[LLVMdev] [lldb-dev] [cfe-dev] What does "debugger tuning" mean?
Adrian Prantl
aprantl at apple.com
Wed May 6 13:20:38 PDT 2015
I don’t think there was a driver patch so far, was there?
-- adrian
> On May 6, 2015, at 1:19 PM, Eric Christopher <echristo at gmail.com> wrote:
>
> Does the patch do all of this?
>
> -eric
>
> On Wed, May 6, 2015 at 1:18 PM Robinson, Paul <Paul_Robinson at playstation.sony.com <mailto:Paul_Robinson at playstation.sony.com>> wrote:
> I just skimmed through the thread again, and I *think* all the main questions have been answered…
>
> It feels like the consensus is "reluctant agreement," with the specific design points being:
>
> - a "debugger tuning" option would have some sort of target-based default
>
> - the "debugger tuning" option would unpack into defaults for individual feature flags
>
> - emitting actual DWARF would test the feature flags not the tuning option
>
> - any command-line options for feature flags would override the tuning-based defaults
>
>
>
> If I missed anything, let me know, otherwise I'll go back go pinging the patch.
>
> Thanks,
>
> --paulr
>
> <>
> From: llvmdev-bounces at cs.uiuc.edu <mailto:llvmdev-bounces at cs.uiuc.edu> [mailto:llvmdev-bounces at cs.uiuc.edu <mailto:llvmdev-bounces at cs.uiuc.edu>] On Behalf Of David Blaikie
> Sent: Tuesday, May 05, 2015 8:21 PM
> To: Adrian Prantl
> Cc: lldb-dev at cs.uiuc.edu <mailto:lldb-dev at cs.uiuc.edu>; Greg Clayton; cfe-dev at cs.uiuc.edu <mailto:cfe-dev at cs.uiuc.edu> Developers (cfe-dev at cs.uiuc.edu <mailto:cfe-dev at cs.uiuc.edu>); LLVM Developers Mailing List (llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>)
> Subject: Re: [LLVMdev] [cfe-dev] [lldb-dev] What does "debugger tuning" mean?
>
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> On Tue, May 5, 2015 at 8:16 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
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>
> On May 5, 2015, at 8:12 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>
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> On Tue, May 5, 2015 at 4:04 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>
>
> > On May 1, 2015, at 2:18 PM, Greg Clayton <gclayton at apple.com <mailto:gclayton at apple.com>> wrote:
> >
> >
> >> On May 1, 2015, at 2:00 PM, Robinson, Paul <Paul_Robinson at playstation.sony.com <mailto: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.
> >
> > by default for darwin, it doesn't do this. For others you must specify -fno-limit-debug-info or some flag like that.
>
> I think the option is -f(no-)standalone-debug-info
>
>
> -fno-limit-debug-info == -fstandalone-debug
> (limit-debug-info was the old name & we had a long discussion and decided standalone-debug more aptly described what it should mean/how it should generalize)
>
>
>
> And if my memory serves correctly, what adds to the confusion is that -flimit-debug-info used to do more than just this particular optimization, but we decided that most of the other optimizations weren’t really helpful, so they were removed.
>
>
> Not quite - I refactored the existing optimizations once I figured out what they did & how it generalized, they are still controlled by the same (both) flags. There are 3 main optimizations:
>
> 1) requires complete type (if a type is referenced, use a declaration unless the type is required to be complete (eg: it was dereferenced somewhere, etc))
> 2) vtable (if a type is dynamic, only emit its definition where the vtable is emitted)
> 3) explicit template instantiation (if a type has an explicit template instantiation declaration, only emit the definition where the explicit template instantiation definition is)
>
> I really should write a blog post about all this. Seems to create endless confusion. (so far as I know, GCC only does (2), perhaps it does some other things that we don't do, but I haven't seen it)
>
>
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> which only emits full definitions of classes in the object file that holds and object’s vtable.
>
> -- adrian
>
> >
> >>> - 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 <mailto: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 <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 <http://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
> >>>>
> >>>>
> >>>> _______________________________________________
> >>>> lldb-dev mailing list
> >>>> lldb-dev at cs.uiuc.edu <mailto:lldb-dev at cs.uiuc.edu>
> >>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev <http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev>
> >>
> >
> >
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