Optimize generic spinlock code and use C11 like atomic macros.

This patch optimizes the generic spinlock code.

The type pthread_spinlock_t is a typedef to volatile int on all archs.
Passing a volatile pointer to the atomic macros which are not mapped to the
C11 atomic builtins can lead to extra stores and loads to stack if such
a macro creates a temporary variable by using "__typeof (*(mem)) tmp;".
Thus, those macros which are used by spinlock code - atomic_exchange_acquire,
atomic_load_relaxed, atomic_compare_exchange_weak - have to be adjusted.
According to the comment from  Szabolcs Nagy, the type of a cast expression is
unqualified (see http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_423.htm):
__typeof ((__typeof (*(mem)) *(mem)) tmp;
Thus from spinlock perspective the variable tmp is of type int instead of
type volatile int.  This patch adjusts those macros in include/atomic.h.
With this construct GCC >= 5 omits the extra stores and loads.

The atomic macros are replaced by the C11 like atomic macros and thus
the code is aligned to it.  The pthread_spin_unlock implementation is now
using release memory order instead of sequentially consistent memory order.
The issue with passed volatile int pointers applies to the C11 like atomic
macros as well as the ones used before.

I've added a glibc_likely hint to the first atomic exchange in
pthread_spin_lock in order to return immediately to the caller if the lock is
free.  Without the hint, there is an additional jump if the lock is free.

I've added the atomic_spin_nop macro within the loop of plain reads.
The plain reads are also realized by C11 like atomic_load_relaxed macro.

The new define ATOMIC_EXCHANGE_USES_CAS determines if the first try to acquire
the spinlock in pthread_spin_lock or pthread_spin_trylock is an exchange
or a CAS.  This is defined in atomic-machine.h for all architectures.

The define SPIN_LOCK_READS_BETWEEN_CMPXCHG is now removed.
There is no technical reason for throwing in a CAS every now and then,
and so far we have no evidence that it can improve performance.
If that would be the case, we have to adjust other spin-waiting loops
elsewhere, too!  Using a CAS loop without plain reads is not a good idea
on many targets and wasn't used by one.  Thus there is now no option to
do so.

Architectures are now using the generic spinlock automatically if they
do not provide an own implementation.  Thus the pthread_spin_lock.c files
in sysdeps folder are deleted.

ChangeLog:

	* NEWS: Mention new spinlock implementation.
	* include/atomic.h:
	(__atomic_val_bysize): Cast type to omit volatile qualifier.
	(atomic_exchange_acq): Likewise.
	(atomic_load_relaxed): Likewise.
	(ATOMIC_EXCHANGE_USES_CAS): Check definition.
	* nptl/pthread_spin_init.c (pthread_spin_init):
	Use atomic_store_relaxed.
	* nptl/pthread_spin_lock.c (pthread_spin_lock):
	Use C11-like atomic macros.
	* nptl/pthread_spin_trylock.c (pthread_spin_trylock):
	Likewise.
	* nptl/pthread_spin_unlock.c (pthread_spin_unlock):
	Use atomic_store_release.
	* sysdeps/aarch64/nptl/pthread_spin_lock.c: Delete File.
	* sysdeps/arm/nptl/pthread_spin_lock.c: Likewise.
	* sysdeps/hppa/nptl/pthread_spin_lock.c: Likewise.
	* sysdeps/m68k/nptl/pthread_spin_lock.c: Likewise.
	* sysdeps/microblaze/nptl/pthread_spin_lock.c: Likewise.
	* sysdeps/mips/nptl/pthread_spin_lock.c: Likewise.
	* sysdeps/nios2/nptl/pthread_spin_lock.c: Likewise.
	* sysdeps/aarch64/atomic-machine.h (ATOMIC_EXCHANGE_USES_CAS): Define.
	* sysdeps/alpha/atomic-machine.h: Likewise.
	* sysdeps/arm/atomic-machine.h: Likewise.
	* sysdeps/i386/atomic-machine.h: Likewise.
	* sysdeps/ia64/atomic-machine.h: Likewise.
	* sysdeps/m68k/coldfire/atomic-machine.h: Likewise.
	* sysdeps/m68k/m680x0/m68020/atomic-machine.h: Likewise.
	* sysdeps/microblaze/atomic-machine.h: Likewise.
	* sysdeps/mips/atomic-machine.h: Likewise.
	* sysdeps/powerpc/powerpc32/atomic-machine.h: Likewise.
	* sysdeps/powerpc/powerpc64/atomic-machine.h: Likewise.
	* sysdeps/s390/atomic-machine.h: Likewise.
	* sysdeps/sparc/sparc32/atomic-machine.h: Likewise.
	* sysdeps/sparc/sparc32/sparcv9/atomic-machine.h: Likewise.
	* sysdeps/sparc/sparc64/atomic-machine.h: Likewise.
	* sysdeps/tile/tilegx/atomic-machine.h: Likewise.
	* sysdeps/tile/tilepro/atomic-machine.h: Likewise.
	* sysdeps/unix/sysv/linux/hppa/atomic-machine.h: Likewise.
	* sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h: Likewise.
	* sysdeps/unix/sysv/linux/nios2/atomic-machine.h: Likewise.
	* sysdeps/unix/sysv/linux/sh/atomic-machine.h: Likewise.
	* sysdeps/x86_64/atomic-machine.h: Likewise.

