[llvm-dev] LLD status update and performance chart

Rui Ueyama via llvm-dev
Mon, 12 Dec 2016 03:04:07 -0000

Hi,

Now that 2016 is almost over, I wanted to look back and summarize the
progress we've made to LLD this year, as I guess most people who are not
looking closely at LLD don't know very well about the current status. I
think I can say that this year was a fantastic year for LLD. Now I'm pretty
sure that that is going to be a serious (and better, in my opinion)
alternative to the existing GNU linkers thanks to all the improvements
we've made this year.

LLD is now able to link most x86-64 userland programs. The FreeBSD project
and we are trying to make LLD the system default linker of the operating
system, and except a few tricky programs such as the kernel or a boot
loader, the linker works mostly fine. We are still working on implementing
long-tail features/bugs, but I'd say that's just a matter of time. LLD
supports x86, x86-64, x32, AArch64, AMDGPU, ARM, PPC64 and MIPS32/64,
though completeness varies.

Looks like there are already a few systems that are using LLD as system
linkers, such as CloudABI or Fuchsia. Chromium and Clang/LLVM itself has
build options to use LLD to build them.

It is hard to argue about the complexity of a program quantitatively, and
of course I'm biased, but I believe we succeeded to maintain LLD code base
clean, easy to read, and easy to add new features. It is just 20k lines of
modern C++ code which is much smaller than GNU linkers.

Even though LLD was fast from day one, LLD got faster this year, despite it
got a lot of new features. Below is a chart of Clang link time for every
commit made to the LLD repository this year. At the beginning of this year,
LLD took about 16 seconds to produce a 1.5 GB clang (debug build)
executable. Now, it takes about 14.5 seconds on single core and 8.5 seconds
on 20 cores (*1). ld.gold takes about 25 seconds and 20 seconds,
respectively, so we've widen the gap. You can see the benchmark results
here (*2). If you have a problem of too long link time, I'd recommend to
try LLD.

Last but not least, a lot of people joined to the LLD development this
year to make LLD better. We are growing as a community, and I'm very happy
about that!

Thanks,
Rui

(*1) My machine has Ivy Bridge Xeon 2.8 GHz 20 physical cores (40
hyper-threading cores). To measure a single-thread performance, I pinned a
process to (physical and hyper-threading) core 0. To measure a multi-thread
performance, I pinned to CPU socket 2, so that a process gets 10 physical
cores (20 hyperthreading cores).

(*2)
https://docs.google.com/spreadsheets/d/1VvOqiU5JvqlxU7aof8gsbh-yweeNchMgtkamyXrwzrA/edit?usp=sharing.
Changes with more than 1% rise or drop compared to the average of previous
5 commits are colored in green or red, respectively.
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