[llvm-dev] Pardon the newbie question

Thu Apr 15 16:57:20 PDT 2021

Is constructor homing this featurette where class di is emitted only for
compunits with reference to constructor? Id speculate therell be problems
with it as i understand the idea by now. Shared type table feels bit more
complex on impl side fir merging etc but feels stronger.

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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