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<a name="JIT-Compilation-Interface"></a>
<h2 class="chapter">29 JIT Compilation Interface</h2>
<a name="index-just_002din_002dtime-compilation"></a>
<a name="index-JIT-compilation-interface"></a>
<p>This chapter documents <small>GDB</small>’s <em>just-in-time</em> (JIT) compilation
interface. A JIT compiler is a program or library that generates native
executable code at runtime and executes it, usually in order to achieve good
performance while maintaining platform independence.
</p>
<p>Programs that use JIT compilation are normally difficult to debug because
portions of their code are generated at runtime, instead of being loaded from
object files, which is where <small>GDB</small> normally finds the program’s symbols
and debug information. In order to debug programs that use JIT compilation,
<small>GDB</small> has an interface that allows the program to register in-memory
symbol files with <small>GDB</small> at runtime.
</p>
<p>If you are using <small>GDB</small> to debug a program that uses this interface, then
it should work transparently so long as you have not stripped the binary. If
you are developing a JIT compiler, then the interface is documented in the rest
of this chapter. At this time, the only known client of this interface is the
LLVM JIT.
</p>
<p>Broadly speaking, the JIT interface mirrors the dynamic loader interface. The
JIT compiler communicates with <small>GDB</small> by writing data into a global
variable and calling a fuction at a well-known symbol. When <small>GDB</small>
attaches, it reads a linked list of symbol files from the global variable to
find existing code, and puts a breakpoint in the function so that it can find
out about additional code.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">• <a href="Declarations.html#Declarations" accesskey="1">Declarations</a>:</td><td> </td><td align="left" valign="top">Relevant C struct declarations
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<tr><td align="left" valign="top">• <a href="Registering-Code.html#Registering-Code" accesskey="2">Registering Code</a>:</td><td> </td><td align="left" valign="top">Steps to register code
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<tr><td align="left" valign="top">• <a href="Unregistering-Code.html#Unregistering-Code" accesskey="3">Unregistering Code</a>:</td><td> </td><td align="left" valign="top">Steps to unregister code
</td></tr>
<tr><td align="left" valign="top">• <a href="Custom-Debug-Info.html#Custom-Debug-Info" accesskey="4">Custom Debug Info</a>:</td><td> </td><td align="left" valign="top">Emit debug information in a custom format
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