[PATCH] Speed up creation of live ranges for physical registers by using a segment set

[Matthias Braun]

As I said: Don't let my comments stop you from committing, fixing the performance regressions right now is important, cleaning the code up/avoiding the extra pointer can be dealt with later I guess.

> On Dec 18, 2014, at 9:32 AM, Quentin Colombet <qcolombet at apple.com> wrote:
> 
> Hi all,
> 
> I agree with Vaidas that moving the set outside of the LiveRange may not be that easy.
Nonetheless we should revisit the code later, I'm still quiet sure that LiveRangeCalc is not used with multiple LiveRanges at the same time as it will break down if you don't reset() it before using it on a new LiveRange...

> 
> Regarding the complexity improvements:
> 
>> for the createDeadDefs() part it may be interesting to simply append the new segments to the vector in any order and later run a sorting algorithm which also eliminates duplicates on the vector. This should reduce the O(n**2) complexity to O(n log n) and I'd imagine it to perform well in the case where only a few segments are used.
> 
> 
> I am not so sure that would be beneficial, indeed, we would have to do this sorting for each live range, i.e., O(m n log n) and I suspect that the (log n) factor will be bigger than the cost of the set for the cases we try to optimize.
So currently we add segments in a random order. As the LiveRange segments sector is kept sorted, this results in an insertion-sort style O(n**2) algorithm for createDeadDefs(), that's why using a set instead of the vector brings this down to O(n log n). However this complicates the code as you suddenly have two datastructures and switch forward and backward between them. I'd guess that is also the reason why using this style for vregs is not beneficial.
Adding the segments in a random fashion to the vector and sorting the complete vector later would give you the same complexity without having to maintain a second datastructure on the side. However this is probably a bigger rewrite as the current LiveRange datastructure does not allow this, I'm also not sure if we have a sorting algorithm that eliminates duplicates at the same time. So this is more of an add to the TODO list proposal.

Greetings
	Matthias