Atomic ops: sync with Chromium.

src/atomicops_internals_arm_gcc.h

Index: src/atomicops_internals_arm_gcc.h
diff --git a/src/atomicops_internals_arm_gcc.h b/src/atomicops_internals_arm_gcc.h
index 6c30256d93dea601b5d3592250c303dbae0dadeb..918920d02ae66e5723c0bb9993473be49f66036b 100644
--- a/src/atomicops_internals_arm_gcc.h
+++ b/src/atomicops_internals_arm_gcc.h
@@ -32,46 +32,197 @@
 #ifndef V8_ATOMICOPS_INTERNALS_ARM_GCC_H_
 #define V8_ATOMICOPS_INTERNALS_ARM_GCC_H_
 
+#if defined(__QNXNTO__)
+#include <sys/cpuinline.h>
+#endif
+
 namespace v8 {
 namespace internal {
 
-// 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;
+// Memory barriers on ARM are funky, but the kernel is here to help:
+//
+// * ARMv5 didn't support SMP, there is no memory barrier instruction at
+//   all on this architecture, or when targeting its machine code.
+//
+// * Some ARMv6 CPUs support SMP. A full memory barrier can be produced by
+//   writing a random value to a very specific coprocessor register.
+//
+// * On ARMv7, the "dmb" instruction is used to perform a full memory
+//   barrier (though writing to the co-processor will still work).
+//   However, on single core devices (e.g. Nexus One, or Nexus S),
+//   this instruction will take up to 200 ns, which is huge, even though
+//   it's completely un-needed on these devices.
+//
+// * There is no easy way to determine at runtime if the device is
+//   single or multi-core. However, the kernel provides a useful helper
+//   function at a fixed memory address (0xffff0fa0), which will always
+//   perform a memory barrier in the most efficient way. I.e. on single
+//   core devices, this is an empty function that exits immediately.
+//   On multi-core devices, it implements a full memory barrier.
+//
+// * This source could be compiled to ARMv5 machine code that runs on a
+//   multi-core ARMv6 or ARMv7 device. In this case, memory barriers
+//   are needed for correct execution. Always call the kernel helper, even
+//   when targeting ARMv5TE.
+//
 
-typedef void (*LinuxKernelMemoryBarrierFunc)(void);
-LinuxKernelMemoryBarrierFunc pLinuxKernelMemoryBarrier __attribute__((weak)) =
-    (LinuxKernelMemoryBarrierFunc) 0xffff0fa0;
+inline void MemoryBarrier() {
+#if defined(__linux__) || defined(__ANDROID__)
+  // Note: This is a function call, which is also an implicit compiler barrier.
+  typedef void (*KernelMemoryBarrierFunc)();
+  ((KernelMemoryBarrierFunc)0xffff0fa0)();
+#elif defined(__QNXNTO__)
+  __cpu_membarrier();
+#else
+#error MemoryBarrier() is not implemented on this platform.
+#endif
+}
 
+// An ARM toolchain would only define one of these depending on which
+// variant of the target architecture is being used. This tests against
+// any known ARMv6 or ARMv7 variant, where it is possible to directly
+// use ldrex/strex instructions to implement fast atomic operations.
+#if defined(__ARM_ARCH_7__) || defined(__ARM_ARCH_7A__) || \
+    defined(__ARM_ARCH_7R__) || defined(__ARM_ARCH_7M__) || \
+    defined(__ARM_ARCH_6__) || defined(__ARM_ARCH_6J__) || \
+    defined(__ARM_ARCH_6K__) || defined(__ARM_ARCH_6Z__) || \
+    defined(__ARM_ARCH_6KZ__) || defined(__ARM_ARCH_6T2__)
 
