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<p class="MsoNormal" style="margin-left:.5in">the DWARF for that file (on non-darwin) will contain a declaration for 'base' and a definition for 'derived'.<br>
<br>
<span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Ah, okay I see it now. So to answer Greg's question, yes that seems like an LLDB tuning thing.<o:p></o:p></span></a></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> David Blaikie [mailto:dblaikie@gmail.com]
<br>
<b>Sent:</b> Friday, May 01, 2015 2:22 PM<br>
<b>To:</b> Robinson, Paul<br>
<b>Cc:</b> Greg Clayton; lldb-dev@cs.uiuc.edu; cfe-dev@cs.uiuc.edu Developers (cfe-dev@cs.uiuc.edu); LLVM Developers Mailing List (llvmdev@cs.uiuc.edu)<br>
<b>Subject:</b> Re: [cfe-dev] [lldb-dev] What does "debugger tuning" mean?<o:p></o:p></span></p>
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<p class="MsoNormal">On Fri, May 1, 2015 at 2:00 PM, Robinson, Paul <<a href="mailto:Paul_Robinson@playstation.sony.com" target="_blank">Paul_Robinson@playstation.sony.com</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">> A few more things that vote for debugger tuning:<br>
><br>
> - LLDB doesn't like to have DWARF that has a class A that inherits from<br>
> class B, but only a forward declaration of class B is provided.<br>
<br>
Hmm do we emit that kind of thing today? In a naïve test, I'm seeing<br>
the full description of class B.<o:p></o:p></p>
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<p class="MsoNormal"><br>
the trick is to make the base class's vtable out of line (by having a key function):<br>
<br>
struct base {<br>
virtual ~base();<br>
};<br>
<br>
struct derived : base {<br>
};<br>
<br>
derived d;<br>
<br>
the DWARF for that file (on non-darwin) will contain a declaration for 'base' and a definition for 'derived'.<br>
<br>
GCC does the same thing.<br>
<o:p></o:p></p>
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<p class="MsoNormal"><br>
> - LLDB wants the .apple_XXX accelerator tables, GDB wants<br>
> .debug_pubnames/.debug_pubtypes<br>
<br>
Agreed.<br>
<br>
> So it would be great to have a "-debugger" flag that could be specified<br>
><br>
> -debugger=lldb<br>
> -debugger=gdb<br>
><br>
> Not sure on the option name, but I do like the idea.<br>
<br>
We'll bikeshed the name later but yes, that's the plan.<br>
Thanks,<br>
--paulr<o:p></o:p></p>
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<p class="MsoNormal"><br>
><br>
> Greg<br>
><br>
> > On May 1, 2015, at 1:06 PM, Robinson, Paul<br>
> <<a href="mailto:Paul_Robinson@playstation.sony.com">Paul_Robinson@playstation.sony.com</a>> wrote:<br>
> ><br>
> > This is basically a reboot of the previous thread titled<br>
> > About the "debugger target"<br>
> > except that "target" was really too strong a term for what I had<br>
> intended<br>
> > to use this feature for. "Debugger tuning" is more like it. You don't<br>
> > need to have read the previous thread, I'll recap here.<br>
> ><br>
> > Fundamentally, Clang/LLVM uses DWARF as the specification for the<br>
> _format_<br>
> > of information provided by the compiler to a variety of "consumers,"<br>
> which<br>
> > primarily means debuggers (but not exclusively). [For a long time it<br>
> was<br>
> > the only format supported by LLVM. Lately, Microsoft debug info has<br>
> started<br>
> > appearing, but being a less widely used format, the issues that DWARF<br>
> runs<br>
> > into aren't a concern for that format. So "debugger tuning" is unlikely<br>
> > to be an issue for Microsoft debug info.]<br>
> ><br>
> > DWARF is a permissive standard, meaning that it does not rigidly require<br>
> > that source-language construct X must be described using the DWARF<br>
> > construct Y. Instead, DWARF says something more like, "If you have a<br>
> > source construct that means something like X, here's a mechanism Y that<br>
> > you could use to describe it." While this gives compilers a lot of nice<br>
> > flexibility, it does mean that there's a lot of wiggle room for how a<br>
> > compiler describes something and in how a debugger interprets that<br>
> > description. Compilers and debuggers therefore need to do a bit of<br>
> > negotiation in determining how the debug-info "contract" will work, when<br>
> > it comes to nitty-gritty details. DWARF itself (the standard, as well<br>
> > as the committee that owns the standard) refuses to get involved in this<br>
> > negotiation, referring to all that as "quality of implementation<br>
> issues."<br>
> ><br>
> > It is readily apparent that different debuggers have different ideas<br>
> > about certain DWARF features, for example whether they are useful or<br>
> > irrelevant, or whether a certain source construct should be described<br>
> > this way or that way. As these generally fall into the QOI realm, the<br>
> > DWARF spec itself is no help, and it comes down to a matter of opinion<br>
> > about whether "the debugger should just know this" or "the compiler<br>
> > really ought to just emit it that way."<br>
> ><br>
> > Clang/LLVM is in the position of being a compiler that wants to support<br>
> > several different debuggers, all of which have slightly different ideas<br>
> > about what they want from the DWARF info for a program. Our first line<br>
> > of defense of course is the DWARF standard itself, but as we've seen,<br>
> > that is not a universally definitive reference.<br>
> ><br>
> > LLVM already emits DWARF slightly differently for different *targets*;<br>
> > primarily Darwin, in a few cases PS4. But in at least some cases, the<br>
> > target is just a (somewhat unreliable) proxy for which *debugger* the<br>
> > compiler expects to be consuming the DWARF. The most instructive case<br>
> > is the exact DWARF expression used to describe the location of a thread-<br>
