LLVM  8.0.0svn
AtomicExpandUtils.h
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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 #ifndef LLVM_CODEGEN_ATOMICEXPANDUTILS_H
11 #define LLVM_CODEGEN_ATOMICEXPANDUTILS_H
12 
13 #include "llvm/ADT/STLExtras.h"
14 #include "llvm/IR/IRBuilder.h"
16 
17 namespace llvm {
18 
19 class AtomicRMWInst;
20 class Value;
21 
22 /// Parameters (see the expansion example below):
23 /// (the builder, %addr, %loaded, %new_val, ordering,
24 /// /* OUT */ %success, /* OUT */ %new_loaded)
26  function_ref<void(IRBuilder<> &, Value *, Value *, Value *, AtomicOrdering,
27  Value *&, Value *&)>;
28 
29 /// Expand an atomic RMW instruction into a loop utilizing
30 /// cmpxchg. You'll want to make sure your target machine likes cmpxchg
31 /// instructions in the first place and that there isn't another, better,
32 /// transformation available (for example AArch32/AArch64 have linked loads).
33 ///
34 /// This is useful in passes which can't rewrite the more exotic RMW
35 /// instructions directly into a platform specific intrinsics (because, say,
36 /// those intrinsics don't exist). If such a pass is able to expand cmpxchg
37 /// instructions directly however, then, with this function, it could avoid two
38 /// extra module passes (avoiding passes by `-atomic-expand` and itself). A
39 /// specific example would be PNaCl's `RewriteAtomics` pass.
40 ///
41 /// Given: atomicrmw some_op iN* %addr, iN %incr ordering
42 ///
43 /// The standard expansion we produce is:
44 /// [...]
45 /// %init_loaded = load atomic iN* %addr
46 /// br label %loop
47 /// loop:
48 /// %loaded = phi iN [ %init_loaded, %entry ], [ %new_loaded, %loop ]
49 /// %new = some_op iN %loaded, %incr
50 /// ; This is what -atomic-expand will produce using this function on i686
51 /// targets:
52 /// %pair = cmpxchg iN* %addr, iN %loaded, iN %new_val
53 /// %new_loaded = extractvalue { iN, i1 } %pair, 0
54 /// %success = extractvalue { iN, i1 } %pair, 1
55 /// ; End callback produced IR
56 /// br i1 %success, label %atomicrmw.end, label %loop
57 /// atomicrmw.end:
58 /// [...]
59 ///
60 /// Returns true if the containing function was modified.
62 
63 } // end namespace llvm
64 
65 #endif // LLVM_CODEGEN_ATOMICEXPANDUTILS_H
Atomic ordering constants.
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
An efficient, type-erasing, non-owning reference to a callable.
Definition: STLExtras.h:107
an instruction that atomically reads a memory location, combines it with another value, and then stores the result back.
Definition: Instructions.h:681
AtomicOrdering
Atomic ordering for LLVM&#39;s memory model.
bool expandAtomicRMWToCmpXchg(AtomicRMWInst *AI, CreateCmpXchgInstFun CreateCmpXchg)
Expand an atomic RMW instruction into a loop utilizing cmpxchg.
LLVM Value Representation.
Definition: Value.h:73