29 files changed, 57 insertions, 167 deletions

diff --git a/sysdeps/aarch64/atomic-machine.h b/sysdeps/aarch64/atomic-machine.h
index a5d2213..eb59a5b 100644
--- a/sysdeps/aarch64/atomic-machine.h
+++ b/sysdeps/aarch64/atomic-machine.h
@@ -38,6 +38,7 @@ typedef uintmax_t uatomic_max_t;
 
 #define __HAVE_64B_ATOMICS 1
 #define USE_ATOMIC_COMPILER_BUILTINS 1
+#define ATOMIC_EXCHANGE_USES_CAS 0
 
 /* Compare and exchange.
    For all "bool" routines, we return FALSE if exchange succesful.  */
diff --git a/sysdeps/aarch64/nptl/pthread_spin_lock.c b/sysdeps/aarch64/nptl/pthread_spin_lock.c
deleted file mode 100644
index fcfcb40..0000000
--- a/sysdeps/aarch64/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,24 +0,0 @@
-/* Copyright (C) 2008-2017 Free Software Foundation, Inc.
-
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public License as
-   published by the Free Software Foundation; either version 2.1 of the
-   License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <http://www.gnu.org/licenses/>.  */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
-   it will resolve to this very file.  Using "sysdeps/.." as reference to the
-   top level directory does the job.  */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/alpha/atomic-machine.h b/sysdeps/alpha/atomic-machine.h
index 06e93f2..2cb2290 100644
--- a/sysdeps/alpha/atomic-machine.h
+++ b/sysdeps/alpha/atomic-machine.h
@@ -45,6 +45,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 1
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 
 #ifdef UP
 # define __MB		/* nothing */
diff --git a/sysdeps/arm/atomic-machine.h b/sysdeps/arm/atomic-machine.h
index eeac7f0..c22d05b 100644
--- a/sysdeps/arm/atomic-machine.h
+++ b/sysdeps/arm/atomic-machine.h
@@ -35,6 +35,7 @@ typedef uintmax_t uatomic_max_t;
 
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 1
 
 void __arm_link_error (void);
 
diff --git a/sysdeps/arm/nptl/pthread_spin_lock.c b/sysdeps/arm/nptl/pthread_spin_lock.c
deleted file mode 100644
index 037b3b8..0000000
--- a/sysdeps/arm/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,23 +0,0 @@
-/* Copyright (C) 2008-2017 Free Software Foundation, Inc.
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library.  If not, see
-   <http://www.gnu.org/licenses/>.  */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
-   it will resolve to this very file.  Using "sysdeps/.." as reference to the
-   top level directory does the job.  */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/hppa/nptl/pthread_spin_lock.c b/sysdeps/hppa/nptl/pthread_spin_lock.c
deleted file mode 100644
index 14f36a6..0000000
--- a/sysdeps/hppa/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,23 +0,0 @@
-/* Copyright (C) 2005-2017 Free Software Foundation, Inc.
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library.  If not, see
-   <http://www.gnu.org/licenses/>.  */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
-   it will resolve to this very file.  Using "sysdeps/.." as reference to the
-   top level directory does the job.  */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/i386/atomic-machine.h b/sysdeps/i386/atomic-machine.h
index 77759f7..0e24200 100644
--- a/sysdeps/i386/atomic-machine.h
+++ b/sysdeps/i386/atomic-machine.h
@@ -56,6 +56,7 @@ typedef uintmax_t uatomic_max_t;
 
