[cfe-users] Should clang++ -g produce debugging info for all types?

[Bob Eastbrook via cfe-users]

On Fedora 30, "clang++ -g main.cc" does not emit debugging information
for types such as std::string.  I can only get complete debugging
information by including "-fno-limit-debug-info".

On Ubuntu 19.04, "clang++ -g" emits debugging info for std::string as expected.

Which behavior of "-g" is correct?  Can anyone explain why each
platform behaves differently?
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Re: [cfe-users] Should clang++ -g produce debugging info for all types?

[Bob Eastbrook via cfe-users]

On Thu, Jul 25, 2019 at 8:35 PM David Blaikie  wrote:

> No, it shouldn't - clang attempts to avoid emitting duplicate debug info 
> across the program (it assumes you built the whole program and all libraries 
> with debug info), gcc assumes the same thing though in slightly 
> different/fewer ways.
>
> The solution is to install the -dbg build of your libstdc++ package (assuming 
> you're using libstdc++), it will include debug info for the standard library, 
> including std::string.

Thanks for the pointer.  Based on what you told me, I was able to dig
deeper and found these relevant links:

https://bugs.llvm.org/show_bug.cgi?id=24202#c1
https://stackoverflow.com/questions/41745527/cannot-view-stdstring-when-compiled-with-clang

I installed the debug version of libstd++ on my Fedora system with
"dnf debuginfo-install libstdc++" and I now see a debug version of
libstdc++:

$ file /usr/lib/debug/lib64/libstdc++.so.6.0.26-9.1.1-1.fc30.x86_64.debug
/usr/lib/debug/lib64/libstdc++.so.6.0.26-9.1.1-1.fc30.x86_64.debug:
ELF 64-bit LSB shared object, x86-64, version 1 (GNU/Linux),
dynamically linked,
BuildID[sha1]=3b2e1aaafd0cb7e1ebd75627d4cde2504c927159, with
debug_info, not stripped

I don't have things working though.  I still don't see std::string
info when debugging.  My executable is linked against the non-debug
version:

$ ldd a.out
linux-vdso.so.1 (0x7ffec03fe000)
libstdc++.so.6 => /lib64/libstdc++.so.6 (0x7f2fccd31000)
libm.so.6 => /lib64/libm.so.6 (0x7f2fccbeb000)
libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x7f2fccbd1000)
libc.so.6 => /lib64/libc.so.6 (0x7f2fcca0b000)
/lib64/ld-linux-x86-64.so.2 (0x7f2fccf45000)

Is that my problem?  Or does LLDB somehow know to use the version in
/usr/lib/debug/lib64?

I'm also puzzled about why the debug version was put in /usr/lib and
not /usr/lib64.
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Re: [cfe-users] Should clang++ -g produce debugging info for all types?

[Bob Eastbrook via cfe-users]

On Mon, Aug 5, 2019 at 1:36 PM David Blaikie  wrote:

> Does it work with gdb?

It works with gdb.  More info:

g++ & gdb -- works, even without debuginfo for libstdc++
clang++ & gdb -- works
g++ & lldb -- works
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?

> 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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