<div dir="ltr">Does the patch do all of this?<br><br>-eric<br></div><br><div class="gmail_quote">On Wed, May 6, 2015 at 1:18 PM Robinson, Paul <<a href="mailto:Paul_Robinson@playstation.sony.com">Paul_Robinson@playstation.sony.com</a>> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div lang="EN-US" link="blue" vlink="purple">
<div>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">I just skimmed through the thread again, and I *think* all the main questions have been answered…<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">It feels like the consensus is "reluctant agreement," with the specific design points being:<u></u><u></u></span></p>
<p><u></u><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><span>-<span style="font:7.0pt "Times New Roman"">
</span></span></span><u></u><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">a "debugger tuning" option would have some sort of target-based default<u></u><u></u></span></p>
<p><u></u><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><span>-<span style="font:7.0pt "Times New Roman"">
</span></span></span><u></u><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">the "debugger tuning" option would unpack into defaults for individual feature flags<u></u><u></u></span></p>
<p><u></u><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><span>-<span style="font:7.0pt "Times New Roman"">
</span></span></span><u></u><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">emitting actual DWARF would test the feature flags not the tuning option<u></u><u></u></span></p>
<p><u></u><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><span>-<span style="font:7.0pt "Times New Roman"">
</span></span></span><u></u><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">any command-line options for feature flags would override the tuning-based defaults<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">If I missed anything, let me know, otherwise I'll go back go pinging the patch.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Thanks,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">--paulr<u></u><u></u></span></p>
<p class="MsoNormal"><a name="msg-f:1500453035452317154__MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></a></p>
<div style="border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt">
<div>
<div style="border:none;border-top:solid #b5c4df 1.0pt;padding:3.0pt 0in 0in 0in">
<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""> <a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">llvmdev-bounces@cs.uiuc.edu</a> [mailto:<a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">llvmdev-bounces@cs.uiuc.edu</a>]
<b>On Behalf Of </b>David Blaikie<br>
<b>Sent:</b> Tuesday, May 05, 2015 8:21 PM<br>
<b>To:</b> Adrian Prantl<br>
<b>Cc:</b> <a href="mailto:lldb-dev@cs.uiuc.edu" target="_blank">lldb-dev@cs.uiuc.edu</a>; Greg Clayton; <a href="mailto:cfe-dev@cs.uiuc.edu" target="_blank">cfe-dev@cs.uiuc.edu</a> Developers (<a href="mailto:cfe-dev@cs.uiuc.edu" target="_blank">cfe-dev@cs.uiuc.edu</a>); LLVM Developers Mailing List (<a href="mailto:llvmdev@cs.uiuc.edu" target="_blank">llvmdev@cs.uiuc.edu</a>)<br>
<b>Subject:</b> Re: [LLVMdev] [cfe-dev] [lldb-dev] What does "debugger tuning" mean?<u></u><u></u></span></p>
</div>
</div></div></div></div><div lang="EN-US" link="blue" vlink="purple"><div><div style="border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt">
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<p class="MsoNormal">On Tue, May 5, 2015 at 8:16 PM, Adrian Prantl <<a href="mailto:aprantl@apple.com" target="_blank">aprantl@apple.com</a>> wrote:<u></u><u></u></p>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal">On May 5, 2015, at 8:12 PM, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>> wrote:<u></u><u></u></p>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<p class="MsoNormal"><br>
<br>
<u></u><u></u></p>
<div>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"LucidaGrande","serif"">On Tue, May 5, 2015 at 4:04 PM, Adrian Prantl <<a href="mailto:aprantl@apple.com" target="_blank">aprantl@apple.com</a>> wrote:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"LucidaGrande","serif""><br>
> On May 1, 2015, at 2:18 PM, Greg Clayton <<a href="mailto:gclayton@apple.com" target="_blank">gclayton@apple.com</a>> wrote:<br>
><br>
><br>
>> On 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:<br>
>><br>
>>> 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.<br>
><br>
> by default for darwin, it doesn't do this. For others you must specify -fno-limit-debug-info or some flag like that.<br>
<br>
I think the option is -f(no-)standalone-debug-info<u></u><u></u></span></p>
<div>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"LucidaGrande","serif""><br>
-fno-limit-debug-info == -fstandalone-debug<br>
(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)<u></u><u></u></span></p>
</div>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
</div>
<div>
<p class="MsoNormal"><br>
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:<br>
<br>
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))<br>
2) vtable (if a type is dynamic, only emit its definition where the vtable is emitted)<br>
3) explicit template instantiation (if a type has an explicit template instantiation declaration, only emit the definition where the explicit template instantiation definition is)<br>
<br>
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)<br>
<u></u><u></u></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<div>
<div>
<div>
<div>
<p class="MsoNormal"><br>
<br>
<u></u><u></u></p>
<div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"LucidaGrande","serif""> <u></u><u></u></span></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"LucidaGrande","serif"">which only emits full definitions of classes in the object file that holds and object’s vtable.<br>
<span style="color:#888888"><br>
-- adrian</span><u></u><u></u></span></p>
<div>
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:9.0pt;font-family:"LucidaGrande","serif"">><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<br>
>><br>
>>><br>
>>> Greg<br>
>>><br>
>>>> On May 1, 2015, at 1:06 PM, Robinson, Paul<br>
>>> <<a href="mailto:Paul_Robinson@playstation.sony.com" target="_blank">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" target="_blank">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>
><br>
> _______________________________________________<br>
> LLVM Developers mailing list<br>
> <a href="mailto:LLVMdev@cs.uiuc.edu" target="_blank">LLVMdev@cs.uiuc.edu</a> <a href="http://llvm.cs.uiuc.edu/" target="_blank">http://llvm.cs.uiuc.edu</a><br>
> <a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a><br>
<br>
<u></u><u></u></span></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"LucidaGrande","serif"">_______________________________________________<br>
cfe-dev mailing list<br>
<a href="mailto:cfe-dev@cs.uiuc.edu" target="_blank">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><u></u><u></u></span></p>
</div>
</div>
</blockquote>
</div>
</div>
</div>
</div>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
</blockquote>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
</div>
</div></div></div>
_______________________________________________<br>
lldb-dev mailing list<br>
<a href="mailto:lldb-dev@cs.uiuc.edu" target="_blank">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>
</blockquote></div>