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 0
 
 
 #define atomic_compare_and_exchange_val_acq(mem, newval, oldval) \
diff --git a/sysdeps/ia64/atomic-machine.h b/sysdeps/ia64/atomic-machine.h
index ecf9750..f04ac05 100644
--- a/sysdeps/ia64/atomic-machine.h
+++ b/sysdeps/ia64/atomic-machine.h
@@ -46,6 +46,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 1
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 0
+
 
 #define __arch_compare_and_exchange_bool_8_acq(mem, newval, oldval) \
   (abort (), 0)
diff --git a/sysdeps/m68k/coldfire/atomic-machine.h b/sysdeps/m68k/coldfire/atomic-machine.h
index 9aeb993..d2e4a1f 100644
--- a/sysdeps/m68k/coldfire/atomic-machine.h
+++ b/sysdeps/m68k/coldfire/atomic-machine.h
@@ -53,6 +53,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 1
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 /* The only basic operation needed is compare and exchange.  */
 #define atomic_compare_and_exchange_val_acq(mem, newval, oldval) \
   ({ __typeof (mem) __gmemp = (mem);				      \
diff --git a/sysdeps/m68k/m680x0/m68020/atomic-machine.h b/sysdeps/m68k/m680x0/m68020/atomic-machine.h
index 00dc22d..83238f5 100644
--- a/sysdeps/m68k/m680x0/m68020/atomic-machine.h
+++ b/sysdeps/m68k/m680x0/m68020/atomic-machine.h
@@ -47,6 +47,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 1
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 #define __arch_compare_and_exchange_val_8_acq(mem, newval, oldval) \
   ({ __typeof (*(mem)) __ret;						      \
      __asm __volatile ("cas%.b %0,%2,%1"				      \
diff --git a/sysdeps/m68k/nptl/pthread_spin_lock.c b/sysdeps/m68k/nptl/pthread_spin_lock.c
deleted file mode 100644
index 62795f4..0000000
--- a/sysdeps/m68k/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,24 +0,0 @@
-/* Copyright (C) 2010-2017 Free Software Foundation, Inc.
-   This file is part of the GNU C Library.
-   Contributed by Maxim Kuvyrkov <maxim@codesourcery.com>, 2010.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library.  If not, see
-   <http://www.gnu.org/licenses/>.  */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
-   it will resolve to this very file.  Using "sysdeps/.." as reference to the
-   top level directory does the job.  */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/microblaze/atomic-machine.h b/sysdeps/microblaze/atomic-machine.h
index dc5309c..d73c69d 100644
--- a/sysdeps/microblaze/atomic-machine.h
+++ b/sysdeps/microblaze/atomic-machine.h
@@ -38,6 +38,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 
 /* Microblaze does not have byte and halfword forms of load and reserve and
    store conditional. So for microblaze we stub out the 8- and 16-bit forms.  */
diff --git a/sysdeps/microblaze/nptl/pthread_spin_lock.c b/sysdeps/microblaze/nptl/pthread_spin_lock.c
deleted file mode 100644
index fcfcb40..0000000
--- a/sysdeps/microblaze/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,24 +0,0 @@
-/* Copyright (C) 2008-2017 Free Software Foundation, Inc.
-
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public License as
-   published by the Free Software Foundation; either version 2.1 of the
-   License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <http://www.gnu.org/licenses/>.  */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
-   it will resolve to this very file.  Using "sysdeps/.." as reference to the
-   top level directory does the job.  */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/mips/atomic-machine.h b/sysdeps/mips/atomic-machine.h
index 54c182b..16fef6b 100644
--- a/sysdeps/mips/atomic-machine.h
+++ b/sysdeps/mips/atomic-machine.h
@@ -92,7 +92,15 @@ typedef uintmax_t uatomic_max_t;
    have no assembly alternative available and want to avoid the __sync_*
    builtins if at all possible.  */
 
-#define USE_ATOMIC_COMPILER_BUILTINS 1
+# define USE_ATOMIC_COMPILER_BUILTINS 1
+
+/* MIPS is an LL/SC machine.  However, XLP has a direct atomic exchange
+   instruction which will be used by __atomic_exchange_n.  */
+# ifdef _MIPS_ARCH_XLP
+#  define ATOMIC_EXCHANGE_USES_CAS 0
+# else
+#  define ATOMIC_EXCHANGE_USES_CAS 1
+# endif
 
 /* Compare and exchange.
    For all "bool" routines, we return FALSE if exchange succesful.  */
@@ -213,7 +221,8 @@ typedef uintmax_t uatomic_max_t;
 /* This implementation using inline assembly will be removed once glibc
    requires GCC 4.8 or later to build.  */
 
