Unambiguously specifying source locations

[Daniel Jacobowitz <drow at mvista dot com>]

This isn't a proposal - I haven't clearly thought out the details - just
some ramblings on a problem.

I would like to have a way to clearly identify a location, not by address. 
Primarily this would be for breakpoints; when we re-read an objfile's
symbols, we need to replace breakpoints somehow, and the more of the user's
intent we can preserve the better.  Obviously this is not a perfectly
solvable problem, but we can do pretty well.

It's an important problem, too.  Not only is recompile/reload a pretty
common thing to do, but IDEs which save breakpoints across sessions would
use this also.  Et cetera.

I think it's only source locations that I need to identify in this way, in
the short term, but we should use an extensible syntax.  For a
way-beyond-current-state example, consider an inlined function with multiple
inlined instances.  One of the simpler things we could do is:

  record the number of instances in this objfile before reloading
  check the number of instances in this objfile after reloading
  if same, then we can preserve things like the breakpoint enabled state
    to the new set
  if not, punt - enable all?  warn?

The goal there being to preserve the status of particular breakpoints across
reload, as best as we can, when unrelated changes are made to the source.
Someone fixes emit_foo () and recompiles and we try not to disturb their
eight disabled and one enabled breakpoint on inlined copies of obstack_alloc
where they were tracking the allocation of the bad object.

We could be much more thorough.  Try this on for size:
  Record the source location of the emitted function which contains each
    inlined instance, as unambiguously as we can.
  Record the inlining path.

The result would look something like (I don't like this syntax, just
illustrating):
  [libfoo.so.2][foo-1.cc:foo_func:75][bar-1.h:inline_one:33]\
    [bar-1.h:inline_two:36]
Then after reload if the path still leads to an inlined copy of inline_two
we can re-establish the breakpoint.  Or there are various fuzzy matching
things we could do.  Et cetera.

Generating such paths would be useful for output anyway; and if we can do
them in such a way as to accept them for input too, that would be useful to
our users.  We could also accept as input ambiguous paths and do basically
wildcarding:
  break [libfoo.so.2][bar-1.h]inline_two

Wouldn't that be nice?

The use of braces is not entirely coincidental.  decode_line_1 currently
does not accept anything that starts with a '[' as far as I can see; ObjC
selectors always have +[ or -[.  Using braces simplifies quoting and parsing
quite a bit.  And it could be extended as necessary without too much
trouble.

There are other problems: for instance, we might want to use linker names
for non-inlined functions where possible, for GDB-generated location
descriptions; that would handle keeping track of which constructor we were
stopped on.  Otherwise we'd need some other method for that.

Another nice thing to have might be an element in that list describing
C++ template instantiation.  Not sure if that's necessary or should just be
added to the function name as needed.

Include paths may also be needed.  This happens in C, with static functions
in headers.  Here's an example that bites me all the time:
  [libbfd.so][elf32-i386.c][elflink.h:2300]
  [libbfd.so][elf32-sparc.c][elflink.h:2300]
or maybe it should be:
  [libbfd.so][elf32-i386.c:elflink.h:2300]
  [libbfd.so][elf32-sparc.c:elflink.h:2300]
The latter, I think.

Right now the equivalent to this is handled in build_canonical_line_spec.
It's a little simple-minded; there are plenty of cases it doesn't handle.
It would be nice to do better.

Any specification of this should be explicit about quoting rules, comma,
dammit.

What other thoughts do you all have?  Am I on the right track, should we
draw up a formal specification for these?

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer