Re: gcj and debugging

On 02/07/2013 10:47 AM, Tom Tromey wrote:
>>>>>> "Michael" == Michael Haupt <[email protected]> writes:
>
> Michael> in a research project, I'm generating DWARF debugging info for machine
> Michael> code generated from Java. I looked at some of the DWARF debug info
> Michael> generated by gcj (including that contained in libgcj.so).
>
> I think the first thing to realize is that the gdb support for Java was
> all written to target gcj specifically, and also written long ago and
> then barely updated.
>
> It isn't very general or done the way it would be done now.
> It was never updated for the gcj binary compatibility ABI.
>
> Michael> Java arrays are all merely declared in the DWARF; structural
> Michael> information about Java array objects is not given. In libgcj's
> Michael> array.h, there are two classes, __JArray and JArray<T> (inheriting
> Michael> from __Jarray), that hold a length field and, in addition, a T*
> Michael> array[0].
>
> Michael> The debug info for libgcj does not contain any structural information
> Michael> about these two classes.
>
> Michael> How does gdb determine that there is a length field, and how many
> Michael> elements there are if the length is assigned dynamically? I reckon the
> Michael> latter can be done by describing the length as a DWARF exprloc, but
> Michael> the description is nowhere to be found.
>
> I thought things like "print array[0]" used to work, but even that seems
> to fail for me now.  There is definitely some code for this in gdb, see
> jv-valprint.c:java_value_print.  It seems to know a bit about array
> layout and then goes from there (though the code looks somewhat bogus to
> me...).  See also jv-lang.c:evaluate_subexp_java for the code that tries
> to do array subscripting.
>
> Without debugging more I couldn't say why it is broken.
>
> Michael> How does the DWARF description of a Java array look?
> Michael> Is this generated at run-time somehow?
> Michael> Do gcj-generated binaries and gdb interact in some other ways than
> Michael> through DWARF when it comes to meta-info exchange?
>
> I believe gdb can also read some of the gcj runtime metadata and
> construct type information from this.  The code here seems pretty
> minimal though.
>
> Michael> Likewise, how does gdb know how to display a java.lang.String as a,
> Michael> well, string?
>
> gdb has special code for this.  It looks for a type named
> "java.lang.String" and then knows what fields it has.
>

Yes.  IIRC, from gdb you have to do:  call degug_????

The exact names of the debug helpers escape me at the moment.

David Daney


> Yucky.
>
> Nowadays I'd be more tempted to implement string-printing using Python.
> _Jv_Utf8Const could be handled this way too.
>
> Tom
>
>