llvm dev - Dec 2017 - [LLD] Slow callstacks in gdb

Martin Richtarsky <s at martinien.de> writes:
> Hi,
>
>> Is the program being built by gcc or clang?
> gcc 6, but I can try clang.
>
>> Are both using --gdb-index? Can you try lld trunk if so?
> No.
>
>> Is any of the programs you tested open source?
> No.
>
> Here is some more info. I enabled "set verbose on" in gdb. With
this
> setting, I'm getting DWARF errors in both versions when executing
"bt"
> (linked by gold and lld), but the lld version has significantly more
> errors. These only appear in the lld version:
>
> (gdb) bt
> Reading in symbols for abc.cpp....
> debug_line address at offset 0x575128 is 0 [in module abc.so]....
> debug_line address at offset 0x575141 is 0 [in module abc.so]....
> cannot get low and high bounds for subprogram DIE at 52844002...
> cannot get low and high bounds for subprogram DIE at 52844042...
> cannot get low and high bounds for subprogram DIE at 52844092...
> const value length mismatch for 'std::ios_constants::boolalpha',
got 4,
> expected 0
> debug info gives command-line macro definition with non-zero line 13:
__MACRO
> macro '__USE_ISOC95' is #undefined twice
> macro `__STDC_LIMIT_MACROS' redefined at...
> Member function "~basic_istream" (offset 28792309) is virtual but
the
> vtable offset is not specified...
>
>
>
> Maybe gdb needs to fall back to slower line number resolution because e.g.
> low and high bounds cannot be retrieved and debug_line_address is 0?
It is hard to know without a reproducible. I tried gdb on clang itself
build with both clang and gcc and linked with gold and lld. I could not
reproduce the slowdown, but I was using trunk lld.

Cheers,
Rafael

Rafael Avila de Espindola wrote :
>> Maybe gdb needs to fall back to slower line number resolution because
>> e.g.
>> low and high bounds cannot be retrieved and debug_line_address is 0?
>
> It is hard to know without a reproducible. I tried gdb on clang itself
> build with both clang and gcc and linked with gold and lld. I could not
> reproduce the slowdown, but I was using trunk lld.
I will retry with clang trunk, when it reproduces I will build some other
large project (that has DSOs) using our compile/link options (they are not
that special, I think).

Best regards,
Martin

-- 
http://www.productive-cpp.com/

Martin Richtarsky <s at martinien.de> writes:
> Rafael Avila de Espindola wrote :
>>> Maybe gdb needs to fall back to slower line number resolution
because
>>> e.g.
>>> low and high bounds cannot be retrieved and debug_line_address is
0?
>>
>> It is hard to know without a reproducible. I tried gdb on clang itself
>> build with both clang and gcc and linked with gold and lld. I could not
>> reproduce the slowdown, but I was using trunk lld.
>
> I will retry with clang trunk, when it reproduces I will build some other
> large project (that has DSOs) using our compile/link options (they are not
> that special, I think).
If you can try lld trunk too that would be awesome.

Cheers,
Rafael