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annotate docs/BranchWeightMetadata.rst @ 148:63bd29f05246

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author Shinji KONO <kono@ie.u-ryukyu.ac.jp>
date Wed, 14 Aug 2019 19:46:37 +0900
parents c2174574ed3a
children
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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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11 Branch Weight Metadata represents branch weights as its likeliness to be taken
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12 (see :doc:`BlockFrequencyTerminology`). Metadata is assigned to an
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13 ``Instruction`` that is a terminator as a ``MDNode`` of the ``MD_prof`` kind.
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14 The first operator is always a ``MDString`` node with the string
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15 "branch_weights". Number of operators depends on the terminator type.
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16
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17 Branch weights might be fetch from the profiling file, or generated based on
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18 `__builtin_expect`_ instruction.
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19
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20 All weights are represented as an unsigned 32-bit values, where higher value
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21 indicates greater chance to be taken.
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22
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23 Supported Instructions
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24 ======================
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25
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26 ``BranchInst``
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27 ^^^^^^^^^^^^^^
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28
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29 Metadata is only assigned to the conditional branches. There are two extra
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30 operands for the true and the false branch.
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31
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32 .. code-block:: none
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33
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34 !0 = metadata !{
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35 metadata !"branch_weights",
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36 i32 <TRUE_BRANCH_WEIGHT>,
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37 i32 <FALSE_BRANCH_WEIGHT>
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38 }
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39
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40 ``SwitchInst``
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41 ^^^^^^^^^^^^^^
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42
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43 Branch weights are assigned to every case (including the ``default`` case which
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44 is always case #0).
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45
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46 .. code-block:: none
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47
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48 !0 = metadata !{
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49 metadata !"branch_weights",
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50 i32 <DEFAULT_BRANCH_WEIGHT>
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51 [ , i32 <CASE_BRANCH_WEIGHT> ... ]
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52 }
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53
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54 ``IndirectBrInst``
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55 ^^^^^^^^^^^^^^^^^^
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56
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57 Branch weights are assigned to every destination.
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58
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59 .. code-block:: none
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60
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61 !0 = metadata !{
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62 metadata !"branch_weights",
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63 i32 <LABEL_BRANCH_WEIGHT>
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64 [ , i32 <LABEL_BRANCH_WEIGHT> ... ]
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65 }
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66
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67 ``CallInst``
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68 ^^^^^^^^^^^^^^^^^^
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69
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70 Calls may have branch weight metadata, containing the execution count of
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71 the call. It is currently used in SamplePGO mode only, to augment the
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72 block and entry counts which may not be accurate with sampling.
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73
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74 .. code-block:: none
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75
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76 !0 = metadata !{
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77 metadata !"branch_weights",
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78 i32 <CALL_BRANCH_WEIGHT>
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79 }
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80
0
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81 Other
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82 ^^^^^
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83
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84 Other terminator instructions are not allowed to contain Branch Weight Metadata.
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85
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86 .. _\__builtin_expect:
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87
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88 Built-in ``expect`` Instructions
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89 ================================
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90
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91 ``__builtin_expect(long exp, long c)`` instruction provides branch prediction
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92 information. The return value is the value of ``exp``.
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93
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94 It is especially useful in conditional statements. Currently Clang supports two
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95 conditional statements:
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96
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97 ``if`` statement
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98 ^^^^^^^^^^^^^^^^
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99
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100 The ``exp`` parameter is the condition. The ``c`` parameter is the expected
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101 comparison value. If it is equal to 1 (true), the condition is likely to be
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102 true, in other case condition is likely to be false. For example:
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103
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104 .. code-block:: c++
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105
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106 if (__builtin_expect(x > 0, 1)) {
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107 // This block is likely to be taken.
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108 }
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109
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110 ``switch`` statement
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111 ^^^^^^^^^^^^^^^^^^^^
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112
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113 The ``exp`` parameter is the value. The ``c`` parameter is the expected
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114 value. If the expected value doesn't show on the cases list, the ``default``
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115 case is assumed to be likely taken.
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116
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117 .. code-block:: c++
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118
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119 switch (__builtin_expect(x, 5)) {
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120 default: break;
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121 case 0: // ...
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122 case 3: // ...
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123 case 5: // This case is likely to be taken.
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124 }
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125
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126 CFG Modifications
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127 =================
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128
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129 Branch Weight Metatada is not proof against CFG changes. If terminator operands'
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130 are changed some action should be taken. In other case some misoptimizations may
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131 occur due to incorrect branch prediction information.
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132
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133 Function Entry Counts
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134 =====================
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135
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136 To allow comparing different functions during inter-procedural analysis and
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137 optimization, ``MD_prof`` nodes can also be assigned to a function definition.
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138 The first operand is a string indicating the name of the associated counter.
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139
121
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140 Currently, one counter is supported: "function_entry_count". The second operand
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141 is a 64-bit counter that indicates the number of times that this function was
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142 invoked (in the case of instrumentation-based profiles). In the case of
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143 sampling-based profiles, this operand is an approximation of how many times
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144 the function was invoked.
95
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145
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146 For example, in the code below, the instrumentation for function foo()
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147 indicates that it was called 2,590 times at runtime.
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148
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149 .. code-block:: llvm
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150
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151 define i32 @foo() !prof !1 {
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152 ret i32 0
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153 }
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154 !1 = !{!"function_entry_count", i64 2590}
121
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155
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156 If "function_entry_count" has more than 2 operands, the later operands are
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157 the GUID of the functions that needs to be imported by ThinLTO. This is only
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158 set by sampling based profile. It is needed because the sampling based profile
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159 was collected on a binary that had already imported and inlined these functions,
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160 and we need to ensure the IR matches in the ThinLTO backends for profile
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161 annotation. The reason why we cannot annotate this on the callsite is that it
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162 can only goes down 1 level in the call chain. For the cases where
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163 foo_in_a_cc()->bar_in_b_cc()->baz_in_c_cc(), we will need to go down 2 levels
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164 in the call chain to import both bar_in_b_cc and baz_in_c_cc.