asmjit-utilities

helper classes for profiling and debugging of jitted code

Profiling with perf on linux

The profiling tool perf on linux can annotate assembly code with profiling information and also map them to source code lines. This can be done for jit code as well if the jit runtime dumps the generated code in a special file format. The class JitDump in the subdirectory perf adds support for this file format.

Instructions can be mapped back to source files and line numbers which helps understanding the profile data. The two example programs in the examples/ subdirectory illustrate how to use it with asmjit::x86::Assembler in sum.cpp, and PerfCompiler which is drop-in replacement for asmjit::x86::Compiler in sumCompiler.cpp.

Usage in application, see examples/sum.cpp:

• create JitDump object right after JitRuntime and call init() method
• when assembling a function get the code size from CodeHolder::codeSize()
• before calling function, dump it with addCodeSegment(name, function_ptr, code_size)
• call close() at the end

Profiling steps:

1. perf record -k 1 ./application
2. perf inject -j -i perf.data -o perf.data.jitted
3. perf report -i perf.data.jitted

In perf report the generated function should appear with the associated name. The annotated view can be reached by pressing 'a'.

Structure definition of dump file for Okteta

For debugging purposes, there is a structure definition of the binary jitdump file in perf/jitdump-structure. The hex editor okteta can load this definition and display all records in the dump file in a nice tree structure. Copy the subdirectory perf/jitdump-structure to ~/.local/share/okteta/structures/ and enable it in the settings screen of the structures panel.

Debugging with GDB on Linux

The debugging capabilities of GDB for jit compiled code is very limited currently as asmjit does not generate normal object files. You only get debugging on assembly level within TUI mode of GDB (disassemble command does not work). Furthermore, to be able to set (pending) breakpoints on a jitted function, you need to patch GDB with the patch included in the gdb subdirectory.

Before calling the generated function in your asmjit application, call GDBJit::addCodeSegment() with the function name, the function pointer and the code size (CodeHolder::codeSize()). This tells GDB about the newly generated function and, if patched, makes it possible to set breakpoints on this function.