[Beignet] [PATCH v2 1/2] GBE: continue to refine interfering check.

[Zhigang Gong]

On Wed, Sep 23, 2015 at 02:33:43AM +0000, Song, Ruiling wrote:
> 
> 
> > -----Original Message-----
> > From: Beignet [mailto:beignet-bounces at lists.freedesktop.org] On Behalf Of
> > Zhigang Gong
> > Sent: Monday, September 7, 2015 8:20 AM
> > To: beignet at lists.freedesktop.org
> > Cc: Gong, Zhigang
> > Subject: [Beignet] [PATCH v2 1/2] GBE: continue to refine interfering check.
> > 
> > More aggresive interfering check, even if both registers are in Livein set or
> > Liveout set, they are still possible not interfering to each other.
> > 
> > v2:
> > Liveout interfering check need to take care those BBs which has only one
> > register defined.
> > 
> > For example:
> > 
> > BBn:
> >   ...
> >   MOV %r1, %src
> >   ...
> > 
> > Both %r1 and %r2 are in the BBn's liveout set, but %r2 is not defined or used in
> > BBn. The previous implementation ignore this BB which is incorrect. As %r1 was
> > modified to a different value, it means %r1 could not be replaced with %r2 in
> > this case.
> I thought of another one: (r0, r1 contain different values)
> BB0:
>   def r0
> 
> BB1:
>   def r1
>   use r1
>   use r0
> 
> How could the algorithm deal with it?
If r1 is a local value in BB1, then it is just ignored in these analysis including
DAG and liveness. Don't worry about the correctness of this case, as in gen_reg_allocation
we will calculate correct interval for it.

If r1 is not a local value of BB1, then it's either in live in of
liveout set of BB1. Either case, this algorithm could deal it correctly.

> And looks like the algorithm is converting the "live-range interference problem" into
> "checking the interference in live-In, live-Out set".
> Any paper talking on this method?
Actually, most of the paper just talk about to use liveness and DAg information to check
interference between the phi and phi copy values to determin whether to coalesce. Not
too much details. I just use our liveness information and DAG to do the job.

> 
> And for same BB interference (r0, r1 defined and used in same BB), although we don't need to support it yet, I think if we can put an assert, it would be better.
Could you describe more detail of this case? local variable or not local variable? Are they in live in set or live out set?

Thanks for the comments,
Zhigang Gong.

> 
> Thanks!
> Ruiling
> 
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