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

tglsfdc · tglsfdc · commit 5cfa8dd3007d · 2012-01-02T00:02:00.000-05:00
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.
diff --git a/configure b/configure
@@ -1635,7 +1635,7 @@ Copyright (C) 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001,
 This configure script is free software; the Free Software Foundation
 gives unlimited permission to copy, distribute and modify it.
 
-Copyright (c) 1996-2011, PostgreSQL Global Development Group
+Copyright (c) 1996-2012, PostgreSQL Global Development Group
 _ACEOF
   exit
 fi
@@ -18207,6 +18207,66 @@ rm -f core conftest.err conftest.$ac_objext conftest_ipa8_conftest.oo \
       conftest$ac_exeext conftest.$ac_ext
 fi
 
+# On PPC, check if assembler supports LWARX instruction's mutex hint bit
+case $host_cpu in
+  ppc*|powerpc*)
+    { $as_echo "$as_me:$LINENO: checking whether assembler supports lwarx hint bit" >&5
+$as_echo_n "checking whether assembler supports lwarx hint bit... " >&6; }
+    cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+
+int
+main ()
+{
+int a = 0; int *p = &a; int r;
+	 __asm__ __volatile__ (" lwarx %0,0,%1,1\n" : "=&r"(r) : "r"(p));
+  ;
+  return 0;
+}
+_ACEOF
+rm -f conftest.$ac_objext
+if { (ac_try="$ac_compile"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval ac_try_echo="\"\$as_me:$LINENO: $ac_try_echo\""
+$as_echo "$ac_try_echo") >&5
+  (eval "$ac_compile") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  $as_echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } && {
+	 test -z "$ac_c_werror_flag" ||
+	 test ! -s conftest.err
+       } && test -s conftest.$ac_objext; then
+  pgac_cv_have_ppc_mutex_hint=yes
+else
+  $as_echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	pgac_cv_have_ppc_mutex_hint=no
+fi
+
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+    { $as_echo "$as_me:$LINENO: result: $pgac_cv_have_ppc_mutex_hint" >&5
+$as_echo "$pgac_cv_have_ppc_mutex_hint" >&6; }
+    if test x"$pgac_cv_have_ppc_mutex_hint" = xyes ; then
+
+cat >>confdefs.h <<\_ACEOF
+#define HAVE_PPC_LWARX_MUTEX_HINT 1
+_ACEOF
+
+    fi
+  ;;
+esac
+
 # Check largefile support.  You might think this is a system service not a
 # compiler characteristic, but you'd be wrong.  We must check this before
 # probing existence of related functions such as fseeko, since the largefile
diff --git a/configure.in b/configure.in
@@ -1170,11 +1170,27 @@ if test "$with_krb5" = yes; then
   AC_MSG_CHECKING(for krb5_free_unparsed_name)
   AC_TRY_LINK([#include <krb5.h>],
               [krb5_free_unparsed_name(NULL,NULL);],
-              [AC_DEFINE(HAVE_KRB5_FREE_UNPARSED_NAME, 1, [Define to 1 if you have krb5_free_unparsed_name])
+              [AC_DEFINE(HAVE_KRB5_FREE_UNPARSED_NAME, 1, [Define to 1 if you have krb5_free_unparsed_name.])
 AC_MSG_RESULT(yes)],
               [AC_MSG_RESULT(no)])
 fi
 
+# On PPC, check if assembler supports LWARX instruction's mutex hint bit
+case $host_cpu in
+  ppc*|powerpc*)
+    AC_MSG_CHECKING([whether assembler supports lwarx hint bit])
+    AC_TRY_COMPILE([],
+	[int a = 0; int *p = &a; int r;
+	 __asm__ __volatile__ (" lwarx %0,0,%1,1\n" : "=&r"(r) : "r"(p));],
+	[pgac_cv_have_ppc_mutex_hint=yes],
+	[pgac_cv_have_ppc_mutex_hint=no])
+    AC_MSG_RESULT([$pgac_cv_have_ppc_mutex_hint])
+    if test x"$pgac_cv_have_ppc_mutex_hint" = xyes ; then
+	AC_DEFINE(HAVE_PPC_LWARX_MUTEX_HINT, 1, [Define to 1 if the assembler supports PPC's LWARX mutex hint bit.])
+    fi
+  ;;
+esac
+
 # Check largefile support.  You might think this is a system service not a
 # compiler characteristic, but you'd be wrong.  We must check this before
 # probing existence of related functions such as fseeko, since the largefile
diff --git 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
@@ -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
  *------------------------------------------------------------------------
  */
@@ -170,6 +173,21 @@
 #define PG_PRINTF_ATTRIBUTE printf
 #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
diff --git 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"
