[llvm-dev] Pardon the newbie question

Fri Apr 16 05:41:35 PDT 2021

Hello,
Splitdwarf worked nicely for me. Big thaks to all contributing and biy
there were many useful hints.

I think that would be useful. I can try to collect them all and brief each
and then somebody could apply the styling if need be or if just pure text
paragraph would suffice I can prepare some file like that. I especially
liked lineinfo+funcaddrs only option. Very useful for stacktraces and
asserts if not planning to use gdb.

Hope to help somehow.

Big thanks and best regards,
Pk

pt., 16.04.2021, 14:18 użytkownik kuter dinel <kuterdinel at gmail.com>
napisał:

> Thanks everyone for great advice, maybe we should mention some of this on
> the https://llvm.org/docs/GettingStarted.html page.
> Maybe under a subtitle like "Development on low-end devices" or something.
> This would be helpful for newcomers who are working with low-end devices.
>
> Also since it was not mentioned, ccache <https://ccache.dev/> can help
> with build times during development.
>
> pawel k. via llvm-dev <llvm-dev at lists.llvm.org>, 16 Nis 2021 Cum, 03:10
> tarihinde şunu yazdı:
>
>> Hello,
>> Not sure i have anything useful to say on discussion on ctorhoming but
>> should it be so ill try to chime in on discussion. On bugs i cant talk.
>> Didnt heavily test it or use it much at all but i could say how it may seem
>> when trying to compete with vc and how it may look from john does sw devel
>> side possibly not a superstar one.
>>
>> Best regards,
>> Pawel Kunio
>>
>> pt., 16.04.2021, 02:03 użytkownik David Blaikie <dblaikie at gmail.com>
>> napisał:
>>
>>> On Thu, Apr 15, 2021 at 4:57 PM pawel k. <pawel.kunio at gmail.com> wrote:
>>> >
>>> > Is constructor homing this featurette where class di is emitted only
>>> for compunits with reference to constructor?
>>>
>>> Yep - both Clang and GCC already do something equivalent for any type
>>> with virtual functions - the debug info for that type is emitted only
>>> where the vtable is emitted (so called "vtable homing") so it's not
>>> without precedent. Yeah, it means potentially missing any type that
>>> isn't constructed (types used only to hold a bunch of static members,
>>> for instance) - but perhaps such a type isn't needed anyway. This
>>> isn't done for types with trivial constructors - since they don't
>>> generate code, etc.
>>>
>>> >  Id speculate therell be problems with it as i understand the idea by
>>> now.
>>>
>>> Certainly open to any critiques/bugs/etc. It's currently accessible
>>> under -fuse-ctor-homing
>>> https://www.mail-archive.com/cfe-commits@lists.llvm.org/msg218269.html
>>>
>>> > Shared type table feels bit more complex on impl side fir merging etc
>>> but feels stronger.
>>>
>>> Tradeoffs to be sure - anything that's shared during compile time's
>>> not really usable in the build environment (massively distributed) I
>>> support, but can work for others (like MSVC does). C++ modules (both
>>> Clang Header Modules and C++20 standard modules) offer another way to
>>> "home" debug info - since modules are then a first class build product
>>> they can be used to home debug info quite reliably (template
>>> instantiations may be duplicated - since they may be instantiated in
>>> multiple unrelated places, but even then any module depending on a
>>> module with an insntantiation need not re-do that instantiation).
>>>
>>> - Dave
>>>
>>> >
>>> > Best regards,
>>> > Pawel Kunio
>>> >
>>> > pt., 16.04.2021, 01:40 użytkownik David Blaikie <dblaikie at gmail.com>
>>> napisał:
>>> >>
>>> >> 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
>>> >> >> >>
>>> >> >> > _______________________________________________
>>> >> >> > LLVM Developers mailing list
>>> >> >> > llvm-dev at lists.llvm.org
>>> >> >> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
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