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1 ===================
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2 Debugging with XRay
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3 ===================
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4
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5 This document shows an example of how you would go about analyzing applications
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6 built with XRay instrumentation. Here we will attempt to debug ``llc``
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7 compiling some sample LLVM IR generated by Clang.
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8
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9 .. contents::
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10 :local:
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11
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12 Building with XRay
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13 ------------------
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14
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15 To debug an application with XRay instrumentation, we need to build it with a
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16 Clang that supports the ``-fxray-instrument`` option. See `XRay <XRay.html>`_
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17 for more technical details of how XRay works for background information.
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18
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19 In our example, we need to add ``-fxray-instrument`` to the list of flags
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20 passed to Clang when building a binary. Note that we need to link with Clang as
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21 well to get the XRay runtime linked in appropriately. For building ``llc`` with
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22 XRay, we do something similar below for our LLVM build:
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23
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24 ::
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25
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26 $ mkdir -p llvm-build && cd llvm-build
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27 # Assume that the LLVM sources are at ../llvm
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28 $ cmake -GNinja ../llvm -DCMAKE_BUILD_TYPE=Release \
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29 -DCMAKE_C_FLAGS_RELEASE="-fxray-instrument" -DCMAKE_CXX_FLAGS="-fxray-instrument" \
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30 # Once this finishes, we should build llc
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31 $ ninja llc
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32
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33
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34 To verify that we have an XRay instrumented binary, we can use ``objdump`` to
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35 look for the ``xray_instr_map`` section.
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36
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37 ::
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38
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39 $ objdump -h -j xray_instr_map ./bin/llc
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40 ./bin/llc: file format elf64-x86-64
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41
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42 Sections:
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43 Idx Name Size VMA LMA File off Algn
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44 14 xray_instr_map 00002fc0 00000000041516c6 00000000041516c6 03d516c6 2**0
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45 CONTENTS, ALLOC, LOAD, READONLY, DATA
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46
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47 Getting Traces
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48 --------------
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49
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50 By default, XRay does not write out the trace files or patch the application
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51 before main starts. If we just run ``llc`` it should just work like a normally
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52 built binary. However, if we want to get a full trace of the application's
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53 operations (of the functions we do end up instrumenting with XRay) then we need
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54 to enable XRay at application start. To do this, XRay checks the
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55 ``XRAY_OPTIONS`` environment variable.
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56
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57 ::
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58
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59 # The following doesn't create an XRay trace by default.
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60 $ ./bin/llc input.ll
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61
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62 # We need to set the XRAY_OPTIONS to enable some features.
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63 $ XRAY_OPTIONS="patch_premain=true xray_mode=xray-basic verbosity=1" ./bin/llc input.ll
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64 ==69819==XRay: Log file in 'xray-log.llc.m35qPB'
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65
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66 At this point we now have an XRay trace we can start analysing.
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67
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68 The ``llvm-xray`` Tool
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69 ----------------------
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70
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71 Having a trace then allows us to do basic accounting of the functions that were
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72 instrumented, and how much time we're spending in parts of the code. To make
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73 sense of this data, we use the ``llvm-xray`` tool which has a few subcommands
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74 to help us understand our trace.
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75
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76 One of the simplest things we can do is to get an accounting of the functions
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77 that have been instrumented. We can see an example accounting with ``llvm-xray
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78 account``:
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79
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80 ::
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81
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82 $ llvm-xray account xray-log.llc.m35qPB -top=10 -sort=sum -sortorder=dsc -instr_map ./bin/llc
