[LLVMdev] Bug? Coalescing & Updating Subreg Intervals

[David A. Greene]

On Wednesday 20 February 2008 07:00:28 pm Evan Cheng wrote:

> > In other words, after coalescing, should it be the case that
> > subregister
> > intervals contain at least all of the range information that was
> > contained
> > in any eliminated intervals when those eliminated intervals were
> > coalesced
> > to the subregister's superregister?
>
> Right. The code is being conservative. It's saying all of the entirety
> of the superregister's live interval must also be in the subregister's
> live interval. i.e. When the superregister is live, the subregister
> must be live.

Ok, but that is NOT what's happening in this case.

Also, LiveIntervalAnalysis doesn't do any subregister checks as
far as I can tell.  It's certainly not the case that subregister
intervals contain all of the information their supperregister's
interval contains.

So is this just supposed to be a conservative update for the
purposes of coalescing?

In any event, it seems not to be working right if what you
describe is supposed to be happening.

Given the two intervals in my example, which should happen
with the two overlappring ranges 

%reg1026:  [458,5168:0 [0])
%reg15: [938,942:1 [0])

My assumption was that after MergeInClobberRanges that %reg15
would contain [458,5168:0 [0]).  But it doesn't.

                                                  -Dave