Forth debugger

[mkobetic]

The debugging story is not great. It is possible to use GDB and it is actually fairly nice with the Forth extensions if you are debugging assembler in codewords, but debugging forth code is painful. There is some progress with building actual Forth backtrace in GDB and there may be a viable path forward there, but at this point it's hard to even put a breakpoint anywhere.

It may make sense to try building a forth side debugger, there is some prior art:

• https://win32forth.sourceforge.net/doc/p-debugging.htm
• https://www.complang.tuwien.ac.at/forth/gforth/Docs-html/Singlestep-Debugger.html

It would probably require some overhead in the inner interpreter but we could set it up so that it can be compiled in when needed.

[mkobetic]

Few quick, probably bad ideas:

• When WANT_DEBUG, Inject a deferred word at DO_NEXT (before loading the next word into FORTHW), initialized to XT_NOP (still need to figure out how to execute the deferred word without it being impacted by the debug infrastructure, global flag?)
• New word debug, does the following:
  • sends count prefixed stack cells, then count prefixed r-stack cells, preceded by a keyword recognized by amshell's read_response
  • waits for command key from amshell
  • based on the key installs a corresponding word into the defer, unless the key is c for continue.
  • resumes the interpreter
• amshell looks for the debug keyword in response, if it sees one, renders the stacks, and sends any input keystroke back to amforth, resumes normal interpreter loop
  • to render the return stack, use the symbol table in build/amforth.sym
  • could try rendering a backtrace based on assembler listing too.
  • if we wanted to be really fancy we could try rendering the source code position using the assembler listings too.
• debug defer actions
  • 's' - step, the action does the same thing as debug word above
  • 'n' - next, the action keeps stepping until the r-stack is back at the same level as when the action started

What I like about this is that it doesn't require any external tools, just amforth and amforth-shell. It also offloads a lot of the work to amforth-shell so the burden on amforth should be minimal.