> 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 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> 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 > > > > > > _______________________________________________ > > lldb-dev mailing list > > lldb-dev@cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev _______________________________________________ lldb-dev mailing list lldb-dev@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
