Re: Thoughts on LLVM and LTO

[Benjamin Kosnik <bkoz at redhat dot com>]

> First off, regardless of what direction we choose to go, I think we
> are in a great position.  Finally, GCC will have all the obvious and
> standard technology that one reads in textbooks.  Not long ago, GCC
> didn't even build a flowgraph, and now here we are deciding what IPA
> technology we want to implement.

I agree. It's cool to see this evolution.

> LLVM is missing a few other features like debugging information and
> vectorization.  Yes, all of it is fixable, but again, we have limited
> resources.  Furthermore, it may be hard to convince our development
> community to add these missing features: "we already implemented
> that!".  It is much easier to entice folks to do something new than to
> re-implement old stuff.

Debugability is essential. I would like to seem some comment from LLVM
people about plans for this, and I fully realize that IMA and the
other proposal also have their own debugability baggage. 

IMHO, some realistic plan to deal with this essential feature is
required.

> The lack of FSF copyright assignment for LLVM is a problem.  It may
> even be a bigger problem than what we think.  Then again, it may not.
> I just don't know.  What I do know is that this must absolutely be
> resolved before we even think of adding LLVM to any branch.  Chris
> said he'd be adding LLVM to the apple branch soon.  I hope the FSF
> assignment is worked out by then.  I understand that even code in
> branches should be under FSF copyright assignment.

This is a solvable problem, and has been pointed out to Chris
repeatedly, by many people, at various venues, for over a year.

http://gcc.gnu.org/ml/gcc/2004-10/msg01146.html

He keeps hand waving, saying that it's possible.

Great. 

I say, enough grandstanding: it's not enough to be possible, it needs
to be actual. If it is indeed actually possible to release LLVM under
the GPL, then he needs to pick a version and GPL it. Then we can get
serious.

Make it so.

He seems to be operating under the mistaken idea that the GCC
community can go ahead and make plans around LLVM being free as
defined by the GNU project, without it actually being so.

> Another minor nit is performance.  Judging by SPEC, LLVM has some
> performance problems.  It's very good for floating point (a 9%
> advantage over GCC), but GCC has a 24% advantage over LLVM 1.2 in
> integer code.  I'm sure that is fixable and I only have data for an
> old release of LLVM.  But is still more work to be done.  Particularly
> for targets not yet supported by LLVM.

What about compile-time performance?

I'd actually like to make this a requirement, regardless of the option
chosen.

-benjamin