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

[Rui Ueyama via llvm-commits]

gold is about 10% slower with --build-id to link Clang. So "okay" is
stricter than that.

On Thu, Mar 10, 2016 at 12:14 PM, David Blaikie <dblaikie at gmail.com> wrote:

> 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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>>
>
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