[PATCH] D18091: ELF: Implement --build-id.

[Rui Ueyama via llvm-commits]

ruiu created this revision.
ruiu added reviewers: emaste, rafael, silvas.
ruiu added a subscriber: llvm-commits.

This patch implements --build-id. After the linker creates an output file
in the memory buffer, it computes the FNV1 hash of the resulting file
and set the hash to the .note section as a build-id.

GNU ld and gold has the same feature, but their default choice of the hash
function is different. Their default is SHA1.

We made a deliberate choice to not use a secure hash function for the
sake of performance. Computing a secure hash is slow -- for example,
MD5 throughput is usually 400 MB/s or so. SHA1 is slower than that.

As a result, if you pass --build-id to gold, then the linker becomes about
10% slower than that without the option. We observed a similar degradation
in an experimental implementation of build-id to LLD. On the other hand,
we observed only 1-2% performance degradation with the FNV hash.

Since build-id is not for digital certificate or anything, we think that
a very small probability of collision is acceptable.

GNU linkers accepts --build-id=<style> option where style is one of
"MD5", "SHA1", or an arbitrary hex string. That option is out of scope
of this patch.

http://reviews.llvm.org/D18091

Files:
  ELF/Config.h
  ELF/Driver.cpp
  ELF/Options.td
  ELF/OutputSections.cpp
  ELF/OutputSections.h
  ELF/Writer.cpp
  test/ELF/build-id.s

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