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

[Rui Ueyama via llvm-dev]

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 <https://lists.debian.org/debian-gcc/2009/07/msg00082.html>).
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.
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