LWLock optimization for PowerPC processors

Alexander Korotkov · Alexander Korotkov · commit b8c6f83f4e0e · 2016-12-16T13:05:13.000+03:00
PowerPC processors have quite specific implementation of atomic operations:
they are implemented optimistically.  If value is changed between our read and
write, then we have to retry.  So, CAS operation on this platform becomes
potentially infinite loop.  Thus, loop of CAS operations becomes two nested
loops.  This patch implements LWLockAttemptLock() as single loop in assembly.
That shows great speedup on high concurrent workloads.
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
@@ -768,6 +768,56 @@ GetLWLockIdentifier(uint8 classId, uint16 eventId)
 	return LWLockTrancheArray[eventId]->name;
 }
 
+#if (defined(__GNUC__) || defined(__INTEL_COMPILER)) && (defined(__ppc__) || defined(__powerpc__) || defined(__ppc64__) || defined(__powerpc64__))
+
+/*
+ * Special optimization for PowerPC processors: put logic dealing with LWLock
+ * state between lwarx/stwcx operations.
+ */
+static bool
+LWLockAttemptLock(LWLock *lock, LWLockMode mode)
+{
+	uint32		mask, increment;
+	bool		result;
+
+	AssertArg(mode == LW_EXCLUSIVE || mode == LW_SHARED);
+
+	if (mode == LW_EXCLUSIVE)
+	{
+		mask = LW_LOCK_MASK;
+		increment = LW_VAL_EXCLUSIVE;
+	}
+	else
+	{
+		mask = LW_VAL_EXCLUSIVE;
+		increment = LW_VAL_SHARED;
+	}
+
+	__asm__ __volatile__(
+"0:	lwarx   3,0,%4		\n"
+"	and     4,3,%2		\n"
+"	cmpwi   4,0			\n"
+"	bne-    1f			\n"
+"	add     3,3,%3		\n"
+"	stwcx.  3,0,%4		\n"
+"	bne-    0b			\n"
+"	li      %0,0		\n"
+"	b       2f			\n"
+"1: li      %0,1		\n"
+#ifdef USE_PPC_LWSYNC
+"2:	lwsync				\n"
+#else
+"2:	isync				\n"
+#endif
+:	"=&r"(result), "+m"(lock->state)
+:	"r"(mask), "r"(increment), "r"(&lock->state)
+:	"memory", "cc", "r3", "r4");
+
+	return result;
+}
+
+#else
+
 /*
  * Internal function that tries to atomically acquire the lwlock in the passed
  * in mode.
@@ -910,6 +960,8 @@ LWLockWaitListUnlock(LWLock *lock)
 	Assert(old_state & LW_FLAG_LOCKED);
 }
 
+#endif
+
 /*
  * Wakeup all the lockers that currently have a chance to acquire the lock.
  */
