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| <h2 class="chapter">29 JIT Compilation Interface</h2> |
| |
| <p><a name="index-just_002din_002dtime-compilation-2172"></a><a name="index-JIT-compilation-interface-2173"></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. |
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