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annotate docs/DebuggingJITedCode.rst @ 107:a03ddd01be7e

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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
parents 95c75e76d11b
children
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1 ==============================
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2 Debugging JIT-ed Code With GDB
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3 ==============================
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4
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5 Background
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6 ==========
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7
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8 Without special runtime support, debugging dynamically generated code with
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9 GDB (as well as most debuggers) can be quite painful. Debuggers generally
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10 read debug information from the object file of the code, but for JITed
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11 code, there is no such file to look for.
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12
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13 In order to communicate the necessary debug info to GDB, an interface for
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14 registering JITed code with debuggers has been designed and implemented for
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15 GDB and LLVM MCJIT. At a high level, whenever MCJIT generates new machine code,
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16 it does so in an in-memory object file that contains the debug information in
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17 DWARF format. MCJIT then adds this in-memory object file to a global list of
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18 dynamically generated object files and calls a special function
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19 (``__jit_debug_register_code``) marked noinline that GDB knows about. When
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20 GDB attaches to a process, it puts a breakpoint in this function and loads all
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21 of the object files in the global list. When MCJIT calls the registration
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22 function, GDB catches the breakpoint signal, loads the new object file from
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23 the inferior's memory, and resumes the execution. In this way, GDB can get the
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24 necessary debug information.
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25
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26 GDB Version
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27 ===========
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28
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29 In order to debug code JIT-ed by LLVM, you need GDB 7.0 or newer, which is
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30 available on most modern distributions of Linux. The version of GDB that
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31 Apple ships with Xcode has been frozen at 6.3 for a while. LLDB may be a
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32 better option for debugging JIT-ed code on Mac OS X.
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33
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34
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35 Debugging MCJIT-ed code
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36 =======================
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37
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38 The emerging MCJIT component of LLVM allows full debugging of JIT-ed code with
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39 GDB. This is due to MCJIT's ability to use the MC emitter to provide full
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40 DWARF debugging information to GDB.
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41
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42 Note that lli has to be passed the ``-use-mcjit`` flag to JIT the code with
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43 MCJIT instead of the old JIT.
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44
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45 Example
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46 -------
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47
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48 Consider the following C code (with line numbers added to make the example
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49 easier to follow):
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50
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51 ..
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52 FIXME:
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53 Sphinx has the ability to automatically number these lines by adding
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54 :linenos: on the line immediately following the `.. code-block:: c`, but
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55 it looks like garbage; the line numbers don't even line up with the
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56 lines. Is this a Sphinx bug, or is it a CSS problem?
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57
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58 .. code-block:: c
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59
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60 1 int compute_factorial(int n)
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61 2 {
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62 3 if (n <= 1)
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63 4 return 1;
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64 5
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65 6 int f = n;
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66 7 while (--n > 1)
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67 8 f *= n;
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68 9 return f;
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69 10 }
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70 11
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71 12
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72 13 int main(int argc, char** argv)
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73 14 {
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74 15 if (argc < 2)
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75 16 return -1;
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76 17 char firstletter = argv[1][0];
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77 18 int result = compute_factorial(firstletter - '0');
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78 19
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79 20 // Returned result is clipped at 255...
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80 21 return result;
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81 22 }
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82
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83 Here is a sample command line session that shows how to build and run this
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84 code via ``lli`` inside GDB:
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85
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86 .. code-block:: bash
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87
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88 $ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c
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89 $ gdb --quiet --args $BINPATH/lli -use-mcjit showdebug.ll 5
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90 Reading symbols from $BINPATH/lli...done.
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91 (gdb) b showdebug.c:6
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92 No source file named showdebug.c.
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93 Make breakpoint pending on future shared library load? (y or [n]) y
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94 Breakpoint 1 (showdebug.c:6) pending.
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95 (gdb) r
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96 Starting program: $BINPATH/lli -use-mcjit showdebug.ll 5
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97 [Thread debugging using libthread_db enabled]
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98
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99 Breakpoint 1, compute_factorial (n=5) at showdebug.c:6
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100 6 int f = n;
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101 (gdb) p n
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102 $1 = 5
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103 (gdb) p f
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104 $2 = 0
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105 (gdb) n
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106 7 while (--n > 1)
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107 (gdb) p f
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108 $3 = 5
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109 (gdb) b showdebug.c:9
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110 Breakpoint 2 at 0x7ffff7ed404c: file showdebug.c, line 9.
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111 (gdb) c
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112 Continuing.
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113
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114 Breakpoint 2, compute_factorial (n=1) at showdebug.c:9
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115 9 return f;
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116 (gdb) p f
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117 $4 = 120
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118 (gdb) bt
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119 #0 compute_factorial (n=1) at showdebug.c:9
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120 #1 0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
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121 #2 0x3500000001652748 in ?? ()
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122 #3 0x00000000016677e0 in ?? ()
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123 #4 0x0000000000000002 in ?? ()
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124 #5 0x0000000000d953b3 in llvm::MCJIT::runFunction (this=0x16151f0, F=0x1603020, ArgValues=...) at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/MCJIT/MCJIT.cpp:161
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125 #6 0x0000000000dc8872 in llvm::ExecutionEngine::runFunctionAsMain (this=0x16151f0, Fn=0x1603020, argv=..., envp=0x7fffffffe040)
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126 at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/ExecutionEngine.cpp:397
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127 #7 0x000000000059c583 in main (argc=4, argv=0x7fffffffe018, envp=0x7fffffffe040) at /home/ebenders_test/llvm_svn_rw/tools/lli/lli.cpp:324
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128 (gdb) finish
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129 Run till exit from #0 compute_factorial (n=1) at showdebug.c:9
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130 0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
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131 18 int result = compute_factorial(firstletter - '0');
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132 Value returned is $5 = 120
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133 (gdb) p result
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134 $6 = 23406408
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135 (gdb) n
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136 21 return result;
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137 (gdb) p result
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138 $7 = 120
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139 (gdb) c
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140 Continuing.
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141
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142 Program exited with code 0170.
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143 (gdb)