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83 Functions with latencies: 29
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84 funcid count [ min, med, 90p, 99p, max] sum function
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85 187 360 [ 0.000000, 0.000001, 0.000014, 0.000032, 0.000075] 0.001596 LLLexer.cpp:446:0: llvm::LLLexer::LexIdentifier()
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86 85 130 [ 0.000000, 0.000000, 0.000018, 0.000023, 0.000156] 0.000799 X86ISelDAGToDAG.cpp:1984:0: (anonymous namespace)::X86DAGToDAGISel::Select(llvm::SDNode*)
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87 138 130 [ 0.000000, 0.000000, 0.000017, 0.000155, 0.000155] 0.000774 SelectionDAGISel.cpp:2963:0: llvm::SelectionDAGISel::SelectCodeCommon(llvm::SDNode*, unsigned char const*, unsigned int)
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88 188 103 [ 0.000000, 0.000000, 0.000003, 0.000123, 0.000214] 0.000737 LLParser.cpp:2692:0: llvm::LLParser::ParseValID(llvm::ValID&, llvm::LLParser::PerFunctionState*)
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89 88 1 [ 0.000562, 0.000562, 0.000562, 0.000562, 0.000562] 0.000562 X86ISelLowering.cpp:83:0: llvm::X86TargetLowering::X86TargetLowering(llvm::X86TargetMachine const&, llvm::X86Subtarget const&)
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90 125 102 [ 0.000001, 0.000003, 0.000010, 0.000017, 0.000049] 0.000471 Verifier.cpp:3714:0: (anonymous namespace)::Verifier::visitInstruction(llvm::Instruction&)
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91 90 8 [ 0.000023, 0.000035, 0.000106, 0.000106, 0.000106] 0.000342 X86ISelLowering.cpp:3363:0: llvm::X86TargetLowering::LowerCall(llvm::TargetLowering::CallLoweringInfo&, llvm::SmallVectorImpl<llvm::SDValue>&) const
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92 124 32 [ 0.000003, 0.000007, 0.000016, 0.000041, 0.000041] 0.000310 Verifier.cpp:1967:0: (anonymous namespace)::Verifier::visitFunction(llvm::Function const&)
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93 123 1 [ 0.000302, 0.000302, 0.000302, 0.000302, 0.000302] 0.000302 LLVMContextImpl.cpp:54:0: llvm::LLVMContextImpl::~LLVMContextImpl()
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94 139 46 [ 0.000000, 0.000002, 0.000006, 0.000008, 0.000019] 0.000138 TargetLowering.cpp:506:0: llvm::TargetLowering::SimplifyDemandedBits(llvm::SDValue, llvm::APInt const&, llvm::APInt&, llvm::APInt&, llvm::TargetLowering::TargetLoweringOpt&, unsigned int, bool) const
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95
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96 This shows us that for our input file, ``llc`` spent the most cumulative time
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97 in the lexer (a total of 1 millisecond). If we wanted for example to work with
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98 this data in a spreadsheet, we can output the results as CSV using the
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99 ``-format=csv`` option to the command for further analysis.
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100
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101 If we want to get a textual representation of the raw trace we can use the
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102 ``llvm-xray convert`` tool to get YAML output. The first few lines of that
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103 output for an example trace would look like the following:
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104
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105 ::
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106
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107 $ llvm-xray convert -f yaml -symbolize -instr_map=./bin/llc xray-log.llc.m35qPB
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108 ---
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109 header:
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110 version: 1
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111 type: 0
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112 constant-tsc: true
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113 nonstop-tsc: true
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114 cycle-frequency: 2601000000
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115 records:
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116 - { type: 0, func-id: 110, function: __cxx_global_var_init.8, cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426023268520 }
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117 - { type: 0, func-id: 110, function: __cxx_global_var_init.8, cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426023523052 }
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118 - { type: 0, func-id: 164, function: __cxx_global_var_init, cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426029925386 }
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119 - { type: 0, func-id: 164, function: __cxx_global_var_init, cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426030031128 }
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120 - { type: 0, func-id: 142, function: '(anonymous namespace)::CommandLineParser::ParseCommandLineOptions(int, char const* const*, llvm::StringRef, llvm::raw_ostream*)', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426046951388 }
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121 - { type: 0, func-id: 142, function: '(anonymous namespace)::CommandLineParser::ParseCommandLineOptions(int, char const* const*, llvm::StringRef, llvm::raw_ostream*)', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426047282020 }
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122 - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426047857332 }
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123 - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426047984152 }
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124 - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426048036584 }
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125 - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426048042292 }
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126 - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426048055056 }
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127 - { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426048067316 }
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128
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129 Controlling Fidelity
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130 --------------------
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131
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132 So far in our examples, we haven't been getting full coverage of the functions
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133 we have in the binary. To get that, we need to modify the compiler flags so
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134 that we can instrument more (if not all) the functions we have in the binary.
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135 We have two options for doing that, and we explore both of these below.
