[llvm-dev] Slow debugger starts of LLVM tools

[David Greene via llvm-dev]

I was under the impression that a generated gdb-index only helps when
loading the file ('file foo' in gdb).  Does it help execution startup
time as well?

I'll give it a try and see if it helps.  Thanks all!

                         -David

David Blaikie via llvm-dev <llvm-dev at lists.llvm.org> writes:

> Yep, as others have mentioned - using a linker-generated gdb-index is
> super helpful (10s for my machine to start debugging an unoptimized
> clang, set a breakpoint at llvm::StringRef::StringRef and run until
> that breakpoint is hit, as opposed to 1m26s without an index)
>
> That's also with/without split DWARF too, but I suspect most of the
> benefit is in the index there. Split DWARF might save you some linking
> time and /maybe/ some debugger performance.
>
> On Thu, Jan 10, 2019 at 1:16 PM Petr Penzin via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>
>         I've ran into the same issue before. I tend to see it as a
>     price we pay for static linking, since it brings all the symbols
>     debugger has to read at the start time. Quick check on launching a
>     tool (fort) in GDB (I usually use it) and LLDB shows that LLDB
>     starts much faster, but takes time to set breakpoints and launch
>     the application, so I think they are roughly on the same page in
>     terms of delays.
>
>         -Petr
>     
>     
>     On 1/9/19 6:17 PM, Zachary Turner via llvm-dev wrote:
>     
>     This is admittedly a longshot, but Can you check whether you
>         experience unusually long run times with LLDB? If there’s
>         something strange about tge binaries we generate, maybe lldb
>         will exhibit some strangeness too and we can more easily
>         profile it.
>         
>         
>         On Wed, Jan 9, 2019 at 2:48 PM Shoaib Meenai via llvm-dev
>         <llvm-dev at lists.llvm.org> wrote:
>         
>         I don't know about the issues with running being slow, but
>             using a GDB index greatly speeds up initial symbol
>             loading. Compile with `-ggnu-pubnames` and link with
>             `-Xlinker --gdb-index` and you should get significantly
>             faster symbol load times. (I'd be curious to see if it
>             helps with the issues with run being slow as well.)
>             
>             On 1/9/19, 2:07 PM, "llvm-dev on behalf of David Greene
>             via llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf
>             of llvm-dev at lists.llvm.org> wrote:
>             
>             
>             David Jones via llvm-dev <llvm-dev at lists.llvm.org> writes:
>             
>             > GDB likes to load all symbols from shared libraries up
>             front. And on
>             > x86_64 your main executable is really just another
>             shared library.
>             
>             Yes, but does gdb reload everything on each execution?
>             Every time I
>             execute "run" I see the same slow behavior. Loading the
>             symbols for
>             small tools like llvm-rc takes hardly any time at all
>             (i.e. "file
>             llvm-rc" is almost instantaneous). But executing it causes
>             gdb to chew
>             up CPU cycles for a while. Every time.
>             
>             -David
>             _______________________________________________
>             LLVM Developers mailing list
>             llvm-dev at lists.llvm.org
>             https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.llvm.
>             org_cgi-2Dbin_mailman_listinfo_llvm-2Ddev&d=DwIGaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=dqw6dQj2Nwimdo2WZ3P_
>             n0WRZBWrrtNACdQfXzCB0Xc&s=psETNIS-WHrFqc7U8d3tg6i1UGx4-CHNUo0FO-
>             FNQJg&e=
>             
>             
>             _______________________________________________
>             LLVM Developers mailing list
>             llvm-dev at lists.llvm.org
>             http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>         
>         _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>     _______________________________________________
>     LLVM Developers mailing list
>     llvm-dev at lists.llvm.org
>     http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev