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[Caml-list] debugging a JIT compiler (from Ocaml bytecode to machine code [x86,etc...])
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Date: -- (:)
From: Basile Starynkevitch <basile.starynkevitch@i...>
Subject: [Caml-list] debugging a JIT compiler (from Ocaml bytecode to machine code [x86,etc...])
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
from http://savannah.gnu.org/projects/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).

Regards.


-- 
Basile STARYNKEVITCH -- basile dot starynkevitch at inria dot fr
Project cristal.inria.fr - phone +33 1 3963 5197 - mobile 6 8501 2359
http://cristal.inria.fr/~starynke --- all opinions are only mine 

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