[llvm-dev] Pardon the newbie question

Thu Apr 15 16:40:19 PDT 2021

On Thu, Apr 15, 2021 at 4:22 PM pawel k. <pawel.kunio at gmail.com> wrote:
>
> Ok for gdb, load time was major pain.

Ah, for large binaries you probably want to compile with
-ggnu-pubnames -Wl,--gdb-index (you must compile with this for Split
DWARF - it relies on the index) that takes, for instance, gdb startup
time debugging clang from 3 minutes to about 3 seconds. (lldb does a
better job lazy loading DWARF - so faster startup time even without an
index (& it can't use the gdb index anyway) trading off some
operations later will be slower as things are lazy loaded later on)

> Type casting was patchy some types missing etc,

Hard to say what's at work there - happy to speculate/help if you have
a specific/isolated example.

> some var tracking especially under optimizations was tricky for example having to cast registers because info this var is stored in this register was missing

Yeah, that's less about the workflow/technology, but more as we like
to say, the "quality of implementation" - keeping track of variables
under optimization is tricky, and different compilers do it
well/poorly - DWARF certainly offers a good variety of ways to express
these things (and gaining more as DWARF improves - it's certainly not
"complete", but I don't think it ever can be - there'll always be more
cunning optimizations and cunning ways to try to recover the value of
user variables under those optimizations).

> hmm also one mire thing that could minimize debug info size could be shared types section with removing local copies per compilation unit.

Yep - that can already be done with DWARF type units
(-fdebug-types-section), but also various strategies to reduce
emitting the types in the first place (hopefully we'll be enabling the
constructor homing strategy by default soon which should reduce
duplicate type information significantly)

> Also global vars etc could be stored in shared segment. Also we could think of somehow indexing the db.

Yeah, as mentioned above, gdb (& lld and gold) support the gdb_index
format and that can be generated at link time with -Wl,--gdb-index
(objects need to be compiled with -ggnu-pubnames (that comes by
default if you specify -gsplit-dwarf I think... ) for this to work).
And there is a DWARFv5 debug_names section, but it's not fully
supported yet (we don't have any linker that supports merging
.debug_names from object files into a unified .debug_names in the
linked binary - nor any debuggers that can consume the .debug_names
index).

> My other idea is indeed distributing stripped binaries and devels keeping debug info dbs snapshots locally or via debug servers. If we dont wanna debug servers, stack traces etc should contain stack dumps maybe or some in the middle solution with lineinfo only possibly hashed etc.
>
> Best regards,
> Pawel Kunio
>
> pt., 16.04.2021, 01:06 użytkownik David Blaikie <dblaikie at gmail.com> napisał:
>>
>> On Thu, Apr 15, 2021 at 2:43 PM pawel k. via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >
>> > Hiya,
>> >
>> > Enormous thanks for all suggests and extensive too. Testing splitdwarf asap.
>> >
>> > Makes me think what i thought previously. We need dwarf go pdbish way.
>>
>> Split DWARF provides something like this, though with some different
>> tradeoffs - for instance Split DWARF is easier to use in a distributed
>> build (since the compiler doesn't have to communicate with a separate
>> server while it's compiling).
>>
>> > Separate dblike possibly adressable by url.
>>
>> Symbol servers are another problem - even using classic DWARF you can
>> then strip the main binary, keep the unstripped binary and put that
>> unstripped binary on some kind of symbol server system (I'm not sure
>> if gdb supports something like that natively, but could be done
>> without DWARF changes).
>>
>> > I was hacking pdb techno before it got sexy. I know a bit how it is organized and could try to help with architecting or designing such solution. Loved dbinfo on vstudio. Gdbish not so much.
>>
>> What sort of problems have you had with gdb or the DWARF debugging
>> workflow more generally?
>>
>> > And i was slaving on my corpo cotton plantation passively using gdb for about a decade.
>>
>> (analogies to slavery aren't suitable for this community)
>>
>> - Dave
>>
>> >
>> > If catches anyones focus, lets discuss the solution.
>> >
>> > Best regards,
>> > Pawel Kunio
>> >
>> > czw., 15.04.2021, 23:37 użytkownik Min-Yih Hsu <minyihh at uci.edu> napisał:
>> >>
>> >> You can use `LLVM_USE_LINKER=lld` CMake variable to adopt LLD (to build LLVM). And yes, LLD takes less memory and runs faster. Here are some other tips to save memory:
>> >> 1. You can use `LLVM_PARALLEL_LINK_JOBS=N` (also a cmake variable) to limit the amount of parallel linker jobs to save some memory.
>> >>
>> >> 2. Build libraries as shared library via `BUILD_SHARED_LIBS=ON` CMake variable can dramatically speed up the linking time and save you some memory.
>> >>
>> >> 3. Since you’re building a Debug build (and you’re building on Linux), `LLVM_USE_SPLIT_DWARF` can dramatically reduce the size of debug info, which, to some extend, also save you some memory during link time.
>> >>
>> >> Best,
>> >> -Min
>> >> > On Apr 15, 2021, at 1:05 PM, pawel k. via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> >> >
>> >> > Hello,
>> >> > Im trying to build trunk clang in debug version on oldish ubuntu with low mem. Linking lli takes ages and fails on low mem. Is there a chance building would succeed if i used lld instead of ld? If so is there an option either to force lld or whole clang toolchain use in cmake instead of default gcc (both gcc and clang are avail on system)? Otherwise I think ill stick with release.
>> >> >
>> >> > Best regards,
>> >> > Pawel Kunio
>> >> > _______________________________________________
>> >> > LLVM Developers mailing list
>> >> > llvm-dev at lists.llvm.org
>> >> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >>
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