XP_DARWIN)
-  union {
-    os_unfair_lock mUnfairLock;
-    OSSpinLock mSpinLock;
-  } mMutex;
+  os_unfair_lock mMutex;
 #else
   pthread_mutex_t mMutex;
 #endif
@@ -63,10 +54,7 @@ struct Mutex {
       return false;
     }
 #elif defined(XP_DARWIN)
-    // The hack below works because both OS_UNFAIR_LOCK_INIT and
-    // OS_SPINLOCK_INIT initialize the lock to 0 and in both case it's a 32-bit
-    // integer.
-    mMutex.mUnfairLock = OS_UNFAIR_LOCK_INIT;
+    mMutex = OS_UNFAIR_LOCK_INIT;
 #elif defined(XP_LINUX) && !defined(ANDROID)
     pthread_mutexattr_t attr;
     if (pthread_mutexattr_init(&attr) != 0) {
@@ -90,19 +78,46 @@ struct Mutex {
 #if defined(XP_WIN)
     EnterCriticalSection(&mMutex);
 #elif defined(XP_DARWIN)
-    if (Mutex::gFallbackToOSSpinLock) {
-      OSSpinLockLock(&mMutex.mSpinLock);
-    } else {
     // We rely on a non-public function to improve performance here.
     // The OS_UNFAIR_LOCK_DATA_SYNCHRONIZATION flag informs the kernel that
     // the calling thread is able to make progress even in absence of actions
     // from other threads and the OS_UNFAIR_LOCK_ADAPTIVE_SPIN one causes the
     // kernel to spin on a contested lock if the owning thread is running on
     // the same physical core (presumably only on x86 CPUs given that ARM
-      // macs don't have cores capable of SMT).
+    // macs don't have cores capable of SMT). On versions of macOS older than
+    // 10.15 the latter is not available and we spin in userspace instead.
+    if (Mutex::gSpinInKernelSpace) {
       os_unfair_lock_lock_with_options(
-          &mMutex.mUnfairLock,
+          &mMutex,
           OS_UNFAIR_LOCK_DATA_SYNCHRONIZATION | OS_UNFAIR_LOCK_ADAPTIVE_SPIN);
+    } else {
+#  if defined(__x86_64__)
+      // On older versions of macOS (10.14 and older) the
+      // `OS_UNFAIR_LOCK_ADAPTIVE_SPIN` flag is not supported by the kernel,
+      // we spin in user-space instead like `OSSpinLock` does:
+      // https://github.com/apple/darwin-libplatform/blob/215b09856ab5765b7462a91be7076183076600df/src/os/lock.c#L183-L198
+      // Note that `OSSpinLock` uses 1000 iterations on x86-64:
+      // https://github.com/apple/darwin-libplatform/blob/215b09856ab5765b7462a91be7076183076600df/src/os/lock.c#L93
+      // ...but we only use 100 like it does on ARM:
+      // https://github.com/apple/darwin-libplatform/blob/215b09856ab5765b7462a91be7076183076600df/src/os/lock.c#L90
+      // We choose this value because it yields the same results in our
+      // benchmarks but is less likely to have detrimental effects caused by
+      // excessive spinning.
+      uint32_t retries = 100;
+
+      do {
+        if (os_unfair_lock_trylock(&mMutex)) {
+          return;
+        }
+
+        __asm__ __volatile__("pause");
+      } while (retries--);
+
+      os_unfair_lock_lock_with_options(&mMutex,
+                                       OS_UNFAIR_LOCK_DATA_SYNCHRONIZATION);
+#  else
+      MOZ_CRASH("User-space spin-locks should never be used on ARM");
+#  endif  // defined(__x86_64__)
     }
 #else
     pthread_mutex_lock(&mMutex);
@@ -113,19 +128,15 @@ struct Mutex {
 #if defined(XP_WIN)
     LeaveCriticalSection(&mMutex);
 #elif defined(XP_DARWIN)
-    if (Mutex::gFallbackToOSSpinLock) {
-      OSSpinLockUnlock(&mMutex.mSpinLock);
-    } else {
-      os_unfair_lock_unlock(&mMutex.mUnfairLock);
-    }
+    os_unfair_lock_unlock(&mMutex);
 #else
     pthread_mutex_unlock(&mMutex);
 #endif
   }
 
 #if defined(XP_DARWIN)
-  static bool UseUnfairLocks();
-  static bool gFallbackToOSSpinLock;
+  static bool SpinInKernelSpace();
+  static bool gSpinInKernelSpace;
 #endif  // XP_DARWIN
 };
 
@@ -153,10 +164,7 @@ struct StaticMutex {
 typedef Mutex StaticMutex;
 
 #  if defined(XP_DARWIN)
-// The hack below works because both OS_UNFAIR_LOCK_INIT and OS_SPINLOCK_INIT
-// initialize the lock to 0 and in both case it's a 32-bit integer.
-#    define STATIC_MUTEX_INIT \
-      { .mUnfairLock = OS_UNFAIR_LOCK_INIT }
+#    define STATIC_MUTEX_INIT OS_UNFAIR_LOCK_INIT
 #  elif defined(XP_LINUX) && !defined(ANDROID)
 #    define STATIC_MUTEX_INIT PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP
 #  else