> > local variable. DWARF v3 defined an operator to find the base address<br>
> > of the thread-local storage area; however, GDB has never learned to<br>
> > recognize it. Therefore, for targets where we "know" GDB isn't used,<br>
> > we can emit the standard operator; for targets where GDB *might* be<br>
> > used, we need to emit the equivalent (non-standard) GNU operator.<br>
> ><br>
> > It would be semantically more meaningful to base decisions like this on<br>
> > whether we expected the debugger to be X or Y or Z. Therefore I've<br>
> > proposed (<a href="http://reviews.llvm.org/D8506" target="_blank">http://reviews.llvm.org/D8506</a>) a "debugger tuning" option that<br>
> > will make the reasoning behind these choices more obvious, and<br>
> ultimately<br>
> > give users a way to control the tuning themselves, when the platform's<br>
> > default isn't what they want. (I'll have a follow-up patch exposing the<br>
> > tuning option to the Clang driver.)<br>
> ><br>
> > So, what kinds of things should be based on the debugger tuning option?<br>
> > Are there still things that should be based on the target platform?<br>
> > Simplest to consider these questions together, because it is often clear<br>
> > which criterion is important if you consider (a) the same debugger run<br>
> > on different targets, versus (b) different debuggers running on the same<br>
> > target. Basically, if the same debugger on different targets wants to<br>
> > have something a certain way, that's probably a debugger-tuning thing.<br>
> > And if different debuggers on the same target doesn't mean you should<br>
> > change how the DWARF looks, that's likely a platform-specific thing.<br>
> ><br>
> > The most obvious example of a debugger-tuning consideration is the TLS<br>
> > operator mentioned above. That's something that GDB insists on having.<br>
> > (It turns out that the standard operator was defined in DWARF 3, so we<br>
> > also have to emit the GNU operator if we're producing DWARF 2. Tuning<br>
> > considerations don't trump what the standard says.)<br>
> ><br>
> > Another example would be .debug_pubnames and .debug_pubtypes sections.<br>
> > Currently these default to omitted for Darwin and PS4, but included<br>
> > everywhere else. My initial patch for "tuning" changes the PS4 platform<br>
> > criterion to the SCE debugger predicate; quite likely the "not Darwin"<br>
> > criterion ought to be "not LLDB" or in other words "on for GDB only."<br>
> > And having the code actually reflect the correct semantic purpose seems<br>
> > like an overall goodness.<br>
> ><br>
> > An example of a target-dependent feature might be the .debug_aranges<br>
> > section. As it happens, we don't emit this section by default, because<br>
> > apparently no debugger finds it useful, although there's a command-line<br>
> > option (-gdwarf-aranges) for it. But, for PS4 we do want to emit it,<br>
> > because we have non-debugger tools that find it useful. We haven't yet<br>
> > done the work to make that change on <a href="http://llvm.org" target="_blank">
llvm.org</a>, but it's on the list.<br>
> > I would conditionalize this on the target, not the debugger, because<br>
> > the debugger is not why we want to generate the section.<br>
> ><br>
> > Okay, so I've been pretty long-winded about all this, can I possibly<br>
> > codify it all into a reasonably succinct set of guidelines? (which<br>
> > ought to be committed to the repo somewhere, although whether it's as<br>
> > a lump of text in a docs webpage or a lump of commentary in some source<br>
> > file is not clear; opinions welcome.)<br>
> ><br>
> > o Emit standard DWARF if possible.<br>
> > o Omitting standard DWARF features that nobody uses is fine.<br>
> > (example: DW_AT_sibling)<br>
> > o Extensions are okay, but think about the circumstances where they<br>
> > would be useful (versus just wasting space). These are probably a<br>
> > debugger tuning decision, but might be a target-based decision.<br>
> > (example: DW_AT_APPLE_* attributes)<br>
> > o If some debugger can't tolerate some piece of standard DWARF, that's<br>
> > a missing feature or a bug in the debugger. Accommodating that in<br>
> > the compiler is a debugger tuning decision.<br>
> > (example: DW_OP_form_tls_address not understood by GDB)<br>
> > o If some debugger has no use for some piece of standard DWARF, and<br>
> > it saves space to omit it, that's a debugger tuning decision.<br>
> > (example: .debug_pubnames/.debug_pubtypes sections)<br>
> > o If a debugger wants things a certain way regardless of the target,<br>
> > that's probably a debugger tuning decision.<br>
> > o If "system" software on a target (other than the debugger) wants<br>
> > things a certain way regardless of which debugger you're using,<br>
> > that's NOT a debugger tuning decision, but a target-based decision.<br>
> > (example: .debug_aranges section)<br>
> ><br>
> > Let me know if this all seems reasonable, and especially if you have<br>
> > a good idea where to keep the guidelines.<br>
> > Thanks,<br>
> > --paulr<br>
> ><br>
> ><br>
> > _______________________________________________<br>
> > lldb-dev mailing list<br>
> > <a href="mailto:lldb-dev@cs.uiuc.edu">lldb-dev@cs.uiuc.edu</a><br>
> > <a href="http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev" target="_blank">
http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev</a><br>
<br>
<br>
_______________________________________________<o:p></o:p></p>
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<p class="MsoNormal">cfe-dev mailing list<br>
<a href="mailto:cfe-dev@cs.uiuc.edu">cfe-dev@cs.uiuc.edu</a><br>
<a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev</a><o:p></o:p></p>
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