On Fri, Aug 9, 2019 at 6:03 PM Bob Eastbrook <baconeater...@gmail.com> wrote:
> On Mon, Aug 5, 2019 at 1:36 PM David Blaikie <dblai...@gmail.com> wrote: > > > Does it work with gdb? > > It works with gdb. More info: > > g++ & gdb -- works, even without debuginfo for libstdc++ > ^ for std::string I'd expect this. For std::fstream, I expect you'd see the same problem/failure (gcc and clang both rely on fstream being homed in the standard library's debug info (because of vtables/key functions), but clang takes it a step further and does this for std::string too (because of template explicit instantiation decl/def)) > clang++ & gdb -- works > g++ & lldb -- works > ^ again, I expect that would fail with std::fstream, demonstrating that the same problem exists for both clang and GCC, just clang takes it a bit further - but it's not fundamentally different between the two. > clang++ & lldb -- fails > > I now notice these warnings in lldb after installing the debuginfo > packages: > > warning: (x86_64) /lib64/libstdc++.so.6 unsupported DW_FORM values: > 0x1f20 0x1f21 > warning: (x86_64) /lib64/libgcc_s.so.1 unsupported DW_FORM values: 0x1f20 > 0x1f21 > > I searched the web for details on that error, but came up empty. > Could it be that the debuginfo packages are just not compatible with > lldb? > Ooh, yeah, that's possible/likely. Unknown forms can't be ignored because forms dictate the encoding, so without recognizing the form, the DWARF can't be parsed (unknown /attributes/ using known forms are fine - because they can be parsed and the values ignored). Yeah, looks like 1f20 (DW_FORM_GNU_ref_alt) and 1f21 (DW_FORM_GNU_strp_alt) are results of the 'dwz' tool that compacts DWARF debug info. Not sure if it'd be enough to teach lldb how to parse but ignore any attribute using these forms - I would guess not, I suspect that would result in holes in the debug info where these forms are used. So you'd probably have to modify/fix/patch lldb to actually be able to understand these forms - which might not be too hard. Hmm, nope, probably fairly involved, since it means reading a new/different file (judging by this patch and the link to the DWARF standards proposal in it: http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150518/277186.html ). Summary: Yeah, short of making some relatively significant changes to lldb, I think you'll probably just have to stick with larger debug info by using -fstandalone-debug, since the debug info provided by your distribution uses GNU extensions that lldb doesn't currently support. :/ > > > I'd try gdb + gcc + binutils ld (or gold) + libstdc++ (use std::fstream > as an example of something that gcc will home to the libstdc++ debug info - > dwarfdump your executable and you'll see it doesn't contain the definition > of basic_fstream, but verify the debugger can still render the full > definition). If that works, swap out various parts of that & see where it > falls apart. > > Good strategy. I don't quite have the hang of dwarfdump yet though, > but I'll continue to investigate. >
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