[llvm-dev] LLD's default --build-id choice

[Sean Silva via llvm-dev]

On Wed, Jun 1, 2016 at 3:41 PM, Rui Ueyama via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> On Wed, Jun 1, 2016 at 3:32 PM, Rafael Espíndola <
> rafael.espindola at gmail.com> wrote:
>
>> On 1 June 2016 at 15:21, Rui Ueyama via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > Nico brought up this topic and made me think whether the current choice
>> of
>> > --build-id was the right one or not.
>> >
>> > Currently, we compute a FNV1 hash for an entire resulting file and
>> store it
>> > to .note.gnu.build-id section. It's one of the slowest parts of the
>> linker
>> > because reading every byte takes time. IIRC, it usually takes about 10%
>> of
>> > total link time.
>> >
>> > In the first place, I believe it was not a good decision to make GCC
>> (and
>> > therefore Clang) to pass --build-id option to the linker by default (it
>> was
>> > done in 2009). Build ID is sometimes useful, particularly when
>> distributing
>> > linked objects to users, but in most cases it is not needed. Spending
>> 10%
>> > more time on usual build-link-debug cycle is a waste of time. It should
>> not
>> > have been added that casually.
>> >
>> > Anyways, the option is there and passed to the linker, so we have to
>> create
>> > and add a build ID if --build-id option is given (we could ignore the
>> option
>> > but that's probably very confusing.)
>> >
>> > So here's my proposal.
>> >
>> >  - Make --build-id=uuid as default for --build-id
>> >
>> > --build-id=uuid sets build-id to a random unique value. It's very fast.
>> > Instead, it breaks build reproducibility because every build has a
>> unique
>> > build ID. But if you want build reproducibility, you can explicitly pass
>> > --build-id=sha1.
>>
>> Please don't, reproducible builds are *really* important.
>>
>> Note that you can disable build-id with -Wl,--build-id=none.
>>
>> Maybe make the default an even simpler hash? Or hash just parts of the
>> file?
>>
>
> I think FNV1 is very fast hash function, so we cannot make it faster by
> replacing it with some other hash function.
>
> We could hash only some part of the file, say the first page of an
> executable. In that way, there's a risk that two executables with the same
> build ID are actually different if they have idential ELF headers, but is
> it going to be a problem?
>

That defeats the whole point of --build-id, I think.

-- Sean Silva


>
>
>> I would also be open to just changing clang to not pass --build-id by
>> default.
>>
>
> I'd be very happy if we do it.
>
>
>>
>> Cheers,
>> Rafael
>>
>
>
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