llvm dev - Mar 2018 - Emiting linkage names for Types to Debuginfo (C++ RTTI support in GDB/LLDB)

>
> I wonder if  abi::__cxa_demangle guarantees unambigous names?
>>
>
> No, it does not.
>
Interesting. Can you give an example of type where it fails?
I'm currently working on hardware construction library for C++ (similar to
Chisel (which is written in Scala)). And since C++ has no standardized
reflection, I use DWARF as a source of reflection metadata. And in case of
G++ 6.3, which seem to emit same name names as abi::__cxa_demangle, it has
never failed so far in my case. And I have very diverse inputs.
In fact I was working on it for about a year, and I was thinking that it
how it supposed to work. Only after I upgraded to g++ 7 I've found out that
both modern g++ and clang do not emit unambiguous debuginfo.

2. The failure that was cited on the gdb mailing list only occurs
on
> polymorphic classes.   If you have it occurring on non-polymorphic classes,
> that seems like a very different problem, and probably related to the fact
> that GDB does not know how to assemble or parse C++ names properly in some
> cases.  Otherwise, this would occur on literally every class you saw in
> GDB, and that's definitely not the case:)

That is correct, these two problems are different. However they can be
solved together.

Consider for example if we had only standardized mangled names in debuginfo:

1) GDB can demangle them and pretty-print in interactive sessions
2) Since it is the same name in debuginfo and RTTI, no problems with
dynamic type identification in debugger
3) Since mangled names is unique for type, we can use it in any other
scenarios. For example I want to analyze state of process in debugger, and
then load it in Clang to do some source-to-source transformations. This is
what I've planned for my hardware construction library.

