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-boun...@cs.uiuc.edu [mailto:llvmdev-boun...@cs.uiuc.edu] On Behalf Of David Blaikie Sent: Tuesday, May 05, 2015 8:21 PM To: Adrian Prantl Cc: lldb-dev@cs.uiuc.edu; Greg Clayton; cfe-...@cs.uiuc.edu Developers (cfe-...@cs.uiuc.edu); LLVM Developers Mailing List (llvm...@cs.uiuc.edu) Subject: Re: [LLVMdev] [cfe-dev] [lldb-dev] What does "debugger tuning" mean? On Tue, May 5, 2015 at 8:16 PM, Adrian Prantl <apra...@apple.com<mailto:apra...@apple.com>> wrote: On May 5, 2015, at 8:12 PM, David Blaikie <dblai...@gmail.com<mailto:dblai...@gmail.com>> wrote: On Tue, May 5, 2015 at 4:04 PM, Adrian Prantl <apra...@apple.com<mailto:apra...@apple.com>> wrote: > On May 1, 2015, at 2:18 PM, Greg Clayton > <gclay...@apple.com<mailto:gclay...@apple.com>> wrote: > > >> On May 1, 2015, at 2:00 PM, Robinson, Paul >> <paul_robin...@playstation.sony.com<mailto:paul_robin...@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) 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_robin...@playstation.sony.com<mailto:paul_robin...@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<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@cs.uiuc.edu<mailto:lldb-dev@cs.uiuc.edu> >>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev >> > > > _______________________________________________ > LLVM Developers mailing list > llvm...@cs.uiuc.edu<mailto:llvm...@cs.uiuc.edu> > http://llvm.cs.uiuc.edu<http://llvm.cs.uiuc.edu/> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev _______________________________________________ cfe-dev mailing list cfe-...@cs.uiuc.edu<mailto:cfe-...@cs.uiuc.edu> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
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