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

[Alexey Samsonov]

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
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