[llvm-dev] Debug symbols are missing in elf

[Nagaraju Mekala via llvm-dev]

On Wed, Apr 22, 2020 at 10:34 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Wed, Apr 22, 2020 at 9:28 AM Nagaraju Mekala via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> On Tue, Apr 21, 2020 at 6:16 PM Robinson, Paul <paul.robinson at sony.com> wrote:
>> >
>> >
>> >
>> > > -----Original Message-----
>> > > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Nagaraju
>> > > Mekala via llvm-dev
>> > > Sent: Tuesday, April 21, 2020 6:04 AM
>> > > To: jh7370.2008 at my.bristol.ac.uk
>> > > Cc: LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
>> > > Subject: Re: [llvm-dev] Debug symbols are missing in elf
>> > >
>> > > Hi James,
>> > >
>> > > Thanks for reply.
>> > > When I looked in to the generated debug dump from the object file I
>> > > found that DW_AT_name is always (indirect string, offset: 0x0): clang
>> > > version 8.0.1, instead of variable names.
>> >
>> > That symptom suggests that relocations from .debug_info to .debug_str
>> > are not being handled correctly.  Either your backend is not emitting
>> > them correctly, or the dumper does not know what to do with them.
>> >
>> > If you are able to dump the .rela.debug_info section and it looks
>> > reasonable, the fault is most likely with the dumper.  If you are
>> > using llvm-dwarfdump then that should not be hard to solve.
>> > --paulr
>> >
>> Hi Paulr,
>> Thanks for the reply.
>> From your suggestion I have used llvm-dwarfdump and was able to figure
>> out that debug_info section was incomplete.
>
>
Hi David,

Thanks for the reply.
> What command did you run, on what file, and what was the output?

I ran llvm-dwarf with -verbose option. It gave me the verbose print as below
0x0000000b: DW_TAG_compile_unit [1] *
              DW_AT_producer [DW_FORM_strp]     (
.debug_str[0x00000000] = "clang version 8.0.1 ")
              DW_AT_language [DW_FORM_data2]    (DW_LANG_C99)
              DW_AT_name [DW_FORM_strp] ( .debug_str[0x00000000] =
"clang version 8.0.1 ")
              DW_AT_stmt_list [DW_FORM_sec_offset]      (0x00000000)
              DW_AT_comp_dir [DW_FORM_strp]     (
.debug_str[0x00000000] = "clang version 8.0.1 ")
Similarly as above snippet .debug_str is always pointing to 0x00000
for everything.
>>
>> Can you please help me in finding which part of the target(Backend
>> files) will construct the dwarf data, especially debug_info.
>
>
> LLVM's DWARF generation is implemented in llvm/lib/CodeGen/AsmPrinter/Dwarf* files
Are there any backend specific functions or files which will effect
dwarf generation of that particular target?
I can find only "SupportsDebugInformation = true;"  in MCAsmInfo.

-Nagaraju
>>
>>
>> Thanks in Advance,
>> Nagaraju
>> > >
>> > > Below is a sample output of the debug dump.
>> > >
>> > >  Compilation Unit @ offset 0x0:
>> > >    Length:        0x6a (32-bit)
>> > >    Version:       4
>> > >    Abbrev Offset: 0x0
>> > >    Pointer Size:  4
>> > >  <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
>> > >     <c>   DW_AT_producer    : (indirect string, offset: 0x0): clang
>> > > version 8.0.1
>> > >     <10>   DW_AT_language    : 12       (ANSI C99)
>> > >     <12>   DW_AT_name        : (indirect string, offset: 0x0): clang
>> > > version 8.0.1
>> > >     <16>   DW_AT_stmt_list   : 0x0
>> > >     <1a>   DW_AT_comp_dir    : (indirect string, offset: 0x0): clang
>> > > version 8.0.1
>> > >     <1e>   DW_AT_low_pc      : 0x0
>> > >     <22>   DW_AT_high_pc     : 0x6c
>> > >  <1><26>: Abbrev Number: 2 (DW_TAG_subprogram)
>> > >     <27>   DW_AT_low_pc      : 0x0
>> > >     <2b>   DW_AT_high_pc     : 0x6c
>> > >     <2f>   DW_AT_frame_base  : 1 byte block: 51         (DW_OP_reg1 (r1))
>> > >     <31>   DW_AT_name        : (indirect string, offset: 0x0): clang
>> > > version 8.0.1
>> > >     <35>   DW_AT_decl_file   : 1
>> > >     <36>   DW_AT_decl_line   : 51
>> > >     <37>   DW_AT_type        : <0x66>
>> > >     <3b>   DW_AT_external    : 1
>> > >
>> > > -Nagaraju
>> >
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