[LLVMdev] Scheduler Roadmap

[dag at cray.com]

Jim Grosbach <grosbach at apple.com> writes:

>> - How difficult do you expect a backport to 3.1 to be?  We have to work
>>  from 3.1.  Trunk is too buggy.

> You've stated that trunk is too buggy for you to work from on multiple
> occasions. Can you elaborate? That doesn't match my experience, as I
> install a new compiler on my workstation from a trunk build every
> night and have only had a problem as a result once in the last year or
> more. It sounds like you've had a much different experience, which is
> unfortunate, and perhaps indicates a hole in our buildbot and nightly
> test infrastructure that could be fixed.

We do a lot of Fortran which doesn't get covered all that well by the
nightly tester.  We also have a completely different frontend and
optimizer meaning that we present code to LLVM that it's never seen
before.  We've seen major scalability problems in the past.  We compile
some absolutely gigantic codes here.  Fortunately it appears most of
these have been fixed but I think LoopStrengthReduce is still a problem
(we've turned it off).  Scheduling, regalloc, instcombine and dagcombine
have all presented problems in the past.  We've sent patches for the
most egregious cases.  Many of those patches get dropped, leading to
less confidence on our side that issues will be fixed quickly.  Others
in the LLVM community have fixed other such problems.

Again, these issues get fixed with new releases (and with some hacking
by us in the interim) but the experience makes me very hesitant to make
our development depend on trunk.  We cannot afford to waste even a few
days of interrupted release development work tracking down a new
regression introduced in trunk.

We also can't afford to have compiler builds break because someone
upstream decided to change an API we rely on.  We make monthly releases.
We have to have some control over when we introduce such changes.

We run hundreds of thousands of tests each week, both simple regression
tests and large applications.  I don't think the nightly tester will
ever be capable of covering that.

I have a long-term goal of setting up a tree that uses LLVM trunk for
our builds both so I can track what's happening on trunk to more easily
know what it means for us and to test out some of the new features, like
the MachineInstr scheduler in this case.  Unfortunately, time is limited
so I have to do that piecemeal 10 minutes here and there.  The git
mirror will make that much easier.

                                -Dave