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diff --git a/include/llvm/CodeGen/AtomicExpandUtils.h b/include/llvm/CodeGen/AtomicExpandUtils.h
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+++ b/include/llvm/CodeGen/AtomicExpandUtils.h
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+//===-- AtomicExpandUtils.h - Utilities for expanding atomic instructions -===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/IR/IRBuilder.h"
+
+namespace llvm {
+class Value;
+class AtomicRMWInst;
+
+
+/// Parameters (see the expansion example below):
+/// (the builder, %addr, %loaded, %new_val, ordering,
+///  /* OUT */ %success, /* OUT */ %new_loaded)
+typedef function_ref<void(IRBuilder<> &, Value *, Value *, Value *,
+                          AtomicOrdering, Value *&, Value *&)> CreateCmpXchgInstFun;
+
+/// \brief Expand an atomic RMW instruction into a loop utilizing
+/// cmpxchg. You'll want to make sure your target machine likes cmpxchg
+/// instructions in the first place and that there isn't another, better,
+/// transformation available (for example AArch32/AArch64 have linked loads).
+///
+/// This is useful in passes which can't rewrite the more exotic RMW
+/// instructions directly into a platform specific intrinsics (because, say,
+/// those intrinsics don't exist). If such a pass is able to expand cmpxchg
+/// instructions directly however, then, with this function, it could avoid two
+/// extra module passes (avoiding passes by `-atomic-expand` and itself). A
+/// specific example would be PNaCl's `RewriteAtomics` pass.
+///
+/// Given: atomicrmw some_op iN* %addr, iN %incr ordering
+///
+/// The standard expansion we produce is:
+///     [...]
+///     %init_loaded = load atomic iN* %addr
+///     br label %loop
+/// loop:
+///     %loaded = phi iN [ %init_loaded, %entry ], [ %new_loaded, %loop ]
+///     %new = some_op iN %loaded, %incr
+/// ; This is what -atomic-expand will produce using this function on i686 targets:
+///     %pair = cmpxchg iN* %addr, iN %loaded, iN %new_val
+///     %new_loaded = extractvalue { iN, i1 } %pair, 0
+///     %success = extractvalue { iN, i1 } %pair, 1
+/// ; End callback produced IR
+///     br i1 %success, label %atomicrmw.end, label %loop
+/// atomicrmw.end:
+///     [...]
+///
+/// Returns true if the containing function was modified.
+bool
+expandAtomicRMWToCmpXchg(AtomicRMWInst *AI, CreateCmpXchgInstFun Factory);
+}