powerpc: Optimize lock elision for pthread_mutex_t

With TLE enabled, the adapt count variable update incurs
an 8% overhead before entering the critical section of an
elided mutex.

Instead, if it is done right after leaving the critical
section, this serialization can be avoided.

This alters the existing behavior of __lll_trylock_elision
as it will only decrement the adapt_count if it successfully
acquires the lock.

	* sysdeps/unix/sysv/linux/powerpc/elision-lock.c
	(__lll_lock_elision): Remove adapt_count decrement...
	* sysdeps/unix/sysv/linux/powerpc/elision-trylock.c
	(__lll_trylock_elision): Likewise.
	* sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
	(__lll_unlock_elision): ... to here. And utilize
	new adapt_count parameter.
	* sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
	(__lll_unlock_elision): Update to include adapt_count
	parameter.
	(lll_unlock_elision): Pass pointer to adapt_count
	variable.

1 files changed, 10 insertions, 2 deletions

diff --git a/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c b/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
index f04c339383..72b893d9f4 100644
--- a/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
+++ b/sysdeps/unix/sysv/linux/powerpc/elision-unlock.c
@@ -21,12 +21,20 @@
 #include "htm.h"
 
 int
-__lll_unlock_elision(int *lock, int pshared)
+__lll_unlock_elision (int *lock, short *adapt_count, int pshared)
 {
   /* When the lock was free we're in a transaction.  */
   if (*lock == 0)
     __builtin_tend (0);
   else
-    lll_unlock ((*lock), pshared);
+    {
+      lll_unlock ((*lock), pshared);
+
+      /* Update the adapt count AFTER completing the critical section.
+         Doing this here prevents unneeded stalling when entering
+         a critical section.  Saving about 8% runtime on P8.  */
+      if (*adapt_count > 0)
+	(*adapt_count)--;
+    }
   return 0;
 }