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Fwd: LLVM collaboration?
- From: Renato Golin <renato dot golin at linaro dot org>
- To: gcc at gcc dot gnu dot org
- Date: Fri, 7 Feb 2014 21:33:58 +0000
- Subject: Fwd: LLVM collaboration?
- Authentication-results: sourceware.org; auth=none
- References: <CAMSE1kdfpeLp6NEc+jnEWqi0KWV-+=Q701UsiLhgcn13X6fYcA at mail dot gmail dot com>
I'm about to do something I've been advised against, but since I
normally don't have good judgement, I'll risk it, because I think it's
worth it. I know some people here share my views and this is the
reason I'm writing this.
For a long time already I've been hearing on the LLVM list people
saying: "oh, ld should not accept this deprecated instruction, but we
can't change that", "that would be a good idea, but we need to talk to
the GCC guys first", and to be honest, nobody ever does.
Worst still, with Clang and LLVM getting more traction recently, and
with a lot of very interesting academic work being done, a lot of new
things are getting into LLVM first (like the sanitizers, or some
specialized pragmas) and we're dangerously close to start having
clang-extensions, which in my humble opinion, would be a nightmare.
We, on the other side of the fence, know very well how hard it is to
keep with legacy undocumented gcc-extensions, and the ARM side is
particularly filled with magical things, so I know very well how you
guys would feel if you, one day, had to start implementing clang stuff
without even participating in the original design just because someone
relies on it.
So, as far as I can see (please, correct me if I'm wrong), there are
two critical problems that we're facing right now:
1. There IS an unnecessary fence between GCC and LLVM.
License arguments are one reason why we can't share code as easily as
we would like, but there is no argument against sharing ideas,
cross-reporting bugs, helping each other implement a better
compiler/linker/assembler/libraries just because of an artificial
wall. We need to break this wall.
I rarely see GCC folks reporting bugs on our side, or people saying
"we should check with the GCC folks" actually doing it. We're not
contagious folks, you know. Talking to GCC engineers won't make me a
lesser LLVM engineer, and vice-versa.
I happen to have a very deep respect for GCC *and* for my preferred
personal license (GPLv3), but I also happen to work with LLVM, and I
like it a lot. There is no contradiction on those statements, and I
wish more people could share my opinion.
2. There are decisions that NEED to be shared.
In the past, GCC implemented a lot of extensions because the standards
weren't good enough. This has changed, but the fact that there will
always be things that don't belong on any other standard, and are very
specific to the toolchain inner workings, hasn't.
It would be beneficial to both toolchains to have a shared forum where
we could not only discuss how to solve problems better, but also keep
track of the results, so we can use it as guidelines when implementing
Further still, other compilers would certainly benefit from such
guidelines, if they want to interact with our toolchains. So, this
wouldn't be just for our sake, but also for future technologies. We
had a hard time figuring out why GCC would do this or that, and in the
end, there was always a reason (mostly good, sometimes, not so much),
but we wasted a lot of time following problems lost in translation.
The Open Source Compiler Initiative
My view is that we're unnecessarily duplicating a lot of the work to
create a powerful toolchain. The license problems won't go away, so I
don't think LLVM will ever disappear. But we're engineers, not
lawyers, so we should solve the bigger technical problem in a way that
we know how: by making things work.
For the last year or two, Clang and GCC are approaching an asymptote
as to what people believe a toolchain should be, but we won't converge
to the same solution unless we talk. If we keep our ideas enclosed
inside our own communities (who has the time to follow both gcc and
llvm lists?), we'll forever fly around the expected target and never
To solve the technical problem of duplicated work we just need to
start talking to each other. This mailing list (or LLVM's) is not a
good place, since the traffic is huge and not every one is interested,
so I think we should have something else (another list? a web page? a
bugzilla?) where we'd record all common problems and proposals for new
features (not present in any standards), so that at least we know what
the problems are.
Getting to fix a problem or accepting a proposal would go a long way
of having them as kosher on both compilers, and that could be
considered as the standard compiler implementation, so other
compilers, even the closed source ones, should follow suit.
I'll be at the GNU Cauldron this year, feel free to come and discuss
this and other ideas. I hope to participate more in the GCC side of
things, and I wish some of you guys would do the same on our side. And
hopefully, in a few years, we'll all be on the same side.
I'll stop here, TL;DR; wise. Please, reply copying me, as I'm not
(yet) subscribing to this list.