-Roman


2018-03-06 8:39 GMT-08:00 Daniel Berlin <dberlin at dberlin.org>:
>
>
> On Mon, Mar 5, 2018 at 11:55 PM, Roman Popov <ripopov at gmail.com>
wrote:
>
>> 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.
>>
> 1. Calling them incorrect is ... not right.  As Andrew quoted on the gdb
> mailing list, this is what DWARF specifies should happen, so they are
> correct by spec. If you believe the spec is wrong, file an issue on the
> DWARF web site and discuss it on the mailing list, and bring back the
> consensus of the committee as to what to do :)
>
> 2. The failure that was cited on the gdb mailing list only occurs on
> polymorphic classes.   If you have it occurring on non-polymorphic classes,
> that seems like a very different problem, and probably related to the fact
> that GDB does not know how to assemble or parse C++ names properly in some
> cases.  Otherwise, this would occur on literally every class you saw in
> GDB, and that's definitely not the case:)
>
> The only reason linkage names would fix that issue is because they provide
> an exact match to GDB's parsing failure.
> You should just fix GDB.
> (GDB already knows how to collect and print out multiple symbols in the
> case they have the same name, FWIW)
>
>
>
>> 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).
>>
>
> I don't understand these use cases well enough to help, but if you
think
> it's a serious issue, again, i'd take it up with the DWARF folks.
>
>
>> I wonder if  abi::__cxa_demangle guarantees unambigous names?
>>
>
> No, it does not.
>
>
>> 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.
>>
>> -Roman
>>
>> 2018-03-05 10:46 GMT-08:00 Daniel Berlin <dberlin at
dberlin.org>:
>>
>>>
>>>
>>> On Mon, Mar 5, 2018, 9:26 AM David Blaikie <dblaikie at
gmail.com> wrote:
>>>
>>>> On Mon, Mar 5, 2018 at 9:09 AM Daniel Berlin <dberlin at
dberlin.org>
>>>> wrote:
>>>>
>>>>> On Mon, Mar 5, 2018 at 8:37 AM, David Blaikie <dblaikie
at gmail.com>
>>>>> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Sat, Mar 3, 2018 at 8:20 PM Daniel Berlin via
llvm-dev <
>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>
>>>>>>> On Fri, Mar 2, 2018 at 3:58 PM, Roman Popov via
llvm-dev <
>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>
>>>>>>>> Hi all,
>>>>>>>>
>>>>>>>> As you may know modern C++ debuggers (GDB and
LLDB) support dynamic
>>>>>>>> type identification for polymorphic objects, by
utilizing C++ RTTI.
>>>>>>>> Unfortunately this feature does not work with
Clang and GDB >= 7.x
>>>>>>>> .  The last compiler that worked well was G++
6.x
>>>>>>>>
>>>>>>>> I've asked about this issue both on GDB and
LLDB maillists.
>>>>>>>> Unfortunately it's hard or impossible to
fix it on debugger side.
>>>>>>>>
>>>>>>>
>>>>>>> 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.
>>>>>>>
>>>>>>
>>>>>> 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.
>>>>>>
>>>>>
>>>>> Right, they produce a named symbol but not debug info.
>>>>>
>>>>> The only thing you need is a single DIE for that symbol,
with a single
>>>>> ref.
>>>>>
>>>>
>>>> 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).
>>>>
>>>
>>> Location and concrete type it belongs to.  That's the minimum
you should
>>> need here.
>>> You don't need the name, though it doesn't hurt.
>>>
>>>
>>>> No name? No other identifying features? I don't think
we've ever really
>>>> produced DIEs like that, though it sounds OK to me.
>>>>
>>>>
>>>>>
>>>>> (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)
>>>>>
>>>>>
>>>>>>
>>>>>> & 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).
>>>>>>
>>>>>
>>>>> I'm confused.
>>>>> This already seems to have  has the same issue?
>>>>> Just because it uses one linker symbol, it still requires
full debug
>>>>> info to print the type anyway.
>>>>>
>>>> So if it's gone, nothing changes.
>>>>>
>>>>
>>>> Sorry, I don't quite understand your comment here - could
you explain
>>>> it in more detail - the steps/issues you're seeing?
>>>>
>>>
>>> I think we are starting from different positions here, so let me
add a
>>> few pieces of data and see how it helps.
>>>
>>> Let's assume the below is true and it won't work on OSX as
described
>>> (i'm certainly in no place to disagree).
>>>
>>> Some data points:
>>>
>>> 1. LLDB works just fine on Darwin (it appears to do the same thing
we
>>> did in gdb, staring at source/Plugins/LanguageRunt
>>> ime/CPlusPlus/ItaniumABI/ItaniumABILanguageRuntime.cpp)
>>>
>>> 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.
>>> You can't even quit most of the time without hitting an assert.
>>> (gdb) q
>>> thread.c:93: internal-error: struct thread_info *inferior_thread():
>>> Assertion `tp' failed.
>>> A problem internal to GDB has been detected,
>>> further debugging may prove unreliable.
>>> Quit this debugging session? (y or n) y
>>>
>>>
>>>
>>> 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.
>>> 4. Ditto LLDB
>>>
>>> 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 :)
>>>
>>> LLDB works, and should work as well as it did with or without this
as
>>> well.
>>>
>>> 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.
>>>
>>> So i wouldn't call it broken, i'd call it, at worst,
inapplicable to
>>> certain situations on OSX, and triggering a fallback :)
>>>
>>>
>>>
>>>> I'll try to do the same:
>>>> 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)
>>>>
>>>> I assumed your proposal would only add this debug info to
describe the
>>>> vtable constant where the vtable is defined. Which would break
OSX.
>>>>
>>>> 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.
>>>>
>>>>
>>>>>
>>>>>> I can go into more detail there - but there are
certainly some
>>>>>> annoying edge cases/questions I have here :/
>>>>>>
>>>>>
>>>>> Constructive alternative?
>>>>>
>>>>
>>>> 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.
>>>>
>>>
>>> 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.
>>>
>>>
>>>>
>>>>
>>>>> 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)
>>>>> That text has to exactly match the text of some other
symbol (which is
>>>>> not guaranteed).
>>>>>
>>>>
>>>> *nod* I agree that the name matching based on demangling is a
bad idea.
>>>>
>>>>
>>>>> That 10 second delay you get sometimes with going to print
a C++
>>>>> symbol in a large binary?
>>>>>
>>>>> That's this lookup.
>>>>>
>>>>> So right now it:
>>>>> 1. Uses a ton of memory
>>>>> 2. Uses a ton of time
>>>>> 3. Doesn't work all the time (depends on demanglers,
and there are
>>>>> very weird edge cases here).
>>>>>
>>>>> Adding linkage names will not change any of these, whereas
adding a
>>>>> DWARF extension fixes all three, forever.
>>>>>
>>>>
>>>> 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).
>>>>
>>>
>>> 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.
>>> (it's a little more nuanced than this, but in practice, not)
>>>
>>>>
>>>>
>>>>> 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".
>>>>>
>>>>
>>>> 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.
>>>>
>>>
>>> At the time, the mangled name was not available anywhere.
>>> It looks like name() is supposed to now return the mangled name in
the
>>> itanium ABI.
>>> 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.
>>>
>>> This will break if you use -fno-rtti, whereas the vtable way
(either
>>> existing or proposed) would still work.
>>>
>>> G++ actually *had* linkage names for types for a long time in the
debug
>>> info, and deliberately removed them due to space usage.
>>>
>>>
>>>> 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)
>>>>
>>>
>>> Yeah, nothing does it yet.
>>> Bug tom tromey,  who did it for Rust, not C++
>>>
>>>
>>>> - Dave
>>>>
>>>>
>>>
>>
>
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On Tue, Mar 6, 2018 at 9:20 AM, Roman Popov <ripopov at gmail.com> wrote:
> I wonder if  abi::__cxa_demangle guarantees unambigous names?
>>>
>>
>> No, it does not.
>>
>
> Interesting. Can you give an example of type where it fails?
>
I can't construct one out of thin air, but i believe someone cited one to
you on the gdb mailing list.  It's entirely possible for the human readable
form of two symbols to be the same when the symbols are different.
I really just don't have the energy to copy the entire discussion on the
other mailing list here.

