The GDB JIT Interface

4 months ago

GDB uses debug information (DWARF format) from compilers like Clang, GCC, and rustc to provide debugging features.
GDB struggles with JIT-compiled functions because the instruction pointer isn't in the debug info tables, leading to '???' in the output.
GDB provides a JIT interface that requires implementing functions like `__jit_debug_register_code` and managing a linked list (`jit_code_entry`).
The JIT interface involves creating ELF/Mach-O objects with DWARF info, which is complex and requires stable memory pointers.
GDB also supports a newer, more flexible interface where custom binary formats can be used, requiring a reader plugin to be loaded into GDB.
The newer interface involves implementing functions like `gdb_read_debug_info` and `gdb_unwind_frame`, but many runtimes leave these stubbed.
There's a suggestion to reuse the Linux perf map interface in GDB for JIT symbols, though it would offer limited functionality.
V8 notes performance issues with the JIT interface due to O(n²) behavior in the linked list and the need to disable moving GC for stability.
ART handles dead code entries by treating the GDB JIT linked list as a weak reference and periodically cleaning it up.

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