-#define USE_ATOMIC_COMPILER_BUILTINS 0
+# define USE_ATOMIC_COMPILER_BUILTINS 0
+# define ATOMIC_EXCHANGE_USES_CAS 1
 
 /* Compare and exchange.  For all of the "xxx" routines, we expect a
    "__prev" and a "__cmp" variable to be provided by the enclosing scope,
diff --git a/sysdeps/mips/nptl/pthread_spin_lock.c b/sysdeps/mips/nptl/pthread_spin_lock.c
deleted file mode 100644
index 19d87a5..0000000
--- a/sysdeps/mips/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,23 +0,0 @@
-/* Copyright (C) 2012-2017 Free Software Foundation, Inc.
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library.  If not, see
-   <http://www.gnu.org/licenses/>.  */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
-   it will resolve to this very file.  Using "sysdeps/.." as reference to the
-   top level directory does the job.  */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/nios2/nptl/pthread_spin_lock.c b/sysdeps/nios2/nptl/pthread_spin_lock.c
deleted file mode 100644
index b203469..0000000
--- a/sysdeps/nios2/nptl/pthread_spin_lock.c
+++ /dev/null
@@ -1,24 +0,0 @@
-/* pthread spin-lock implementation for Nios II.
-   Copyright (C) 2005-2017 Free Software Foundation, Inc.
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library.  If not, see
-   <http://www.gnu.org/licenses/>.  */
-
-#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
-
-/* We can't use the normal "#include <nptl/pthread_spin_lock.c>" because
-   it will resolve to this very file.  Using "sysdeps/.." as reference to the
-   top level directory does the job.  */
-#include <sysdeps/../nptl/pthread_spin_lock.c>
diff --git a/sysdeps/powerpc/powerpc32/atomic-machine.h b/sysdeps/powerpc/powerpc32/atomic-machine.h
index 20d5e85..96c7d81 100644
--- a/sysdeps/powerpc/powerpc32/atomic-machine.h
+++ b/sysdeps/powerpc/powerpc32/atomic-machine.h
@@ -35,6 +35,7 @@
 
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 1
 
 /*
  * The 32-bit exchange_bool is different on powerpc64 because the subf
diff --git a/sysdeps/powerpc/powerpc64/atomic-machine.h b/sysdeps/powerpc/powerpc64/atomic-machine.h
index 40c308e..46df488 100644
--- a/sysdeps/powerpc/powerpc64/atomic-machine.h
+++ b/sysdeps/powerpc/powerpc64/atomic-machine.h
@@ -35,6 +35,7 @@
 
 #define __HAVE_64B_ATOMICS 1
 #define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 1
 
 /* The 32-bit exchange_bool is different on powerpc64 because the subf
    does signed 64-bit arithmetic while the lwarx is 32-bit unsigned
diff --git a/sysdeps/s390/atomic-machine.h b/sysdeps/s390/atomic-machine.h
index 690d2e3..adaca40 100644
--- a/sysdeps/s390/atomic-machine.h
+++ b/sysdeps/s390/atomic-machine.h
@@ -67,6 +67,8 @@ typedef uintmax_t uatomic_max_t;
 # define __HAVE_64B_ATOMICS 0
 #endif
 
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 /* Implement some of the non-C11 atomic macros from include/atomic.h
    with help of the C11 atomic builtins.  The other non-C11 atomic macros
    are using the macros defined here.  */
diff --git a/sysdeps/sparc/sparc32/atomic-machine.h b/sysdeps/sparc/sparc32/atomic-machine.h
index acd029e..a2fe848 100644
--- a/sysdeps/sparc/sparc32/atomic-machine.h
+++ b/sysdeps/sparc/sparc32/atomic-machine.h
@@ -50,6 +50,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 
 /* We have no compare and swap, just test and set.
    The following implementation contends on 64 global locks
diff --git a/sysdeps/sparc/sparc32/sparcv9/atomic-machine.h b/sysdeps/sparc/sparc32/sparcv9/atomic-machine.h
index 7f7895e..2b36dfc 100644
--- a/sysdeps/sparc/sparc32/sparcv9/atomic-machine.h
+++ b/sysdeps/sparc/sparc32/sparcv9/atomic-machine.h
@@ -47,6 +47,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 0
+
 
