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[Caml-list] debugging a JIT compiler (from Ocaml bytecode to machine code [x86,etc...])
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Date: -- (:)
From: Walid Taha <taha@c...>
Subject: Re: [Caml-list] debugging a JIT compiler (from Ocaml bytecode to machine code [x86,etc...])


This sounds like a very interesting and useful project.

What size examples are you looking for?  If you're looking for small
programs (up to depth 5, say), you can write a small program that
enumerates all them.  After that point, there's just way too many of them.  
Do you want just lambda constructs or more ocaml constructs?

Also, with the small benchmarks that you already have, do you run them
just once or do you put them in a loop?  It could be interesting to see if
putting them in a loop also leads to the eventual seg-fault.

Finally, the MetaOCaml example directory (called "mex") has a test
programs of various sizes.


On Fri, 19 Mar 2004, Basile Starynkevitch wrote:

|Dear All,
|As you might have noticed on my home page below, I coded (in C, using
|the GNU lightning library) a JIT translator (or compiler) which
|interprets Ocaml bytecode by translating it to machine code, using the
|GNU lightning library. You'll need the latest CVS version of lightning
|The intended use should be to replace ocaml's byterun/interp.c with my
|jitinterp.c and recompile all the runtime. Details are given in my
|homepage below. 
|**this program is coded but still buggy** so don't use it *yet*
|(except for helping me).
|Debugging such a machine code generating program is painful. All
|trivial tests (those under CVS in test/testinterp/) passes but a bug
|still remain, which causes a segmentation violation (later on... - not
|at the faulty JIT codepoint!).
|Currently, I debugged most of it using a mixture of following
|techniques (enabled only with the -DDEBUG flag).
|1. the generated machine code can be disassembled
|2. the JIT translator is able to write on a pipe, originally to a Ruby
|script (hence the JML_RBYPRINTF name in the C code). (you need a
|special startup.c to open this pipe)
|3. a specific tiny debugger (using the ptrace system call) has been
|coded to st breakpoint appropriately (in the generated machine code).
|4. I instrumented also the bytecode interpreter to print its stack and
|registers (ie bytecode program counter, stack pointer, accumulator,
|..)  and manually compare it with traces from my debugger.
|5. the bytecode is expected to stay fixed (this is false for C
|callbacks). If it is freed, the generated code should be freed also
|(which should be easy to code, since most of the stuff is there).
|My problem is that all simple tests run ok, and the few tests that
|crash have to run a significant amount, so the trace files are huge.
|I suspect that only one or two bug remains, like e.g. a wrong return
|from the GC on allocation, which corrupt the (Caml) stack ... The
|problem is that I lack of simple programs to exhibit it, and that the
|bug don't appear on trivial samples.
|I probably won't have time to work on it in the next few weeks, but
|any insight or hint is helpful. If you happen to have small test
|programs which uses a small fraction of the standard library, it
|should help also.
|If you would be interested by a JIT ocamlrunj program (with speedup of
|at most a factor of 2 w.r.t. to ocamlrun), please tell.
|If as a researcher or hacker you happen to write interpreters from
|scratch for a new super-duper language, consider using GNU lightning,
|it is very interesting and provide good results (which considerably
|easier to code with than generating machine code directly).


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