Fix atomic ops on ARM to compile in NaCl untrusted targets.

base/atomicops_internals_gcc.h

Index: base/atomicops_internals_gcc.h
===================================================================
--- base/atomicops_internals_gcc.h	(revision 152192)
+++ base/atomicops_internals_gcc.h	(working copy)
@@ -2,39 +2,34 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// This file is an internal atomic implementation, use base/atomicops.h instead.
-//
-// LinuxKernelCmpxchg and Barrier_AtomicIncrement are from Google Gears.
+// This file is an internal atomic implementation, include base/atomicops.h
+// instead. This file is for platforms that use GCC intrinsics rather than
+// platform-specific assembly code for atomic operations.
 
-#ifndef BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_
-#define BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_
+#ifndef BASE_ATOMICOPS_INTERNALS_GCC_H_
+#define BASE_ATOMICOPS_INTERNALS_GCC_H_
 
 namespace base {
 namespace subtle {
 
-// 0xffff0fc0 is the hard coded address of a function provided by
-// the kernel which implements an atomic compare-exchange. On older
-// ARM architecture revisions (pre-v6) this may be implemented using
-// a syscall. This address is stable, and in active use (hard coded)
-// by at least glibc-2.7 and the Android C library.
-typedef Atomic32 (*LinuxKernelCmpxchgFunc)(Atomic32 old_value,
-                                           Atomic32 new_value,
-                                           volatile Atomic32* ptr);
-LinuxKernelCmpxchgFunc pLinuxKernelCmpxchg __attribute__((weak)) =
-    (LinuxKernelCmpxchgFunc) 0xffff0fc0;
+namespace {
 
-typedef void (*LinuxKernelMemoryBarrierFunc)(void);
-LinuxKernelMemoryBarrierFunc pLinuxKernelMemoryBarrier __attribute__((weak)) =
-    (LinuxKernelMemoryBarrierFunc) 0xffff0fa0;
+bool Barrier_CompareAndSwap(volatile Atomic32* ptr,
+                            Atomic32 old_value,
+                            Atomic32 new_value) {
+  // The GCC intrinsic for compare-and-swap is a full memory barrier.
+  return __sync_bool_compare_and_swap(const_cast<Atomic32*>(ptr),

[Roland McGrath, 2012/08/21 21:52:59]

What's this cast for?

[bbudge, 2012/08/21 22:06:12]

It's a vestige of the old file. As you point out, it's not needed. Since that
was the only reason to wrap this up in its own function, I removed it just call
the intrinsic directly at the 3 call sites. I did add a comment in the Acquire_
function explaining what's going on.

+                                      old_value, new_value);
+}
 
+}  // namespace
 
 inline Atomic32 NoBarrier_CompareAndSwap(volatile Atomic32* ptr,
                                          Atomic32 old_value,
                                          Atomic32 new_value) {
   Atomic32 prev_value = *ptr;
   do {
-    if (!pLinuxKernelCmpxchg(old_value, new_value,
-                             const_cast<Atomic32*>(ptr))) {
+    if (Barrier_CompareAndSwap(ptr, old_value, new_value)) {
       return old_value;
     }
     prev_value = *ptr;
@@ -47,8 +42,7 @@
   Atomic32 old_value;
   do {
     old_value = *ptr;
-  } while (pLinuxKernelCmpxchg(old_value, new_value,
-                               const_cast<Atomic32*>(ptr)));
+  } while (!Barrier_CompareAndSwap(ptr, old_value, new_value));
   return old_value;
 }
 
@@ -63,14 +57,12 @@
     // Atomic exchange the old value with an incremented one.
     Atomic32 old_value = *ptr;
     Atomic32 new_value = old_value + increment;
-    if (pLinuxKernelCmpxchg(old_value, new_value,
-                            const_cast<Atomic32*>(ptr)) == 0) {
+    if (!Barrier_CompareAndSwap(ptr, old_value, new_value) == 0) {
       // The exchange took place as expected.
       return new_value;
     }
     // Otherwise, *ptr changed mid-loop and we need to retry.
   }
-
 }
 
 inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr,
@@ -90,7 +82,7 @@
 }
 
 inline void MemoryBarrier() {
-  pLinuxKernelMemoryBarrier();
+  __sync_synchronize();
 }
 
 inline void Acquire_Store(volatile Atomic32* ptr, Atomic32 value) {
@@ -118,7 +110,8 @@
   return *ptr;
 }
 
-} // namespace base::subtle
-} // namespace base
+}  // namespace base::subtle
+}  // namespace base
 
-#endif  // BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_
+#endif  // BASE_ATOMICOPS_INTERNALS_GCC_H_
+