<div dir="ltr"><div>I don't understand how extra vtable ref DIE will help in case on non-polymorphic classes. If you remove virtual destructor from example, vtable won't be generated for class, but DWARF will still have incorrect ambiguous names for types.</div><div>It will become a problem when you need to use debuginfo as a C++ runtime reflection (I've already seen this in a couple of projects). Or when you need to go back from LLVM IR to Clang AST (I've already encountered this problem).</div><div><br></div><div>I wonder if abi::__cxa_demangle guarantees unambigous names? If so, then I can just replace current incorrect names that Clang generates, with names from demangler. In this case I don't even need to patch gdb, it will work as is.</div><div><br></div>-Roman</div><div class="gmail_extra"><br><div class="gmail_quote">2018-03-05 10:46 GMT-08:00 Daniel Berlin <span dir="ltr"><<a href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><span class=""><br><br><div class="gmail_quote"><div dir="ltr">On Mon, Mar 5, 2018, 9:26 AM David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div dir="ltr">On Mon, Mar 5, 2018 at 9:09 AM Daniel Berlin <<a href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Mon, Mar 5, 2018 at 8:37 AM, David Blaikie <span dir="ltr"><<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><br><br><div class="gmail_quote"><span><div dir="ltr">On Sat, Mar 3, 2018 at 8:20 PM Daniel Berlin via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Fri, Mar 2, 2018 at 3:58 PM, Roman Popov via llvm-dev <span dir="ltr"><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr">Hi all,<div><br></div><div>As you may know modern C++ debuggers (GDB and LLDB) support dynamic type identification for polymorphic objects, by utilizing C++ RTTI. </div><div>Unfortunately this feature does not work with Clang and GDB >= 7.x . The last compiler that worked well was G++ 6.x</div><div><br></div><div>I've asked about this issue both on GDB and LLDB maillists. Unfortunately it's hard or impossible to fix it on debugger side.</div></div></blockquote><div><br></div></div></div></div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>Errr, i posited a solution on the gdb mailing list that i haven't seen shot down so far, that doesn't require linkage names, it only requires one new attribute that is a DW_FORM_ref, and very cheap.</div></div></div></div></blockquote></span><div><br>FWIW, for C++ at least, neither Clang nor GCC (6.3) produce any DWARF to describe the vtable itself (they describe the vtable pointer inside the struct, but not the constant vtable array) - so it'll be a bit more than one attribute, but the bytes describe the vtable (as a global variable? Do we give it a name? (if so, we're back to paying that cost)) first, then to add the reference from that to the type.<br></div></div></div></blockquote><div><br></div></div></div></div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>Right, they produce a named symbol but not debug info.</div><div><br></div><div>The only thing you need is a single DIE for that symbol, with a single ref.</div></div></div></div></blockquote></div></div><div dir="ltr"><div class="gmail_quote"><div><br>When you say "a single DIE" what attributes are you picturing that DIE having? If it has a single attribute, a ref_addr to the type, that doesn't seem to provide anything useful. Presumably this DIE would need a DW_AT_location with the address of the vtable (with a relocation to resolve that address, etc).<br></div></div></div></blockquote></div><div><br></div></span><div>Location and concrete type it belongs to. That's the minimum you should need here.</div><div>You don't need the name, though it doesn't hurt.</div><div><br></div><div class="gmail_quote"><span class=""><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div><br>No name? No other identifying features? I don't think we've ever really produced DIEs like that, though it sounds OK to me.<br> </div></div></div><div dir="ltr"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><br></div><div>(IE they just need to be able to say "find me the DIE for this address range", have it get to the vtable DIE, and get to the concrete type die)</div></div></div></div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div><br>& I'm not sure what Apple would do or anyone else that has libraries without debug info shipped & users have to debug them (this is what broke -fno-standalone-debug for Apple - their driver API which ships without debug info of its own, has strong vtables in it).<br></div></div></div></blockquote><div><br></div></div></div></div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>I'm confused.</div><div>This already seems to have has the same issue?<br>Just because it uses one linker symbol, it still requires full debug info to print the type anyway. </div></div></div></div></blockquote><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>So if it's gone, nothing changes. </div></div></div></div></blockquote></div></div><div dir="ltr"><div class="gmail_quote"><div><br>Sorry, I don't quite understand your comment here - could you explain it in more detail - the steps/issues you're seeing?<br></div></div></div></blockquote><div><br></div></span><div>I think we are starting from different positions here, so let me add a few pieces of data and see how it helps.</div><div><br></div><div>Let's assume the below is true and it won't work on OSX as described (i'm certainly in no place to disagree).</div><div><br></div><div>Some data points:</div><div><br></div><div>1. LLDB works just fine on Darwin (it appears to do the same thing we did in gdb, staring at source/Plugins/<wbr>LanguageRuntime/CPlusPlus/<wbr>ItaniumABI/<wbr>ItaniumABILanguageRuntime.cpp)</div><div><br></div><div>2. GDB does not work on Darwin at all for any real debugging right now (You can't debug llvm with it, for example). There are barely working versions here and there. The startup time to debug an "opt" binary from llvm is well over 2 minutes alone to get to a prompt just from typing "gdb bin/opt". It requires 4 gigs of ram. It usually fails to print most symbols/types/crashes calling functions, blah blah blah.</div><div>You can't even quit most of the time without hitting an assert.<br><div>(gdb) q</div><div>thread.c:93: internal-error: struct thread_info *inferior_thread(): Assertion `tp' failed.</div><div>A problem internal to GDB has been detected,</div><div>further debugging may prove unreliable.</div><div>Quit this debugging session? (y or n) y</div></div><div><br></div><div><br></div><div><br></div><div>3. On every platform, GDB will have to continue to use what it does now as a fallback anyway, as all existing binaries will not be rebuilt.