[LLVMdev] [cfe-dev] [lldb-dev] What does "debugger tuning" mean?

[David Blaikie]

On Fri, May 1, 2015 at 2:00 PM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> wrote:

> > A few more things that vote for debugger tuning:
> >
> > - LLDB doesn't like to have DWARF that has a class A that inherits from
> > class B, but only a forward declaration of class B is provided.
>
> Hmm do we emit that kind of thing today?  In a naïve test, I'm seeing
> the full description of class B.
>

the trick is to make the base class's vtable out of line (by having a key
function):

struct base {
  virtual ~base();
};

struct derived : base {
};

derived d;

the DWARF for that file (on non-darwin) will contain a declaration for
'base' and a definition for 'derived'.

GCC does the same thing.


>
> > - LLDB wants the .apple_XXX accelerator tables, GDB wants
> > .debug_pubnames/.debug_pubtypes
>
> Agreed.
>
> > So it would be great to have a "-debugger" flag that could be specified
> >
> > -debugger=lldb
> > -debugger=gdb
> >
> > Not sure on the option name, but I do like the idea.
>
> We'll bikeshed the name later but yes, that's the plan.
> Thanks,
> --paulr
>
> >
> > Greg
> >
> > > On May 1, 2015, at 1:06 PM, Robinson, Paul
> > <Paul_Robinson at playstation.sony.com> wrote:
> > >
> > > This is basically a reboot of the previous thread titled
> > >  About the "debugger target"
> > > except that "target" was really too strong a term for what I had
> > intended
> > > to use this feature for.  "Debugger tuning" is more like it.  You don't
> > > need to have read the previous thread, I'll recap here.
> > >
> > > Fundamentally, Clang/LLVM uses DWARF as the specification for the
> > _format_
> > > of information provided by the compiler to a variety of "consumers,"
> > which
> > > primarily means debuggers (but not exclusively).  [For a long time it
> > was
> > > the only format supported by LLVM. Lately, Microsoft debug info has
> > started
> > > appearing, but being a less widely used format, the issues that DWARF
> > runs
> > > into aren't a concern for that format.  So "debugger tuning" is
> unlikely
> > > to be an issue for Microsoft debug info.]
> > >
> > > DWARF is a permissive standard, meaning that it does not rigidly
> require
> > > that source-language construct X must be described using the DWARF
> > > construct Y.  Instead, DWARF says something more like, "If you have a
> > > source construct that means something like X, here's a mechanism Y that
> > > you could use to describe it."  While this gives compilers a lot of
> nice
> > > flexibility, it does mean that there's a lot of wiggle room for how a
> > > compiler describes something and in how a debugger interprets that
> > > description.  Compilers and debuggers therefore need to do a bit of
> > > negotiation in determining how the debug-info "contract" will work,
> when
> > > it comes to nitty-gritty details.  DWARF itself (the standard, as well
> > > as the committee that owns the standard) refuses to get involved in
> this
> > > negotiation, referring to all that as "quality of implementation
> > issues."
> > >
> > > It is readily apparent that different debuggers have different ideas
> > > about certain DWARF features, for example whether they are useful or
> > > irrelevant, or whether a certain source construct should be described
> > > this way or that way.  As these generally fall into the QOI realm, the
> > > DWARF spec itself is no help, and it comes down to a matter of opinion
> > > about whether "the debugger should just know this" or "the compiler
> > > really ought to just emit it that way."
> > >
> > > Clang/LLVM is in the position of being a compiler that wants to support
> > > several different debuggers, all of which have slightly different ideas
> > > about what they want from the DWARF info for a program.  Our first line
> > > of defense of course is the DWARF standard itself, but as we've seen,
> > > that is not a universally definitive reference.
> > >
> > > LLVM already emits DWARF slightly differently for different *targets*;
> > > primarily Darwin, in a few cases PS4.  But in at least some cases, the
> > > target is just a (somewhat unreliable) proxy for which *debugger* the
> > > compiler expects to be consuming the DWARF.  The most instructive case
> > > is the exact DWARF expression used to describe the location of a
> thread-
> > > local variable.  DWARF v3 defined an operator to find the base address
> > > of the thread-local storage area; however, GDB has never learned to
> > > recognize it.  Therefore, for targets where we "know" GDB isn't used,
> > > we can emit the standard operator; for targets where GDB *might* be
> > > used, we need to emit the equivalent (non-standard) GNU operator.
