Rework PNaCl memory ordering

include/llvm/IR/NaClAtomicIntrinsics.h

 //===-- llvm/IR/NaClAtomicIntrinsics.h - NaCl Atomic Intrinsics -*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file describes atomic intrinsic functions that are specific to NaCl.
@@ skipping 82 matching lines @@
 /// Do not reorder these values: their order offers forward
 /// compatibility of bitcode targeted to NaCl.
 enum MemoryOrder {
   MemoryOrderInvalid = 0, // Invalid, keep first.
   MemoryOrderRelaxed,
   MemoryOrderConsume,
   MemoryOrderAcquire,
   MemoryOrderRelease,
   MemoryOrderAcquireRelease,
   MemoryOrderSequentiallyConsistent,
+  // The following memory order is specific to PNaCl and is used to
+  // support non-C11/C++11 legacy code which uses the __sync_synchronize
+  // GCC-style builtin and the asm("":::"memory") compiler fence
+  // idiom. It's equivalent to MemoryOrderSequentiallyConsistent
+  // surrounded by compiler barriers, which means that in practice is
+  // should prevent reordering of memory accesses to objects which may
+  // escape.
+  MemoryOrderSequentiallyConsistentAll,
   MemoryOrderNum // Invalid, keep last.
 };
 
 } // End NaCl namespace
 
 } // End llvm namespace
 
 #endif