 inline Atomic32 NoBarrier_CompareAndSwap(volatile Atomic32* ptr,
                                          Atomic32 old_value,
                                          Atomic32 new_value) {
-  Atomic32 prev_value = *ptr;
+  Atomic32 prev_value;
+  int reloop;
   do {
-    if (!pLinuxKernelCmpxchg(old_value, new_value,
-                             const_cast<Atomic32*>(ptr))) {
-      return old_value;
-    }
-    prev_value = *ptr;
-  } while (prev_value == old_value);
+    // The following is equivalent to:
+    //
+    //   prev_value = LDREX(ptr)
+    //   reloop = 0
+    //   if (prev_value != old_value)
+    //      reloop = STREX(ptr, new_value)
+    __asm__ __volatile__("    ldrex %0, [%3]\n"
+                         "    mov %1, #0\n"
+                         "    cmp %0, %4\n"
+#ifdef __thumb2__
+                         "    it eq\n"
+#endif
+                         "    strexeq %1, %5, [%3]\n"
+                         : "=&r"(prev_value), "=&r"(reloop), "+m"(*ptr)
+                         : "r"(ptr), "r"(old_value), "r"(new_value)
+                         : "cc", "memory");
+  } while (reloop != 0);
   return prev_value;
 }
 
+inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr,
+                                       Atomic32 old_value,
+                                       Atomic32 new_value) {
+  Atomic32 result = NoBarrier_CompareAndSwap(ptr, old_value, new_value);
+  MemoryBarrier();
+  return result;
+}
+
+inline Atomic32 Release_CompareAndSwap(volatile Atomic32* ptr,
+                                       Atomic32 old_value,
+                                       Atomic32 new_value) {
+  MemoryBarrier();
+  return NoBarrier_CompareAndSwap(ptr, old_value, new_value);
+}
+
+inline Atomic32 NoBarrier_AtomicIncrement(volatile Atomic32* ptr,
+                                          Atomic32 increment) {
+  Atomic32 value;
+  int reloop;
+  do {
+    // Equivalent to:
+    //
+    //  value = LDREX(ptr)
+    //  value += increment
+    //  reloop = STREX(ptr, value)
+    //
+    __asm__ __volatile__("    ldrex %0, [%3]\n"
+                         "    add %0, %0, %4\n"
+                         "    strex %1, %0, [%3]\n"
+                         : "=&r"(value), "=&r"(reloop), "+m"(*ptr)
+                         : "r"(ptr), "r"(increment)
+                         : "cc", "memory");
+  } while (reloop);
+  return value;
+}
+
+inline Atomic32 Barrier_AtomicIncrement(volatile Atomic32* ptr,
+                                        Atomic32 increment) {
+  // TODO(digit): Investigate if it's possible to implement this with
+  // a single MemoryBarrier() operation between the LDREX and STREX.
+  // See http://crbug.com/246514
+  MemoryBarrier();
+  Atomic32 result = NoBarrier_AtomicIncrement(ptr, increment);
+  MemoryBarrier();
+  return result;
+}
+
+inline Atomic32 NoBarrier_AtomicExchange(volatile Atomic32* ptr,
+                                         Atomic32 new_value) {
+  Atomic32 old_value;
+  int reloop;
+  do {
+    // old_value = LDREX(ptr)
+    // reloop = STREX(ptr, new_value)
+    __asm__ __volatile__("   ldrex %0, [%3]\n"
+                         "   strex %1, %4, [%3]\n"
+                         : "=&r"(old_value), "=&r"(reloop), "+m"(*ptr)
+                         : "r"(ptr), "r"(new_value)
+                         : "cc", "memory");
+  } while (reloop != 0);
+  return old_value;
+}
+
+// This tests against any known ARMv5 variant.
+#elif defined(__ARM_ARCH_5__) || defined(__ARM_ARCH_5T__) || \
+      defined(__ARM_ARCH_5TE__) || defined(__ARM_ARCH_5TEJ__)
+
+// The kernel also provides a helper function to perform an atomic
+// compare-and-swap operation at the hard-wired address 0xffff0fc0.
+// On ARMv5, this is implemented by a special code path that the kernel
+// detects and treats specially when thread pre-emption happens.
+// On ARMv6 and higher, it uses LDREX/STREX instructions instead.
+//
+// Note that this always perform a full memory barrier, there is no
+// need to add calls MemoryBarrier() before or after it. It also
+// returns 0 on success, and 1 on exit.
+//
+// Available and reliable since Linux 2.6.24. Both Android and ChromeOS
+// use newer kernel revisions, so this should not be a concern.
+namespace {
+
+inline int LinuxKernelCmpxchg(Atomic32 old_value,
+                              Atomic32 new_value,
+                              volatile Atomic32* ptr) {
+  typedef int (*KernelCmpxchgFunc)(Atomic32, Atomic32, volatile Atomic32*);
+  return ((KernelCmpxchgFunc)0xffff0fc0)(old_value, new_value, ptr);
+}
+
+}  // namespace
+
+inline Atomic32 NoBarrier_CompareAndSwap(volatile Atomic32* ptr,
+                                         Atomic32 old_value,
+                                         Atomic32 new_value) {
+  Atomic32 prev_value;
+  for (;;) {
+    prev_value = *ptr;
+    if (prev_value != old_value)
+      return prev_value;
+    if (!LinuxKernelCmpxchg(old_value, new_value, ptr))
+      return old_value;
+  }
+}
+
 inline Atomic32 NoBarrier_AtomicExchange(volatile Atomic32* ptr,
                                          Atomic32 new_value) {
   Atomic32 old_value;
   do {
     old_value = *ptr;
-  } while (pLinuxKernelCmpxchg(old_value, new_value,
-                               const_cast<Atomic32*>(ptr)));
+  } while (LinuxKernelCmpxchg(old_value, new_value, ptr));
   return old_value;
 }
 