> > >
> > > It would be semantically more meaningful to base decisions like this on
> > > whether we expected the debugger to be X or Y or Z.  Therefore I've
> > > proposed (http://reviews.llvm.org/D8506) a "debugger tuning" option
> that
> > > will make the reasoning behind these choices more obvious, and
> > ultimately
> > > give users a way to control the tuning themselves, when the platform's
> > > default isn't what they want. (I'll have a follow-up patch exposing the
> > > tuning option to the Clang driver.)
> > >
> > > So, what kinds of things should be based on the debugger tuning option?
> > > Are there still things that should be based on the target platform?
> > > Simplest to consider these questions together, because it is often
> clear
> > > which criterion is important if you consider (a) the same debugger run
> > > on different targets, versus (b) different debuggers running on the
> same
> > > target.  Basically, if the same debugger on different targets wants to
> > > have something a certain way, that's probably a debugger-tuning thing.
> > > And if different debuggers on the same target doesn't mean you should
> > > change how the DWARF looks, that's likely a platform-specific thing.
> > >
> > > The most obvious example of a debugger-tuning consideration is the TLS
> > > operator mentioned above. That's something that GDB insists on having.
> > > (It turns out that the standard operator was defined in DWARF 3, so we
> > > also have to emit the GNU operator if we're producing DWARF 2.  Tuning
> > > considerations don't trump what the standard says.)
> > >
> > > Another example would be .debug_pubnames and .debug_pubtypes sections.
> > > Currently these default to omitted for Darwin and PS4, but included
> > > everywhere else. My initial patch for "tuning" changes the PS4 platform
> > > criterion to the SCE debugger predicate; quite likely the "not Darwin"
> > > criterion ought to be "not LLDB" or in other words "on for GDB only."
> > > And having the code actually reflect the correct semantic purpose seems
> > > like an overall goodness.
> > >
> > > An example of a target-dependent feature might be the .debug_aranges
> > > section. As it happens, we don't emit this section by default, because
> > > apparently no debugger finds it useful, although there's a command-line
> > > option (-gdwarf-aranges) for it.  But, for PS4 we do want to emit it,
> > > because we have non-debugger tools that find it useful.  We haven't yet
> > > done the work to make that change on llvm.org, but it's on the list.
> > > I would conditionalize this on the target, not the debugger, because
> > > the debugger is not why we want to generate the section.
> > >
> > > Okay, so I've been pretty long-winded about all this, can I possibly
> > > codify it all into a reasonably succinct set of guidelines?  (which
> > > ought to be committed to the repo somewhere, although whether it's as
> > > a lump of text in a docs webpage or a lump of commentary in some source
> > > file is not clear; opinions welcome.)
> > >
> > > o Emit standard DWARF if possible.
> > > o Omitting standard DWARF features that nobody uses is fine.
> > >  (example: DW_AT_sibling)
> > > o Extensions are okay, but think about the circumstances where they
> > >  would be useful (versus just wasting space).  These are probably a
> > >  debugger tuning decision, but might be a target-based decision.
> > >  (example: DW_AT_APPLE_* attributes)
> > > o If some debugger can't tolerate some piece of standard DWARF, that's
> > >  a missing feature or a bug in the debugger.  Accommodating that in
> > >  the compiler is a debugger tuning decision.
> > >  (example: DW_OP_form_tls_address not understood by GDB)
> > > o If some debugger has no use for some piece of standard DWARF, and
> > >  it saves space to omit it, that's a debugger tuning decision.
> > >  (example: .debug_pubnames/.debug_pubtypes sections)
> > > o If a debugger wants things a certain way regardless of the target,
> > >  that's probably a debugger tuning decision.
> > > o If "system" software on a target (other than the debugger) wants
> > >  things a certain way regardless of which debugger you're using,
> > >  that's NOT a debugger tuning decision, but a target-based decision.
> > >  (example: .debug_aranges section)
> > >
> > > Let me know if this all seems reasonable, and especially if you have
> > > a good idea where to keep the guidelines.
> > > Thanks,
> > > --paulr
> > >
> > >
> > > _______________________________________________
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> > > lldb-dev at cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>
>
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