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Commita4b09af

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committed
Micro optimize LWLockAttemptLock() a bit.
LWLockAttemptLock pointlessly read the lock's state in every loopiteration, even though pg_atomic_compare_exchange_u32() returns the oldvalue. Instead do that only once before the loop iteration.Additionally there's no need to have the expected_state variable,old_state mostly had the same value anyway.Noticed-By: Heikki LinnakangasBackpatch: 9.5, no reason to let the branches diverge at this point
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+12
-8
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1 file changed

+12
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‎src/backend/storage/lmgr/lwlock.c

Lines changed: 12 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -582,29 +582,33 @@ LWLockInitialize(LWLock *lock, int tranche_id)
582582
staticbool
583583
LWLockAttemptLock(LWLock*lock,LWLockModemode)
584584
{
585+
uint32old_state;
586+
585587
AssertArg(mode==LW_EXCLUSIVE||mode==LW_SHARED);
586588

589+
/*
590+
* Read once outside the loop, later iterations will get the newer value
591+
* via compare & exchange.
592+
*/
593+
old_state=pg_atomic_read_u32(&lock->state);
594+
587595
/* loop until we've determined whether we could acquire the lock or not */
588596
while (true)
589597
{
590-
uint32old_state;
591-
uint32expected_state;
592598
uint32desired_state;
593599
boollock_free;
594600

595-
old_state=pg_atomic_read_u32(&lock->state);
596-
expected_state=old_state;
597-
desired_state=expected_state;
601+
desired_state=old_state;
598602

599603
if (mode==LW_EXCLUSIVE)
600604
{
601-
lock_free= (expected_state&LW_LOCK_MASK)==0;
605+
lock_free= (old_state&LW_LOCK_MASK)==0;
602606
if (lock_free)
603607
desired_state+=LW_VAL_EXCLUSIVE;
604608
}
605609
else
606610
{
607-
lock_free= (expected_state&LW_VAL_EXCLUSIVE)==0;
611+
lock_free= (old_state&LW_VAL_EXCLUSIVE)==0;
608612
if (lock_free)
609613
desired_state+=LW_VAL_SHARED;
610614
}
@@ -620,7 +624,7 @@ LWLockAttemptLock(LWLock *lock, LWLockMode mode)
620624
* Retry if the value changed since we last looked at it.
621625
*/
622626
if (pg_atomic_compare_exchange_u32(&lock->state,
623-
&expected_state,desired_state))
627+
&old_state,desired_state))
624628
{
625629
if (lock_free)
626630
{

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