[PATCH] D18055: ELF: Implement --build-id.

[David Blaikie via llvm-commits]

Do you have a sense of how other linkers implement this feature and/or how
much it costs in performance/link time? You mention what an "okay" cost for
this would be, but it's not clear to me how you determined that cost.

On Thu, Mar 10, 2016 at 11:23 AM, Rui Ueyama via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> ruiu created this revision.
> ruiu added reviewers: rafael, emaste.
> ruiu added a subscriber: llvm-commits.
>
> This is not ready for submission.
>
> This patch implements --build-id in a naive way. After the linker creates
> an output file in the memory buffer, it computes the md5sum of the
> resulting
> file and set the hash to the .note section as a build-id.
>
> The problem is that its performance impact is too large. Computing a secure
> hash is a computational intensive task (our md5sum function seems a bit too
> slow, though). Here are some numbers (milliseconds to link):
>
> LLD:     713.78 ->  883.44 (+23.8%)
> Scylla: 5005.64 -> 5437.84 (+8.6%)
>
> Even if we replace MD5 with a faster hash function, it cannot be within
> an "okay" range, which is, say, +3%.
>
> Do you guys have any opnion on this? I have a few ideas.
>
>  - Use CRC32 instead of MD5. Non-secure hash should be much faster.
>
>  - Make clang to calculate a secure hash for each section.
>    This is basically moving the workload from the linker to the compiler,
>    but we can use the hash for ICF in the linker,
>    so it might be a overall win.
>
>  - Compute a build-id from input files' timestamps. This makes builds
>    non-reproducible if you touch a file, so I don't think we want this.
>
>  - Build-id is not needed for program execution in general.
>    So we may be able to let the linker exit as soon as it's done with
> linking,
>    and backfill the hash value in a background process which is kicked in
>    by the linker. (It's probably unrealistic plan, though.)
>
>  - Do not support build-id and let the user pass --build-id=<hash value>.
>
> http://reviews.llvm.org/D18055
>
> Files:
>   ELF/Config.h
>   ELF/Driver.cpp
>   ELF/Options.td
>   ELF/OutputSections.cpp
>   ELF/OutputSections.h
>   ELF/Writer.cpp
>
>
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