[LLVMdev] RFC: Using zlib to decompress debug info sections.

[Alexey Samsonov]

Just in case - do we want to link with libz.so installed in the system, or
be self-contained and copy sources to LLVM repo?


On Tue, Apr 16, 2013 at 10:48 PM, Eric Christopher <echristo at gmail.com>wrote:

> On Tue, Apr 16, 2013 at 9:37 AM, Alexey Samsonov <samsonov at google.com>
> wrote:
> >
> > On Tue, Apr 16, 2013 at 8:31 PM, Michael Spencer <bigcheesegs at gmail.com>
> > wrote:
> >>
> >> On Tue, Apr 16, 2013 at 2:37 AM, Alexey Samsonov <samsonov at google.com>
> >> wrote:
> >>>
> >>> Hi!
> >>>
> >>> TL;DR WDYT of adding zlib decompression capabilities to LLVMObject
> >>> library?
> >>>
> >>>
> >>> ld.gold from GNU binutils has --compress-debug-sections=zlib option,
> >>> which uses zlib to compress .debug_xxx sections and renames them to
> >>> .zdebug_xxx.
> >>> binutils (and GDB) support this properly, while LLVM command line tools
> >>> don't:
> >>>
> >>> $ ld --version
> >>> GNU gold (GNU Binutils for Ubuntu 2.22) 1.11
> >>> $ ./bin/clang++ -g a.cc -Wl,--compress-debug-sections=zlib
> >>> $ objdump -h a.out | grep debug
> >>>  26 .debug_info   00000066  0000000000000000  0000000000000000
>  00002010
> >>> 2**0
> >>>  27 .debug_abbrev 00000048  0000000000000000  0000000000000000
>  00002068
> >>> 2**0
> >>>  28 .debug_aranges 00000000  0000000000000000  0000000000000000
>  000020bb
> >>> 2**0
> >>>  29 .debug_macinfo 00000000  0000000000000000  0000000000000000
>  000020cf
> >>> 2**0
> >>>  30 .debug_line   00000053  0000000000000000  0000000000000000
>  000020e3
> >>> 2**0
> >>>  31 .debug_loc    00000000  0000000000000000  0000000000000000
>  0000213e
> >>> 2**0
> >>>  32 .debug_pubtypes 00000000  0000000000000000  0000000000000000
> >>> 00002152  2**0
> >>>  33 .debug_str    00000069  0000000000000000  0000000000000000
>  00002166
> >>> 2**0
> >>>  34 .debug_ranges 00000000  0000000000000000  0000000000000000
>  000021d9
> >>> 2**0
> >>> $ ./bin/llvm-objdump -h a.out | grep debug
> >>>  27 .zdebug_info  00000058 0000000000000000
> >>>  28 .zdebug_abbrev 00000053 0000000000000000
> >>>  29 .zdebug_aranges 00000014 0000000000000000
> >>>  30 .zdebug_macinfo 00000014 0000000000000000
> >>>  31 .zdebug_line  0000005b 0000000000000000
> >>>  32 .zdebug_loc   00000014 0000000000000000
> >>>  33 .zdebug_pubtypes 00000014 0000000000000000
> >>>  34 .zdebug_str   00000073 0000000000000000
> >>>  35 .zdebug_ranges 00000014 0000000000000000
> >>>
> >>> Decompression and proper handling of debug info sections may be needed
> >>> in llvm-dwarfdump and llvm-symbolizer tools. We can implement this by:
> >>> 1) Checking if zlib is present in the system during configuration.
> >>> 2) Adding zlib decompression to llvm::MemoryBuffer, and section
> >>> decompression to LLVMObject (this would require optional linking with
> -lz).
> >>> 3) Using the methods in LLVM tools where needed.
> >>>
> >>> Does this make sense to you?
> >>>
> >>> --
> >>> Alexey Samsonov, MSK
> >>
> >>
> >> I'm not sure MemoryBuffer is the right place to do this either. I'm also
> >> not sure if we want debug info decompression to be transparent in
> LLVMObject
> >> or not. I'm leaning towards no since it's not part of the standard yet,
> >
> >
> > Yeah, I also think that decompression should be explicitly requested by
> the
> > user of LLVMObject.
> >
> >>
> >> unless gold is actually using the SHF_COMPRESSED flag.
> >>
> >>
> >> I think it should be part of Object, but as an external API that is used
> >> when you find a section you know from external factors (the name matches
> >> some list) is compressed.
> >>
>
> Definitely want the feature :)
>
> I don't see SHF_COMPRESSED (unless readelf just isn't showing it to
> me), but it wouldn't be too hard to get binutils to mark them as such.
> Right now the convention is .z<foo> are compressed, but that's not as
> precise as we'd like it to be. There's been some talk on the binutils
> list about it, but it hasn't been implemented yet.
>
> -eric
>



-- 
Alexey Samsonov, MSK
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