[LLVMdev] Scheduler Roadmap

[Evan Cheng]

On May 9, 2012, at 2:00 PM, dag at cray.com wrote:

> 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.

This has nothing to do with the scheduler roadmap. If you want to engage the community on API compatibility discussion, it should be a different thread. But given how long you have been involved in LLVM, you should know the philosophy: LLVM development is optimized for people who stay on trunk.

Evan

> 
> 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
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