 #define __arch_compare_and_exchange_val_8_acq(mem, newval, oldval) \
   (abort (), (__typeof (*mem)) 0)
diff --git a/sysdeps/sparc/sparc64/atomic-machine.h b/sysdeps/sparc/sparc64/atomic-machine.h
index 44a43ff..7200932 100644
--- a/sysdeps/sparc/sparc64/atomic-machine.h
+++ b/sysdeps/sparc/sparc64/atomic-machine.h
@@ -47,6 +47,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 1
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 
 #define __arch_compare_and_exchange_val_8_acq(mem, newval, oldval) \
   (abort (), (__typeof (*mem)) 0)
diff --git a/sysdeps/tile/tilegx/atomic-machine.h b/sysdeps/tile/tilegx/atomic-machine.h
index 6345251..e77f670 100644
--- a/sysdeps/tile/tilegx/atomic-machine.h
+++ b/sysdeps/tile/tilegx/atomic-machine.h
@@ -31,6 +31,7 @@
 #endif
 
 #define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 0
 
 /* Pick appropriate 8- or 4-byte instruction. */
 #define __atomic_update(mem, v, op)                                     \
diff --git a/sysdeps/tile/tilepro/atomic-machine.h b/sysdeps/tile/tilepro/atomic-machine.h
index 33a8b85..45e36de 100644
--- a/sysdeps/tile/tilepro/atomic-machine.h
+++ b/sysdeps/tile/tilepro/atomic-machine.h
@@ -23,6 +23,7 @@
 
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
+#define ATOMIC_EXCHANGE_USES_CAS 0
 
 /* 32-bit integer compare-and-exchange. */
 static __inline __attribute__ ((always_inline))
diff --git a/sysdeps/unix/sysv/linux/hppa/atomic-machine.h b/sysdeps/unix/sysv/linux/hppa/atomic-machine.h
index 2cd2235..59581bd 100644
--- a/sysdeps/unix/sysv/linux/hppa/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/hppa/atomic-machine.h
@@ -39,6 +39,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 /* prev = *addr;
    if (prev == old)
      *addr = new;
diff --git a/sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h b/sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h
index 6755a05..1daac51 100644
--- a/sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/m68k/coldfire/atomic-machine.h
@@ -39,6 +39,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 /* The only basic operation needed is compare and exchange.  */
 /* For ColdFire we'll have to trap into the kernel mode anyway,
    so trap from the library rather then from the kernel wrapper.  */
diff --git a/sysdeps/unix/sysv/linux/nios2/atomic-machine.h b/sysdeps/unix/sysv/linux/nios2/atomic-machine.h
index 6111ccf..d4dd025 100644
--- a/sysdeps/unix/sysv/linux/nios2/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/nios2/atomic-machine.h
@@ -34,6 +34,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 #define __arch_compare_and_exchange_val_8_acq(mem, newval, oldval)	\
   (abort (), (__typeof (*mem)) 0)
 #define __arch_compare_and_exchange_val_16_acq(mem, newval, oldval)	\
diff --git a/sysdeps/unix/sysv/linux/sh/atomic-machine.h b/sysdeps/unix/sysv/linux/sh/atomic-machine.h
index 3c58b70..5a7e953 100644
--- a/sysdeps/unix/sysv/linux/sh/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/sh/atomic-machine.h
@@ -47,6 +47,9 @@ typedef uintmax_t uatomic_max_t;
 #define __HAVE_64B_ATOMICS 0
 #define USE_ATOMIC_COMPILER_BUILTINS 0
 
+/* XXX Is this actually correct?  */
+#define ATOMIC_EXCHANGE_USES_CAS 1
+
 /* SH kernel has implemented a gUSA ("g" User Space Atomicity) support
    for the user space atomicity. The atomicity macros use this scheme.
 
diff --git a/sysdeps/x86_64/atomic-machine.h b/sysdeps/x86_64/atomic-machine.h
index 2e8a9aa..c454734 100644
--- a/sysdeps/x86_64/atomic-machine.h
+++ b/sysdeps/x86_64/atomic-machine.h
@@ -59,6 +59,7 @@ typedef uintmax_t uatomic_max_t;
 
 #define __HAVE_64B_ATOMICS 1
 #define USE_ATOMIC_COMPILER_BUILTINS 1
+#define ATOMIC_EXCHANGE_USES_CAS 0
 
 #define atomic_compare_and_exchange_val_acq(mem, newval, oldval) \
   __sync_val_compare_and_swap (mem, oldval, newval)