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

Robinson, Paul Paul_Robinson at playstation.sony.com
Fri May 1 14:00:54 PDT 2015

> 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.

> - LLDB wants the .apple_XXX accelerator tables, GDB wants
> .debug_pubnames/.debug_pubtypes


> 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.

> 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
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