[llvm-commits] [PATCH] Make dependency DAG construction to use AA

[Hal Finkel]

On Tue, 08 May 2012 20:13:58 -0700
Andrew Trick <atrick at apple.com> wrote:

> 
> On May 8, 2012, at 7:12 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> >> +  int64_t MinOffset = std::min(MMOa->getOffset(),
> >> MMOb->getOffset());
> >> +  int64_t Overlapa = MMOa->getSize() + MMOa->getOffset() -
> >> MinOffset;
> >> +  int64_t Overlapb = MMOb->getSize() + MMOb->getOffset() -
> >> MinOffset;
> >> 
> >> It took me a while to figure out what you're doing and why. We can
> >> hide this behind an API eventually (I can do it post checkin), but
> >> for now can you add comments with any cases you could think of?
> >> Please also comment that it's taken from DAGCombiner. Apparently
> >> DAGCombiner thinks it's a good idea to normalize offsets relative
> >> to each other by subtracting MinOffset. I don't understand why
> >> this is a good idea and can't prove to myself that it is safe. For
> >> example, what if we have an IR-level offset from the same base
> >> pointer smaller than the MachineOperand offset? We would seem to
> >> miss the overlap. Maybe Dan can comment on this.
> >> 
> >> Hal also mentioned negative offsets, but it looks to me like that
> >> would work assuming positive offsets work!
> > 
> > I was actually concerned about one positive offset and one negative.
> > Based on your second e-mail, I'll assume that this is fine too.
> > 
> > For example:
> > A: p + 2 (size = 4)
> > B: p - 8 (size = 2)
> > Here, MinOffset will be -6, and so Overlapa will be 4 + 2 - 8 = -2.
> > That does not seem right, I thought that the size in AA::Location
> > would need to be positive.
> 
> 
> Size definitely needs to be positive.
> 
> I may not be reading your example right. I get:
> 
> OffsetA = 2; SizeA = 4
> OffsetB = -8; SizeB = 2
> MinOffset = min(2, -8) = -8
> OverlapA = 4 + 2 + 8 = 14
> OverlapB = 2 - 8 + 8 = 2

Indeed, you're correct. My mistake.

Thanks again,
Hal

> 
> In general, the overlap with min offset is simply the size. The other
> overlap's offset is always signed greater than min offset, to the
> result of subtraction is positive. As Dan said, "assuming no
> overflow" :)
> 
> I think more comments could help.
> 
> -Andy



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory