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SpeculateAroundPHIs.h
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1 //===- SpeculateAroundPHIs.h - Speculate around PHIs ------------*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8
9 #ifndef LLVM_TRANSFORMS_SCALAR_SPECULATEAROUNDPHIS_H
10 #define LLVM_TRANSFORMS_SCALAR_SPECULATEAROUNDPHIS_H
11
14 #include "llvm/IR/Dominators.h"
15 #include "llvm/IR/Function.h"
16 #include "llvm/IR/PassManager.h"
17 #include "llvm/Support/Compiler.h"
18 #include <vector>
19
20 namespace llvm {
21
22 /// This pass handles simple speculating of instructions around PHIs when
23 /// doing so is profitable for a particular target despite duplicated
24 /// instructions.
25 ///
26 /// The motivating example are PHIs of constants which will require
27 /// materializing the constants along each edge. If the PHI is used by an
28 /// instruction where the target can materialize the constant as part of the
29 /// instruction, it is profitable to speculate those instructions around the
30 /// PHI node. This can reduce dynamic instruction count as well as decrease
31 /// register pressure.
32 ///
33 /// Consider this IR for example:
34 /// 
35 /// entry:
36 /// br i1 %flag, label %a, label %b
37 ///
38 /// a:
39 /// br label %exit
40 ///
41 /// b:
42 /// br label %exit
43 ///
44 /// exit:
45 /// %p = phi i32 [ 7, %a ], [ 11, %b ]
46 /// %sum = add i32 %arg, %p
47 /// ret i32 %sum
48 /// 
49 /// To materialize the inputs to this PHI node may require an explicit
50 /// instruction. For example, on x86 this would turn into something like
51 /// 
52 /// testq %eax, %eax
53 /// movl $7, %rNN 54 /// jne .L 55 /// movl$11, %rNN
56 /// .L:
58 /// movl %rNN, %eax
59 /// retq
60 /// 
61 /// When these constants can be folded directly into another instruction, it
62 /// would be preferable to avoid the potential for register pressure (above we
63 /// can easily avoid it, but that isn't always true) and simply duplicate the
64 /// instruction using the PHI:
65 /// 
66 /// entry:
67 /// br i1 %flag, label %a, label %b
68 ///
69 /// a:
70 /// %sum.1 = add i32 %arg, 7
71 /// br label %exit
72 ///
73 /// b:
74 /// %sum.2 = add i32 %arg, 11
75 /// br label %exit
76 ///
77 /// exit:
78 /// %p = phi i32 [ %sum.1, %a ], [ %sum.2, %b ]
79 /// ret i32 %p
80 /// 
81 /// Which will generate something like the following on x86:
82 /// 
83 /// testq %eax, %eax
84 /// addl $7, %edi 85 /// jne .L 86 /// addl$11, %edi
87 /// .L:
88 /// movl %edi, %eax
89 /// retq
90 /// 
91 ///
92 /// It is important to note that this pass is never intended to handle more
93 /// complex cases where speculating around PHIs allows simplifications of the
94 /// IR itself or other subsequent optimizations. Those can and should already
95 /// be handled before this pass is ever run by a more powerful analysis that
96 /// can reason about equivalences and common subexpressions. Classically, those
97 /// cases would be handled by a GVN-powered PRE or similar transform. This
98 /// pass, in contrast, is *only* interested in cases where despite no
99 /// simplifications to the IR itself, speculation is *faster* to execute. The
100 /// result of this is that the cost models which are appropriate to consider
101 /// here are relatively simple ones around execution and codesize cost, without
102 /// any need to consider simplifications or other transformations.
103 struct SpeculateAroundPHIsPass : PassInfoMixin<SpeculateAroundPHIsPass> {
104  /// Run the pass over the function.
106 };
107
108 } // end namespace llvm
109
110 #endif // LLVM_TRANSFORMS_SCALAR_SPECULATEAROUNDPHIS_H
This class represents lattice values for constants.
Definition: AllocatorList.h:23
F(f)
This pass handles simple speculating of instructions around PHIs when doing so is profitable for a pa...
A CRTP mix-in to automatically provide informational APIs needed for passes.
Definition: PassManager.h:373
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:154
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM)
Run the pass over the function.
A container for analyses that lazily runs them and caches their results.
This header defines various interfaces for pass management in LLVM.