Use mutex hint bit in PPC LWARX instructions, where possible.

The hint bit makes for a small but measurable performance improvement
in access to contended spinlocks.

On the other hand, some PPC chips give an illegal-instruction failure.
There doesn't seem to be a completely bulletproof way to tell whether the
hint bit will cause an illegal-instruction failure other than by trying
it; but most if not all 64-bit PPC machines should accept it, so follow
the Linux kernel's lead and assume it's okay to use it in 64-bit builds.
Of course we must also check whether the assembler accepts the command,
since even with a recent CPU the toolchain could be old.

Patch by Manabu Ori, significantly modified by me.

3 files changed, 26 insertions, 1 deletions

diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index ef467b73866..e1b7fea49fc 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -275,7 +275,7 @@
 /* Define to 1 if `text.data' is member of `krb5_error'. */
 #undef HAVE_KRB5_ERROR_TEXT_DATA
 
-/* Define to 1 if you have krb5_free_unparsed_name */
+/* Define to 1 if you have krb5_free_unparsed_name. */
 #undef HAVE_KRB5_FREE_UNPARSED_NAME
 
 /* Define to 1 if `client' is member of `krb5_ticket'. */
@@ -384,6 +384,9 @@
 /* Define to 1 if you have the POSIX signal interface. */
 #undef HAVE_POSIX_SIGNALS
 
+/* Define to 1 if the assembler supports PPC's LWARX mutex hint bit. */
+#undef HAVE_PPC_LWARX_MUTEX_HINT
+
 /* Define to 1 if you have the `pstat' function. */
 #undef HAVE_PSTAT
 
diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index ac434fabbc4..810be27e813 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -6,6 +6,9 @@
  * for developers.	If you edit any of these, be sure to do a *full*
  * rebuild (and an initdb if noted).
  *
+ * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
  * src/include/pg_config_manual.h
  *------------------------------------------------------------------------
  */
@@ -171,6 +174,21 @@
 #endif
 
 /*
+ * On PPC machines, decide whether to use the mutex hint bit in LWARX
+ * instructions.  Setting the hint bit will slightly improve spinlock
+ * performance on POWER6 and later machines, but does nothing before that,
+ * and will result in illegal-instruction failures on some pre-POWER4
+ * machines.  By default we use the hint bit when building for 64-bit PPC,
+ * which should be safe in nearly all cases.  You might want to override
+ * this if you are building 32-bit code for a known-recent PPC machine.
+ */
+#ifdef HAVE_PPC_LWARX_MUTEX_HINT /* must have assembler support in any case */
+#if defined(__ppc64__) || defined(__powerpc64__)
+#define USE_PPC_LWARX_MUTEX_HINT
+#endif
+#endif
+
+/*
  *------------------------------------------------------------------------
  * The following symbols are for enabling debugging code, not for
  * controlling user-visible features or resource limits.
diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.h
index 98c12db3ab3..cc67be81eb3 100644
--- a/src/include/storage/s_lock.h
+++ b/src/include/storage/s_lock.h
@@ -372,7 +372,11 @@ tas(volatile slock_t *lock)
 	int _res;
 
 	__asm__ __volatile__(
+#ifdef USE_PPC_LWARX_MUTEX_HINT
+"	lwarx   %0,0,%3,1	\n"
+#else
 "	lwarx   %0,0,%3		\n"
+#endif
 "	cmpwi   %0,0		\n"
 "	bne     1f			\n"
 "	addi    %0,%0,1		\n"