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<h2 class="chapter">29 JIT Compilation Interface</h2>

<p><a name="index-just_002din_002dtime-compilation-3150"></a><a name="index-JIT-compilation-interface-3151"></a>
This chapter documents <span class="sc">gdb</span>'s <dfn>just-in-time</dfn> (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>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 <span class="sc">gdb</span> normally finds the program's symbols
and debug information.  In order to debug programs that use JIT compilation,
<span class="sc">gdb</span> has an interface that allows the program to register in-memory
symbol files with <span class="sc">gdb</span> at runtime.

   <p>If you are using <span class="sc">gdb</span> 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>Broadly speaking, the JIT interface mirrors the dynamic loader interface.  The
JIT compiler communicates with <span class="sc">gdb</span> by writing data into a global
variable and calling a fuction at a well-known symbol.  When <span class="sc">gdb</span>
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.

<ul class="menu">
<li><a accesskey="1" href="Declarations.html#Declarations">Declarations</a>:                 Relevant C struct declarations
<li><a accesskey="2" href="Registering-Code.html#Registering-Code">Registering Code</a>:             Steps to register code
<li><a accesskey="3" href="Unregistering-Code.html#Unregistering-Code">Unregistering Code</a>:           Steps to unregister code
<li><a accesskey="4" href="Custom-Debug-Info.html#Custom-Debug-Info">Custom Debug Info</a>:            Emit debug information in a custom format
</ul>

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