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136
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137 Instruction Threshold
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138 `````````````````````
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139
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140 The first "blunt" way of doing this is by setting the minimum threshold for
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141 function bodies to 1. We can do that with the
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142 ``-fxray-instruction-threshold=N`` flag when building our binary. We rebuild
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143 ``llc`` with this option and observe the results:
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144
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145 ::
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146
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147 $ rm CMakeCache.txt
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148 $ cmake -GNinja ../llvm -DCMAKE_BUILD_TYPE=Release \
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149 -DCMAKE_C_FLAGS_RELEASE="-fxray-instrument -fxray-instruction-threshold=1" \
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150 -DCMAKE_CXX_FLAGS="-fxray-instrument -fxray-instruction-threshold=1"
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151 $ ninja llc
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152 $ XRAY_OPTIONS="patch_premain=true" ./bin/llc input.ll
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153 ==69819==XRay: Log file in 'xray-log.llc.5rqxkU'
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154
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155 $ llvm-xray account xray-log.llc.5rqxkU -top=10 -sort=sum -sortorder=dsc -instr_map ./bin/llc
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156 Functions with latencies: 36652
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157 funcid count [ min, med, 90p, 99p, max] sum function
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158 75 1 [ 0.672368, 0.672368, 0.672368, 0.672368, 0.672368] 0.672368 llc.cpp:271:0: main
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159 78 1 [ 0.626455, 0.626455, 0.626455, 0.626455, 0.626455] 0.626455 llc.cpp:381:0: compileModule(char**, llvm::LLVMContext&)
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160 139617 1 [ 0.472618, 0.472618, 0.472618, 0.472618, 0.472618] 0.472618 LegacyPassManager.cpp:1723:0: llvm::legacy::PassManager::run(llvm::Module&)
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161 139610 1 [ 0.472618, 0.472618, 0.472618, 0.472618, 0.472618] 0.472618 LegacyPassManager.cpp:1681:0: llvm::legacy::PassManagerImpl::run(llvm::Module&)
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162 139612 1 [ 0.470948, 0.470948, 0.470948, 0.470948, 0.470948] 0.470948 LegacyPassManager.cpp:1564:0: (anonymous namespace)::MPPassManager::runOnModule(llvm::Module&)
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163 139607 2 [ 0.147345, 0.315994, 0.315994, 0.315994, 0.315994] 0.463340 LegacyPassManager.cpp:1530:0: llvm::FPPassManager::runOnModule(llvm::Module&)
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164 139605 21 [ 0.000002, 0.000002, 0.102593, 0.213336, 0.213336] 0.463331 LegacyPassManager.cpp:1491:0: llvm::FPPassManager::runOnFunction(llvm::Function&)
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165 139563 26096 [ 0.000002, 0.000002, 0.000037, 0.000063, 0.000215] 0.225708 LegacyPassManager.cpp:1083:0: llvm::PMDataManager::findAnalysisPass(void const*, bool)
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166 108055 188 [ 0.000002, 0.000120, 0.001375, 0.004523, 0.062624] 0.159279 MachineFunctionPass.cpp:38:0: llvm::MachineFunctionPass::runOnFunction(llvm::Function&)
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167 62635 22 [ 0.000041, 0.000046, 0.000050, 0.126744, 0.126744] 0.127715 X86TargetMachine.cpp:242:0: llvm::X86TargetMachine::getSubtargetImpl(llvm::Function const&) const
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168
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169
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170 Instrumentation Attributes
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171 ``````````````````````````
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172
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173 The other way is to use configuration files for selecting which functions
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174 should always be instrumented by the compiler. This gives us a way of ensuring
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175 that certain functions are either always or never instrumented by not having to
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176 add the attribute to the source.
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177
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178 To use this feature, you can define one file for the functions to always
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179 instrument, and another for functions to never instrument. The format of these
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180 files are exactly the same as the SanitizerLists files that control similar
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181 things for the sanitizer implementations. For example, we can have two
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182 different files like below:
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183
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184 ::
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185
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186 # always-instrument.txt
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187 # always instrument functions that match the following filters:
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188 fun:main
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189
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190 # never-instrument.txt
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191 # never instrument functions that match the following filters:
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192 fun:__cxx_*
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193
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194 Given the above two files we can re-build by providing those two files as
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195 arguments to clang as ``-fxray-always-instrument=always-instrument.txt`` or
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196 ``-fxray-never-instrument=never-instrument.txt``.
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197
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198 The XRay stack tool
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199 -------------------
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200
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201 Given a trace, and optionally an instrumentation map, the ``llvm-xray stack``
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202 command can be used to analyze a call stack graph constructed from the function
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203 call timeline.
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204
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205 The simplest way to use the command is simply to output the top stacks by call
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206 count and time spent.
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207
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208 ::
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209
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210 $ llvm-xray stack xray-log.llc.5rqxkU -instr_map ./bin/llc
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211
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212 Unique Stacks: 3069
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213 Top 10 Stacks by leaf sum:
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214
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215 Sum: 9633790
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216 lvl function count sum
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217 #0 main 1 58421550
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218 #1 compileModule(char**, llvm::LLVMContext&) 1 51440360
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219 #2 llvm::legacy::PassManagerImpl::run(llvm::Module&) 1 40535375
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220 #3 llvm::FPPassManager::runOnModule(llvm::Module&) 2 39337525
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221 #4 llvm::FPPassManager::runOnFunction(llvm::Function&) 6 39331465
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222 #5 llvm::PMDataManager::verifyPreservedAnalysis(llvm::Pass*) 399 16628590
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223 #6 llvm::PMTopLevelManager::findAnalysisPass(void const*) 4584 15155600
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224 #7 llvm::PMDataManager::findAnalysisPass(void const*, bool) 32088 9633790
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225
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226 ..etc..
