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

[Nick Lewycky]

On 04/16/2013 02:37 AM, Alexey Samsonov wrote:
> Hi!
>
> TL;DR WDYT of adding zlib decompression capabilities to LLVMObject library?

Yes, I want this.

> 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?

Yes, exactly. I'm not certain that MemoryBuffer and LLVMObject are the 
right places, but it doesn't sound wrong.

Nick