More to the point, the ABI literally does not guarantee it, and different
demanglers for the Itanium ABI (there are a bunch) do different things for
human readable names.
You cite below gcc vs gcc, which is different versions of the same
demangler.  There are a bunch of Itanium C++ ABI implementations, including
demanglers and compilers, and i'd strongly caution you to remember that all
the world is not clang and GNU.




> I'm currently working on hardware construction library for C++ (similar
to
> Chisel (which is written in Scala)). And since C++ has no standardized
> reflection, I use DWARF as a source of reflection metadata. And in case of
> G++ 6.3, which seem to emit same name names as abi::__cxa_demangle, it has
> never failed so far in my case. And I have very diverse inputs.
> In fact I was working on it for about a year, and I was thinking that it
> how it supposed to work. Only after I upgraded to g++ 7 I've found out
that
> both modern g++ and clang do not emit unambiguous debuginfo.
>
This seems to be a different question than i thought you asked.
If you are asking "where will the demangled name between what
abi::__cxa_demangle and what GCC outputs in the debug info differ",
it's
unlikely to differ if you use the same versions of both ;)

But it will in some cases. Some bugs, some not.
Mangled names are not a panacea.  I think you also wildly underestimate the
cost of demangling every symbol in a large binary, for example, which would
required for your suggestion, as well as the size of these symbols, etc.
It's enough that people wrote a fast demangler, for example.  That's
just
one issue.

As for the rest, you are taking a perspective that is pretty strongly
focused on your use cases, and currently, DWARF is pretty focused on the
other ones.
If you want to convince the committee/others that it should give up on the
part of the best practices i cited, go for it.

But you started by claiming this was necessary/important to fix the GDB
problems here, and it's simply not.  In fact, it would not fix most of them
without a serious change in the way these things operate, and at high cost.
Suggesting to change gdb, gcc, clang, to fit your non-debugger use case is
taking a very big hammer and saying "it can also pound these nails".
While true, that doesn't mean you should.

I'd strongly suggest, if you have concerns about the ability of DWARF to
handle your use cases without linkage names, that you go to the DWARF
mailing list and start a discussion about, rather than just proposing a
solution.

