Add atomics support for 32-bit PPC.

This was originally submitted as a patch on the Google Code issue
tracker, but the original author didn't submit a pull request. They did,
however, give permission for somebody else to do so. I cleaned up a few
things myself and am now submitting it.

I can't find any record of what the original patch was, but there is a
copy of the discussion at
<https://groups.google.com/forum/#!topic/protobuf/SMwF5fYHIi0>.

Fixes google/protobuf#581

src/google/protobuf/stubs/atomicops.h

@@ -212,6 +212,8 @@ Atomic64 Release_Load(volatile const Atomic64* ptr);
 #include <google/protobuf/stubs/atomicops_internals_mips_gcc.h>
 #elif defined(__native_client__)
 #include <google/protobuf/stubs/atomicops_internals_pnacl.h>
+#elif defined(GOOGLE_PROTOBUF_ARCH_PPC)
+#include <google/protobuf/stubs/atomicops_internals_ppc_gcc.h>
 #elif (((__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)) || (__GNUC__ > 4))
 #include <google/protobuf/stubs/atomicops_internals_generic_gcc.h>
 #elif defined(__clang__)

src/google/protobuf/stubs/atomicops_internals_ppc_gcc.h

+// Protocol Buffers - Google's data interchange format
+// Copyright 2015 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Author: ogabbay@advaoptical.com (Oded Gabbay)
+// Cleaned up by: bsilver16384@gmail.com (Brian Silverman)
+//
+// This file is an internal atomic implementation, use atomicops.h instead.
+
+#ifndef GOOGLE_PROTOBUF_ATOMICOPS_INTERNALS_PPC_GCC_H_
+#define GOOGLE_PROTOBUF_ATOMICOPS_INTERNALS_PPC_GCC_H_
+
+#define ATOMICOPS_COMPILER_BARRIER() __asm__ __volatile__("" : : : "memory")
+
+namespace google {
+namespace protobuf {
+namespace internal {
+
+inline Atomic32 NoBarrier_CompareAndSwap(volatile Atomic32 *ptr,
+                                         Atomic32 old_value,
+                                         Atomic32 new_value) {
+  Atomic32 prev;
+
+  __asm__ __volatile__(
+      "0:                                  \n\t"
+      "lwarx %[prev],0,%[ptr]              \n\t"
+      "cmpw 0,%[prev],%[old_value]         \n\t"
+      "bne- 1f                             \n\t"
+      "stwcx. %[new_value],0,%[ptr]        \n\t"
+      "bne- 0b                             \n\t"
+      "1:                                  \n\t"
+      : [prev] "=&r"(prev), "+m"(*ptr)
+      : [ptr] "r"(ptr), [old_value] "r"(old_value), [new_value] "r"(new_value)
+      : "cc", "memory");
+
+  return prev;
+}
+
+inline Atomic32 NoBarrier_AtomicExchange(volatile Atomic32 *ptr,
+                                         Atomic32 new_value) {
+  Atomic32 old;
+
+  __asm__ __volatile__(
+      "0:                                  \n\t"
+      "lwarx %[old],0,%[ptr]               \n\t"
+      "stwcx. %[new_value],0,%[ptr]        \n\t"
+      "bne- 0b                             \n\t"
+      : [old] "=&r"(old), "+m"(*ptr)
+      : [ptr] "r"(ptr), [new_value] "r"(new_value)
+      : "cc", "memory");
+
+  return old;
+}
+
+inline Atomic32 NoBarrier_AtomicIncrement(volatile Atomic32 *ptr,
+                                          Atomic32 increment) {
+  Atomic32 temp;
+
+  __asm__ __volatile__(
+      "0:                                  \n\t"
+      "lwarx %[temp],0,%[ptr]              \n\t"
+      "add %[temp],%[increment],%[temp]    \n\t"
+      "stwcx. %[temp],0,%[ptr]             \n\t"
+      "bne- 0b                             \n\t"
+      : [temp] "=&r"(temp)
+      : [increment] "r"(increment), [ptr] "r"(ptr)
+      : "cc", "memory");
+
+  return temp;
+}
+
+inline Atomic32 Barrier_AtomicIncrement(volatile Atomic32 *ptr,
+                                        Atomic32 increment) {
+  MemoryBarrier();
+  Atomic32 res = NoBarrier_AtomicIncrement(ptr, increment);
+  MemoryBarrier();
+  return res;
+}
+
+inline Atomic32 Acquire_CompareAndSwap(volatile Atomic32 *ptr,
+                                       Atomic32 old_value, Atomic32 new_value) {
+  Atomic32 res = NoBarrier_CompareAndSwap(ptr, old_value, new_value);
+  MemoryBarrier();
+  return res;
+}
+
+inline Atomic32 Release_CompareAndSwap(volatile Atomic32 *ptr,
+                                       Atomic32 old_value, Atomic32 new_value) {
+  MemoryBarrier();
+  Atomic32 res = NoBarrier_CompareAndSwap(ptr, old_value, new_value);
+  return res;
+}
+
+inline void NoBarrier_Store(volatile Atomic32 *ptr, Atomic32 value) {
+  *ptr = value;
+}
+
+inline void MemoryBarrier() { __asm__ __volatile__("sync" : : : "memory"); }
+
+inline void Acquire_Store(volatile Atomic32 *ptr, Atomic32 value) {
+  *ptr = value;
+  MemoryBarrier();
+}
+
+inline void Release_Store(volatile Atomic32 *ptr, Atomic32 value) {
+  MemoryBarrier();
+  *ptr = value;
+}
+
+inline Atomic32 NoBarrier_Load(volatile const Atomic32 *ptr) { return *ptr; }
+
+inline Atomic32 Acquire_Load(volatile const Atomic32 *ptr) {
+  Atomic32 value = *ptr;
+  MemoryBarrier();
+  return value;
+}
+
+inline Atomic32 Release_Load(volatile const Atomic32 *ptr) {
+  MemoryBarrier();
+  return *ptr;
+}
+
+}  // namespace internal
+}  // namespace protobuf
+}  // namespace google
+
+#undef ATOMICOPS_COMPILER_BARRIER
+
+#endif  // GOOGLE_PROTOBUF_ATOMICOPS_INTERNALS_PPC_GCC_H_

src/google/protobuf/stubs/platform_macros.h

@@ -73,6 +73,9 @@
 #elif defined(_POWER)
 #define GOOGLE_PROTOBUF_ARCH_POWER 1
 #define GOOGLE_PROTOBUF_ARCH_64_BIT 1
+#elif defined(__PPC__)
+#define GOOGLE_PROTOBUF_ARCH_PPC 1
+#define GOOGLE_PROTOBUF_ARCH_32_BIT 1
 #elif defined(__GNUC__)
 # if (((__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)) || (__GNUC__ > 4))
 // We fallback to the generic Clang/GCC >= 4.7 implementation in atomicops.h