LLVM  9.0.0svn
AtomicExpandUtils.h
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1 //===- AtomicExpandUtils.h - Utilities for expanding atomic instructions --===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #ifndef LLVM_CODEGEN_ATOMICEXPANDUTILS_H
10 #define LLVM_CODEGEN_ATOMICEXPANDUTILS_H
11 
12 #include "llvm/ADT/STLExtras.h"
13 #include "llvm/IR/IRBuilder.h"
15 
16 namespace llvm {
17 
18 class AtomicRMWInst;
19 class Value;
20 
21 /// Parameters (see the expansion example below):
22 /// (the builder, %addr, %loaded, %new_val, ordering,
23 /// /* OUT */ %success, /* OUT */ %new_loaded)
25  function_ref<void(IRBuilder<> &, Value *, Value *, Value *, AtomicOrdering,
26  Value *&, Value *&)>;
27 
28 /// Expand an atomic RMW instruction into a loop utilizing
29 /// cmpxchg. You'll want to make sure your target machine likes cmpxchg
30 /// instructions in the first place and that there isn't another, better,
31 /// transformation available (for example AArch32/AArch64 have linked loads).
32 ///
33 /// This is useful in passes which can't rewrite the more exotic RMW
34 /// instructions directly into a platform specific intrinsics (because, say,
35 /// those intrinsics don't exist). If such a pass is able to expand cmpxchg
36 /// instructions directly however, then, with this function, it could avoid two
37 /// extra module passes (avoiding passes by `-atomic-expand` and itself). A
38 /// specific example would be PNaCl's `RewriteAtomics` pass.
39 ///
40 /// Given: atomicrmw some_op iN* %addr, iN %incr ordering
41 ///
42 /// The standard expansion we produce is:
43 /// [...]
44 /// %init_loaded = load atomic iN* %addr
45 /// br label %loop
46 /// loop:
47 /// %loaded = phi iN [ %init_loaded, %entry ], [ %new_loaded, %loop ]
48 /// %new = some_op iN %loaded, %incr
49 /// ; This is what -atomic-expand will produce using this function on i686
50 /// targets:
51 /// %pair = cmpxchg iN* %addr, iN %loaded, iN %new_val
52 /// %new_loaded = extractvalue { iN, i1 } %pair, 0
53 /// %success = extractvalue { iN, i1 } %pair, 1
54 /// ; End callback produced IR
55 /// br i1 %success, label %atomicrmw.end, label %loop
56 /// atomicrmw.end:
57 /// [...]
58 ///
59 /// Returns true if the containing function was modified.
61 
62 } // end namespace llvm
63 
64 #endif // LLVM_CODEGEN_ATOMICEXPANDUTILS_H
Atomic ordering constants.
This class represents lattice values for constants.
Definition: AllocatorList.h:23
An efficient, type-erasing, non-owning reference to a callable.
Definition: STLExtras.h:116
an instruction that atomically reads a memory location, combines it with another value, and then stores the result back.
Definition: Instructions.h:691
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:72