1 //===-- AtomicExpandUtils.h - Utilities for expanding atomic instructions -===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 
10 #include "llvm/ADT/STLExtras.h"
11 #include "llvm/IR/IRBuilder.h"
12 
13 namespace llvm {
14 class Value;
15 class AtomicRMWInst;
16 
17 
18 /// Parameters (see the expansion example below):
19 /// (the builder, %addr, %loaded, %new_val, ordering,
20 ///  /* OUT */ %success, /* OUT */ %new_loaded)
21 typedef function_ref<void(IRBuilder<> &, Value *, Value *, Value *,
22                           AtomicOrdering, Value *&, Value *&)> CreateCmpXchgInstFun;
23 
24 /// \brief Expand an atomic RMW instruction into a loop utilizing
25 /// cmpxchg. You'll want to make sure your target machine likes cmpxchg
26 /// instructions in the first place and that there isn't another, better,
27 /// transformation available (for example AArch32/AArch64 have linked loads).
28 ///
29 /// This is useful in passes which can't rewrite the more exotic RMW
30 /// instructions directly into a platform specific intrinsics (because, say,
31 /// those intrinsics don't exist). If such a pass is able to expand cmpxchg
32 /// instructions directly however, then, with this function, it could avoid two
33 /// extra module passes (avoiding passes by `-atomic-expand` and itself). A
34 /// specific example would be PNaCl's `RewriteAtomics` pass.
35 ///
36 /// Given: atomicrmw some_op iN* %addr, iN %incr ordering
37 ///
38 /// The standard expansion we produce is:
39 ///     [...]
40 ///     %init_loaded = load atomic iN* %addr
41 ///     br label %loop
42 /// loop:
43 ///     %loaded = phi iN [ %init_loaded, %entry ], [ %new_loaded, %loop ]
44 ///     %new = some_op iN %loaded, %incr
45 /// ; This is what -atomic-expand will produce using this function on i686 targets:
46 ///     %pair = cmpxchg iN* %addr, iN %loaded, iN %new_val
47 ///     %new_loaded = extractvalue { iN, i1 } %pair, 0
48 ///     %success = extractvalue { iN, i1 } %pair, 1
49 /// ; End callback produced IR
50 ///     br i1 %success, label %atomicrmw.end, label %loop
51 /// atomicrmw.end:
52 ///     [...]
53 ///
54 /// Returns true if the containing function was modified.
55 bool
56 expandAtomicRMWToCmpXchg(AtomicRMWInst *AI, CreateCmpXchgInstFun Factory);
57 }
58