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annotate docs/BranchWeightMetadata.rst @ 33:e4204d083e25 LLVM3.5

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LLVM 3.5
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Thu, 12 Dec 2013 14:32:10 +0900
parents 95c75e76d11b
children 54457678186b
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1 ===========================
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2 LLVM Branch Weight Metadata
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3 ===========================
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4
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5 .. contents::
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6 :local:
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7
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8 Introduction
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9 ============
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10
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11 Branch Weight Metadata represents branch weights as its likeliness to be
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12 taken. Metadata is assigned to the ``TerminatorInst`` as a ``MDNode`` of the
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13 ``MD_prof`` kind. The first operator is always a ``MDString`` node with the
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14 string "branch_weights". Number of operators depends on the terminator type.
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15
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16 Branch weights might be fetch from the profiling file, or generated based on
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17 `__builtin_expect`_ instruction.
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18
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19 All weights are represented as an unsigned 32-bit values, where higher value
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20 indicates greater chance to be taken.
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21
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22 Supported Instructions
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23 ======================
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24
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25 ``BranchInst``
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26 ^^^^^^^^^^^^^^
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27
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28 Metadata is only assigned to the conditional branches. There are two extra
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29 operarands for the true and the false branch.
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30
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31 .. code-block:: llvm
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32
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33 !0 = metadata !{
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34 metadata !"branch_weights",
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35 i32 <TRUE_BRANCH_WEIGHT>,
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36 i32 <FALSE_BRANCH_WEIGHT>
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37 }
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38
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39 ``SwitchInst``
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40 ^^^^^^^^^^^^^^
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41
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42 Branch weights are assigned to every case (including the ``default`` case which
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43 is always case #0).
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44
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45 .. code-block:: llvm
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46
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47 !0 = metadata !{
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48 metadata !"branch_weights",
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49 i32 <DEFAULT_BRANCH_WEIGHT>
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50 [ , i32 <CASE_BRANCH_WEIGHT> ... ]
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51 }
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52
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53 ``IndirectBrInst``
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54 ^^^^^^^^^^^^^^^^^^
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55
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56 Branch weights are assigned to every destination.
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57
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58 .. code-block:: llvm
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59
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60 !0 = metadata !{
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61 metadata !"branch_weights",
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62 i32 <LABEL_BRANCH_WEIGHT>
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63 [ , i32 <LABEL_BRANCH_WEIGHT> ... ]
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64 }
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65
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66 Other
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67 ^^^^^
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68
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69 Other terminator instructions are not allowed to contain Branch Weight Metadata.
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70
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71 .. _\__builtin_expect:
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72
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73 Built-in ``expect`` Instructions
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74 ================================
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75
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76 ``__builtin_expect(long exp, long c)`` instruction provides branch prediction
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77 information. The return value is the value of ``exp``.
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78
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79 It is especially useful in conditional statements. Currently Clang supports two
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80 conditional statements:
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81
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82 ``if`` statement
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83 ^^^^^^^^^^^^^^^^
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84
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85 The ``exp`` parameter is the condition. The ``c`` parameter is the expected
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86 comparison value. If it is equal to 1 (true), the condition is likely to be
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87 true, in other case condition is likely to be false. For example:
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88
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89 .. code-block:: c++
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90
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91 if (__builtin_expect(x > 0, 1)) {
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92 // This block is likely to be taken.
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93 }
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94
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95 ``switch`` statement
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96 ^^^^^^^^^^^^^^^^^^^^
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97
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98 The ``exp`` parameter is the value. The ``c`` parameter is the expected
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99 value. If the expected value doesn't show on the cases list, the ``default``
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100 case is assumed to be likely taken.
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101
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102 .. code-block:: c++
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103
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104 switch (__builtin_expect(x, 5)) {
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105 default: break;
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106 case 0: // ...
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107 case 3: // ...
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108 case 5: // This case is likely to be taken.
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109 }
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110
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111 CFG Modifications
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112 =================
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113
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114 Branch Weight Metatada is not proof against CFG changes. If terminator operands'
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115 are changed some action should be taken. In other case some misoptimizations may
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116 occur due to incorrent branch prediction information.