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  57. <a name="JIT-Interface"></a>
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  60. Next: <a href="In_002dProcess-Agent.html#In_002dProcess-Agent" accesskey="n" rel="next">In-Process Agent</a>, Previous: <a href="Annotations.html#Annotations" accesskey="p" rel="prev">Annotations</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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  63. <a name="JIT-Compilation-Interface"></a>
  64. <h2 class="chapter">29 JIT Compilation Interface</h2>
  65. <a name="index-just_002din_002dtime-compilation"></a>
  66. <a name="index-JIT-compilation-interface"></a>
  67. <p>This chapter documents <small>GDB</small>&rsquo;s <em>just-in-time</em> (JIT) compilation
  68. interface. A JIT compiler is a program or library that generates native
  69. executable code at runtime and executes it, usually in order to achieve good
  70. performance while maintaining platform independence.
  71. </p>
  72. <p>Programs that use JIT compilation are normally difficult to debug because
  73. portions of their code are generated at runtime, instead of being loaded from
  74. object files, which is where <small>GDB</small> normally finds the program&rsquo;s symbols
  75. and debug information. In order to debug programs that use JIT compilation,
  76. <small>GDB</small> has an interface that allows the program to register in-memory
  77. symbol files with <small>GDB</small> at runtime.
  78. </p>
  79. <p>If you are using <small>GDB</small> to debug a program that uses this interface, then
  80. it should work transparently so long as you have not stripped the binary. If
  81. you are developing a JIT compiler, then the interface is documented in the rest
  82. of this chapter. At this time, the only known client of this interface is the
  83. LLVM JIT.
  84. </p>
  85. <p>Broadly speaking, the JIT interface mirrors the dynamic loader interface. The
  86. JIT compiler communicates with <small>GDB</small> by writing data into a global
  87. variable and calling a function at a well-known symbol. When <small>GDB</small>
  88. attaches, it reads a linked list of symbol files from the global variable to
  89. find existing code, and puts a breakpoint in the function so that it can find
  90. out about additional code.
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