[llvm-dev] Live range priority in Greedy RA

[jingu kang via llvm-dev]

As far as I know, there is a good comment about priority on enqueue as below.

  /// Allow the target to reverse allocation order of local live ranges. This
  /// will generally allocate shorter local live ranges first. For targets with
  /// many registers, this could reduce regalloc compile time by a large
  /// factor. It is disabled by default for three reasons:
  /// (1) Top-down allocation is simpler and easier to debug for targets that
  /// don't benefit from reversing the order.
  /// (2) Bottom-up allocation could result in poor evicition decisions on some
  /// targets affecting the performance of compiled code.
  /// (3) Bottom-up allocation is no longer guaranteed to optimally color.
  virtual bool reverseLocalAssignment() const { return false; }

If you are implementing your custom target, you could control the
priority of enqueue with 'reverseLocalAssignment()'. However, the
allocated registers can be evicted with weight comparison and the live
ranges can be split when the all of physical registers are consumed.
The priority on enqueue does not guarantee  allocation of physical
register. I am not code owner of register allocation so I might be
wrong.

Cheers,
JinGu Kang
2018년 11월 8일 (목) 오전 10:02, Dangeti Tharun kumar
<cs15mtech11002 at iith.ac.in>님이 작성:
>
> Hi,
>
> I had a look at this  'calculateSpillWeightAndHint' function, spill weights are being calculated considering no of uses, defs, loops etc.
> Why not these weights be used as priority, seems they are good enough to serve as priority, instead doing it by their sizes?
>
> On Tue, Oct 30, 2018 at 5:39 PM jingu kang <jaykang10 at gmail.com> wrote:
>>
>> Hi Dangeti,
>>
>> As far as I know, 'calculateSpillWeightAndHint' considers loop
>> induction variable and the spill weight affects evict and spill. If it
>> is not enough for you, I guess you can add some heuristic code on the
>> function.
>>
>> Thanks,
>> JinGu Kang
>> 2018년 10월 30일 (화) 오전 11:51, Dangeti Tharun kumar via llvm-dev
>> <llvm-dev at lists.llvm.org>님이 작성:
>> >
>> > Hi,
>> >
>> > In the Greedy RA, I see that the enqueue method adds higher priority to the live intervals based on their sizes.
>> >
>> > Isn't it makes sense to give priority to live intervals that start and end in a loop? Please let me know if the code is already achieving it in some way.
>> > Also consider correcting me, if this approach is wrong.
>> >
>> > --
>> > Regards,
>> > DTharun
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
> --
> Regards,
> DTharun