In my experience, the people there have thought a lot about all of these
use cases, and you may in fact find a solution that doesn't require doing
anything at all.
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On Tue, Mar 6, 2018 at 9:49 AM Daniel Berlin <dberlin at dberlin.org>
wrote:
> On Tue, Mar 6, 2018 at 9:20 AM, Roman Popov <ripopov at gmail.com>
wrote:
>
>> I wonder if  abi::__cxa_demangle guarantees unambigous names?
>>>>
>>>
>>> No, it does not.
>>>
>>
>> Interesting. Can you give an example of type where it fails?
>>
>
> I can't construct one out of thin air, but i believe someone cited one
to
> you on the gdb mailing list.  It's entirely possible for the human
readable
> form of two symbols to be the same when the symbols are different.
> I really just don't have the energy to copy the entire discussion on
the
> other mailing list here.
>
> More to the point, the ABI literally does not guarantee it, and different
> demanglers for the Itanium ABI (there are a bunch) do different things for
> human readable names.
> You cite below gcc vs gcc, which is different versions of the same
> demangler.  There are a bunch of Itanium C++ ABI implementations, including
> demanglers and compilers, and i'd strongly caution you to remember that
all
> the world is not clang and GNU.
>
>
>
>
>
>> I'm currently working on hardware construction library for C++
(similar
>> to Chisel (which is written in Scala)). And since C++ has no
standardized
>> reflection, I use DWARF as a source of reflection metadata. And in case
of
>> G++ 6.3, which seem to emit same name names as abi::__cxa_demangle, it
has
>> never failed so far in my case. And I have very diverse inputs.
>> In fact I was working on it for about a year, and I was thinking that
it
>> how it supposed to work. Only after I upgraded to g++ 7 I've found
out that
>> both modern g++ and clang do not emit unambiguous debuginfo.
>>
>
> This seems to be a different question than i thought you asked.
> If you are asking "where will the demangled name between what
> abi::__cxa_demangle and what GCC outputs in the debug info differ",
it's
> unlikely to differ if you use the same versions of both ;)
>
> But it will in some cases. Some bugs, some not.
> Mangled names are not a panacea.  I think you also wildly underestimate
> the cost of demangling every symbol in a large binary, for example, which
> would required for your suggestion, as well as the size of these symbols,
> etc. It's enough that people wrote a fast demangler, for example. 
That's
> just one issue.
>
> As for the rest, you are taking a perspective that is pretty strongly
> focused on your use cases, and currently, DWARF is pretty focused on the
> other ones.
> If you want to convince the committee/others that it should give up on the
> part of the best practices i cited, go for it.
>
> But you started by claiming this was necessary/important to fix the GDB
> problems here, and it's simply not.  In fact, it would not fix most of
them
> without a serious change in the way these things operate, and at high cost.
> Suggesting to change gdb, gcc, clang, to fit your non-debugger use case
> is  taking a very big hammer and saying "it can also pound these
nails".
> While true, that doesn't mean you should.
>
> I'd strongly suggest, if you have concerns about the ability of DWARF
to
> handle your use cases without linkage names,
>
I think the only reason Roman's discussing ways that would work without
linkage names is because you pretty firmly said that adding linkage names
was a bad idea (for the original issue of dynamic class identification).

Which, sure, it's an idea with some issues - but the alternative (vtable
DIEs with address/ref) isn't without complications too - not to dismiss it,
but to suggest having some real conversation about the tradeoffs seems
worthwhile.

& while the vtable DIE solution addresses the dynamic class identification
case, it doesn't cover the other use cases Roman has in mind - other use
cases that sound like they would be solved by having the linkage name of a
type provided in the DWARF.

> that you go to the DWARF mailing list and start a discussion about, rather
> than just proposing a solution.
>
> In my experience, the people there have thought a lot about all of these
> use cases, and you may in fact find a solution that doesn't require
doing
> anything at all.
>
*nod* fair, might be worth it for the broader set of issues Roman seems to
be dealing with (beyond the dynamic type identification issues that GDB
demonstrates).
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