Llvm-objcopy --compress-sections

[MaskRay]

ELF linkers and objcopy/llvm-objcopy provide --compress-debug-sections=[zlib|zstd] to compress .debug_* sections. A natural extension is to compress arbitrary sections, which can be used with a number of sections not requiring runtime introspection.

I recently added

ld.lld --compress-sections <sections-glob>=[zlib|zstd]

To complement ld.lld, similar functionality for decompressing sections would be valuable in llvm-objcopy. I have created [llvm-objcopy] Add --compress-sections by MaskRay · Pull Request #85036 · llvm/llvm-project · GitHub

llvm-objcopy --compress-sections <sections-glob>=[zlib|zstd]  # compress sections
llvm-objcopy --compress-sections <sections-glob>=none         # decompress sections

I find arbitrary section compression handy for several tasks:

• Analysis of compressed __llvm_covmap
• Analysis of compressed .strtab ([RFC] Compressed SHT_SYMTAB/SHT_STRTAB for ELF) in exe/dso or an alternative that places `.strtab in a file referenced by .gnu_debuglink
• Analysis of the compression ratio of CREL [RFC] CREL: A compact relocation format for ELF (Applying a generic compression algorithm is a good way to measure the efficiency of an encoding format, as a dense format can hardly be further compressed by a generic algorithm)

I’m curious to see how others are creatively applying compression techniques!