[PATCH] D32563: Improve code placement algorithm in Reassociate pass.

[Daniel Berlin via llvm-commits]

>
>
> > Maybe?
> >
> > Remember that this reassociate has *zero* global view of expressions.
> > It only makes locally good choices.
> >
> > IE given:
> >      bar(a + b);
> >      bar((a + 2) + b);
> >
> >
> > This reassociate will process the a + b and a + 2 + b separately,
>
> This patch will not change the original handling of this case because
> both a and b has >1 uses.
>
>
Sure, i was just pointing out it is only handling local tree-level stuff,
and the way it forms trees will not necessarily guarantee that the overall
live range of the values is reduced (precisely because the trees it forms
do not contain all uses).

IE it is guaranteed to try to shrink a live range locally, and either
succeed or do nothing.
That may or may not shrink a live range globally, and may increase the live
range locally.


IE
       A
        |
A     |
|       |
|       |
A      A

A
|
|
A

->

A
|
|
A   A
      |
      |
A   A
|
|
A


This has shrunk the live range of A locally, but made it worse globally.
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