Debugging

Trace optimisation

Trace optimisation can make it difficult to understand why a yk interpreter has behaved in the way it does. It is worth trying to run your code with the optimiser turned off. You can do this with the YKD_OPT environment variable, which takes the following values:

• 1: turn the optimiser on. Default if not otherwise specified.
• 0: turn the optimiser off.

Debugging JITted code

Often you will find the need to inspect JITted code with a debugger. If the problem trace comes from a C test (i.e. one of the test cases under tests/c), then you can use the gdb_c_test tool.

The tool automates the compilation and invocation of the resulting binary under GDB.

The simplest invocation of gdb_c_test (from the top-level of the yk repo) would look like:

bin/gdb_c_test simple.c

This will automatically compile and run the tests/c/simple.c test under GDB. This would be ideal if you have a crashing trace, as it will dump you into a GDB shell at the time of the crash.

The tool has some other switches which are useful for other situations, e.g.:

bin/gdb_c_test -j -s -b10 simple.c

compiles and runs tests/c/simple.c test under GDB with JIT state debugging enabled, with compilation serialised, setting a breakpoint on the first 10 traces compiled.

For a list of all switches available, run:

bin/gdb_c_test --help

For help on using GDB, see the GDB documentation.

GDB plugin

Yk comes with a GDB plugin that allows the debugger to show higher-level information in the source view window.

The plugin is built by default and put in target/yk_gdb_plugin.so.

To use it, put this line in ~/.gdbinit:

jit-reader-load /path/to/yk/target/yk_gdb_plugin.so

Then when you run GDB, you should see:

Yk JIT support loaded.

When you are inside JITted code, the source view will show higher-level debugging information. You can show the assembler and source views on one GDB screen using the "split" layout. Type:

la spl