[LLVMdev] Greedy register allocation

[Andrew Trick]

On May 3, 2011, at 1:30 PM, Jakob Stoklund Olesen wrote:

> 
> On May 3, 2011, at 12:03 PM, David A. Greene wrote:
> 
>>> 
>>> I am working my way through the regressions, looking for things the
>>> allocator did wrong. Any help is appreciated, please file bugs if you
>>> find examples of stupid register allocation.
>> 
>> Certainly.  I would ask that we keep linearscan around, if possible, as
>> long as there are significant regressions like this.  Our customers tend
>> to really, really care about performance.
> 
> That's reasonable, and it is also useful to keep it around as a reference when greedy breaks.
> 
> On the other hand, I really want to clean up the code surrounding register allocation, and that is much easier to do after linear scan is gone. There is a good chance it won't make it to the 3.0 release.

We designed the basic allocator as a baseline and intend to maintain it as a reference allocator. It factors the complexity out of the greedy allocator but otherwise provides the same interface to surrounding code. I don't think it will be long before LinearScan is more broken than GreedyRA, and BasicRA is the safer reference. Maybe Jakob didn't mention this because BasicRA is *temporarily* broken, but in a trivial way.

The regressions occurred in highly unstable benchmarks. It's not as if we were monotonically improving these scores, and GreedyRA suddenly caused a regression. The point of looking into the regressions is to finally take control of this noise now that we have a better framework for doing so.

-Andy