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

[Alexey Samsonov]

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.
>
> - Michael Spencer
>
>


-- 
Alexey Samsonov, MSK
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