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227
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228 In the default mode, identical stacks on different threads are independently
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229 aggregated. In a multithreaded program, you may end up having identical call
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230 stacks fill your list of top calls.
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231
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232 To address this, you may specify the ``-aggregate-threads`` or
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233 ``-per-thread-stacks`` flags. ``-per-thread-stacks`` treats the thread id as an
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234 implicit root in each call stack tree, while ``-aggregate-threads`` combines
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235 identical stacks from all threads.
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236
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237 Flame Graph Generation
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238 ----------------------
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239
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240 The ``llvm-xray stack`` tool may also be used to generate flamegraphs for
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241 visualizing your instrumented invocations. The tool does not generate the graphs
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242 themselves, but instead generates a format that can be used with Brendan Gregg's
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243 FlameGraph tool, currently available on `github
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244 <https://github.com/brendangregg/FlameGraph>`_.
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245
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246 To generate output for a flamegraph, a few more options are necessary.
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247
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248 - ``-all-stacks`` - Emits all of the stacks instead of just the top stacks.
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249 - ``-stack-format`` - Choose the flamegraph output format 'flame'.
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250 - ``-aggregation-type`` - Choose the metric to graph.
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251
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252 You may pipe the command output directly to the flamegraph tool to obtain an
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253 svg file.
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254
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255 ::
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256
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257 $llvm-xray stack xray-log.llc.5rqxkU -instr_map ./bin/llc -stack-format=flame -aggregation-type=time -all-stacks | \
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258 /path/to/FlameGraph/flamegraph.pl > flamegraph.svg
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259
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260 If you open the svg in a browser, mouse events allow exploring the call stacks.
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261
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262 Further Exploration
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263 -------------------
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264
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265 The ``llvm-xray`` tool has a few other subcommands that are in various stages
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266 of being developed. One interesting subcommand that can highlight a few
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267 interesting things is the ``graph`` subcommand. Given for example the following
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268 toy program that we build with XRay instrumentation, we can see how the
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269 generated graph may be a helpful indicator of where time is being spent for the
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270 application.
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271
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272 .. code-block:: c++
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273
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274 // sample.cc
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275 #include <iostream>
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276 #include <thread>
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277
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134
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278 [[clang::xray_always_instrument]] void f() {
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121
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279 std::cerr << '.';
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280 }
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281
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134
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282 [[clang::xray_always_instrument]] void g() {
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121
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283 for (int i = 0; i < 1 << 10; ++i) {
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284 std::cerr << '-';
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285 }
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286 }
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287
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288 int main(int argc, char* argv[]) {
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289 std::thread t1([] {
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290 for (int i = 0; i < 1 << 10; ++i)
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291 f();
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292 });
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293 std::thread t2([] {
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294 g();
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295 });
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296 t1.join();
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297 t2.join();
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298 std::cerr << '\n';
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299 }
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300
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301 We then build the above with XRay instrumentation:
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302
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303 ::
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304
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305 $ clang++ -o sample -O3 sample.cc -std=c++11 -fxray-instrument -fxray-instruction-threshold=1
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306 $ XRAY_OPTIONS="patch_premain=true" ./sample
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307
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308 We can then explore the graph rendering of the trace generated by this sample
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309 application. We assume you have the graphviz toosl available in your system,
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310 including both ``unflatten`` and ``dot``. If you prefer rendering or exploring
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311 the graph using another tool, then that should be feasible as well. ``llvm-xray
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312 graph`` will create DOT format graphs which should be usable in most graph
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313 rendering applications. One example invocation of the ``llvm-xray graph``
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314 command should yield some interesting insights to the workings of C++
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315 applications:
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316
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317 ::
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318
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319 $ llvm-xray graph xray-log.sample.* -m sample -color-edges=sum -edge-label=sum \
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320 | unflatten -f -l10 | dot -Tsvg -o sample.svg
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321
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322 Next Steps
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323 ----------
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324
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325 If you have some interesting analyses you'd like to implement as part of the
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326 llvm-xray tool, please feel free to propose them on the llvm-dev@ mailing list.
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327 The following are some ideas to inspire you in getting involved and potentially
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328 making things better.
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329
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330 - Implement a query/filtering library that allows for finding patterns in the
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331 XRay traces.
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332 - A conversion from the XRay trace onto something that can be visualised
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333 better by other tools (like the Chrome trace viewer for example).
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334 - Collecting function call stacks and how often they're encountered in the
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335 XRay trace.
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336
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337
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