</div><div>4. Ditto LLDB</div><div><br></div><div>So for GDB, it doesn't really matter whether it breaks OSX, to start. Even if it did, it will still work as well or as not well as it has in the past :)</div><div><br></div><div>LLDB works, and should work as well as it did with or without this as well.</div><div><br></div><div>Given all that: No matter what we do, LLDB and GDB will continue to work exactly as well or as broken as they have before on OSX. Nothing will change.</div><div><br></div><div>So i wouldn't call it broken, i'd call it, at worst, inapplicable to certain situations on OSX, and triggering a fallback :)</div><span class=""><div><br></div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div><br>I'll try to do the same:<br>Currently the DWARF type information (the actual DW_TAG_class_type DIE with the full definition of the class - its members, etc) on OSX goes everywhere the type is used (rather than only in the object files where the vtable is defined) to ensure that types defined in objects built without debug info, but used in objects built with debug info can still be debugged. (whereas on other platforms, like Linux, the assumption is made that the whole program is built with debug info - OSX is different because it has these system libraries for drivers that break this convention (& because LLDB can't handle this situation) - so, because the system itself breaks the assumption, the default is to turn off the assumption)<br><br>I assumed your proposal would only add this debug info to describe the vtable constant where the vtable is defined. Which would break OSX.<br><br>If the idea would be to, in OSX (& other -fstandalone-debug situations/platforms/users) would be to include this vtable DIE even where the vtable is not defined - that adds a bit more debug info & also it means debug info describing the declaration of a variable, also something we haven't really done in LLVM before - again, technically possible, but a nuance I'd call out/want to be aware of/think about/talk about (hence this conversation), etc.<br> </div></div></div><div dir="ltr"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div><br>I can go into more detail there - but there are certainly some annoying edge cases/questions I have here :/</div></div></div></blockquote><div><br></div></div></div></div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>Constructive alternative?<br></div></div></div></div></blockquote></div></div><div dir="ltr"><div class="gmail_quote"><div><br>Not sure - not saying what your proposing isn't workable - but I do want to understand the practical/implementation details a bit to see how it plays out - hence the conversation above.<br></div></div></div></blockquote><div><br></div></span><div>FWIW, i don't have a lot of time/energy to push this, so i'm pretty much going to bow out at this point and leave folks to their own devices. I just wanted to point out there are other solutions that would likely work a lot better over time.</div><span class=""><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div> </div></div></div><div dir="ltr"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>Right now, relying on *more* names, besides being huge in a lot of binaries, relies on the demangler producing certain text (which is not guaranteed)<br></div><div>That text has to exactly match the text of some other symbol (which is not guaranteed).</div></div></div></div></blockquote></div></div><div dir="ltr"><div class="gmail_quote"><div><br>*nod* I agree that the name matching based on demangling is a bad idea.<br> </div></div></div><div dir="ltr"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>That 10 second delay you get sometimes with going to print a C++ symbol in a large binary?<br></div><div><br></div><div>That's this lookup.</div><div><br></div><div>So right now it:<br>1. Uses a ton of memory</div><div>2. Uses a ton of time</div><div>3. Doesn't work all the time (depends on demanglers, and there are very weird edge cases here).</div><div><br></div><div>Adding linkage names will not change any of these, whereas adding a DWARF extension fixes all three, forever.</div></div></div></div></blockquote></div></div><div dir="ltr"><div class="gmail_quote"><div><br>Not sure I follow this - debuggers do lots of name lookups, I would've thought linkage name<>linkage name lookup could be somewhat practical (without all the fuzzy matching logic).<br></div></div></div></blockquote><div><br></div></span><div>You'd think it would be optimized for this, but for GDB, it will now pull in every symbol table looking for the name, until it finds it. It does not, for example, build a global index of names so it knows what CU to go read from or anything smart like that.</div><div>(it's a little more nuanced than this, but in practice, not) </div><span class=""><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div> </div></div></div><div dir="ltr"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>I don't even care about the details of the extension, my overriding constraint is "please don't extend this hack further given the above".<br></div></div></div></div></blockquote></div></div><div dir="ltr"><div class="gmail_quote"><div><br>Mangled to demangled name matching seems like a hack - matching the mangled names doesn't seem like such a hack to me - but, yeah, I'm totally open to an address based solution as you're suggesting, just trying to figure out the details/issues.<br></div></div></div></blockquote><div><br></div></span><div>At the time, the mangled name was not available anywhere. </div><div>It looks like name() is supposed to now return the mangled name in the itanium ABI.</div><div>So theoretically, you could just change GDB to call the name function(), look that up in the minimal symbol tables (name->address mappings, without debug info), and go to the full symbol table info for that address. This avoids needing the DW_AT_name in the debuginfo to match, only the name in the symbol table.</div><div> </div><div>This will break if you use -fno-rtti, whereas the vtable way (either existing or proposed) would still work.</div><div><br></div><div>G++ actually *had* linkage names for types for a long time in the debug info, and deliberately removed them due to space usage.</div><span class=""><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div><br>Have you got a link/steps to a sample/way to get GCC to produce this sort of debug info? (at least with 6.3 using C++ I don't see any debug info like this describing a vtable)<br></div></div></div></blockquote><div><br></div></span><div>Yeah, nothing does it yet.</div><div>Bug tom tromey, who did it for Rust, not C++</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div><br>- Dave<br> </div></div></div>
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