@@ -86,8 +237,7 @@ inline Atomic32 Barrier_AtomicIncrement(volatile Atomic32* ptr,
     // 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 (!LinuxKernelCmpxchg(old_value, new_value, ptr)) {
       // The exchange took place as expected.
       return new_value;
     }
@@ -98,23 +248,46 @@ inline Atomic32 Barrier_AtomicIncrement(volatile Atomic32* ptr,
 inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32* ptr,
                                        Atomic32 old_value,
                                        Atomic32 new_value) {
-  return NoBarrier_CompareAndSwap(ptr, old_value, new_value);
+  Atomic32 prev_value;
+  for (;;) {
+    prev_value = *ptr;
+    if (prev_value != old_value) {
+      // Always ensure acquire semantics.
+      MemoryBarrier();
+      return prev_value;
+    }
+    if (!LinuxKernelCmpxchg(old_value, new_value, ptr))
+      return old_value;
+  }
 }
 
 inline Atomic32 Release_CompareAndSwap(volatile Atomic32* ptr,
                                        Atomic32 old_value,
                                        Atomic32 new_value) {
-  return NoBarrier_CompareAndSwap(ptr, old_value, new_value);
+  // This could be implemented as:
+  //    MemoryBarrier();
+  //    return NoBarrier_CompareAndSwap();
+  //
+  // But would use 3 barriers per succesful CAS. To save performance,
+  // use Acquire_CompareAndSwap(). Its implementation guarantees that:
+  // - A succesful swap uses only 2 barriers (in the kernel helper).
+  // - An early return due to (prev_value != old_value) performs
+  //   a memory barrier with no store, which is equivalent to the
+  //   generic implementation above.
+  return Acquire_CompareAndSwap(ptr, old_value, new_value);
 }
 
+#else
+#  error "Your CPU's ARM architecture is not supported yet"
+#endif
+
+// NOTE: Atomicity of the following load and store operations is only
+// guaranteed in case of 32-bit alignement of |ptr| values.
+
 inline void NoBarrier_Store(volatile Atomic32* ptr, Atomic32 value) {
   *ptr = value;
 }
 
-inline void MemoryBarrier() {
-  pLinuxKernelMemoryBarrier();
-}
-
 inline void Acquire_Store(volatile Atomic32* ptr, Atomic32 value) {
   *ptr = value;
   MemoryBarrier();
@@ -125,9 +298,7 @@ inline void Release_Store(volatile Atomic32* ptr, Atomic32 value) {
   *ptr = value;
 }
 
-inline Atomic32 NoBarrier_Load(volatile const Atomic32* ptr) {
-  return *ptr;
-}
+inline Atomic32 NoBarrier_Load(volatile const Atomic32* ptr) { return *ptr; }
 
 inline Atomic32 Acquire_Load(volatile const Atomic32* ptr) {
   Atomic32 value = *ptr;