[llvm] r228265 - Implement new heuristic for complete loop unrolling.

[Michael Zolotukhin]

Hi Chandler,

I prepared a new version of the patch, do you mind taking a look at it?

To me it looks even simpler than the original implementation, and additionally can handle branches (I added two tests for that).



Thanks,
Michael

> On Feb 13, 2015, at 11:58 AM, Chandler Carruth <chandlerc at google.com> wrote:
> 
> 

> On Fri, Feb 13, 2015 at 11:52 AM, Michael Zolotukhin <mzolotukhin at apple.com <mailto:mzolotukhin at apple.com>> wrote:
> Thank you for the work and pointing out to the problems! It’s good that we found them early.
> 
> I think that we would solve these issues if we do the following:
> 1. When looking for simplified instructions, visit each instruction only once. That’ll be achieved by visiting blocks in pre-order.
> 2. When looking for dead-instructions, also visit them only once - here we would need to visit blocks in reverse-order.
> 3. SCEV indeed might be too slow for this, but we actually only interested in expressions that become constants. That means, we need SCEV for expressions that either feed a load, or have constant Start and Step SCEV-expressions. We can scan all insns once and cache results for such potentially-constant instructions, other instructions don’t need SCEV-expression. So, we won’t need to talk to SCEV after this point.
> 4. As for using threshold for early-exit - that’s a really good idea, and it should integrate here naturally.
> 
> What do you think about this? Am I missing something, or does it sound right?
> 
> Generally yes, but I would start without trying to handle dead instructions, and following the sketch of how to build it I gave. I would then layer dead instructions on as a follow-up. There are several different approaches to handling dead instructions, and I think it will be useful to see how things look without handling them first.
> 
> Note that the outline I gave naturally will skip dead *control flow*, because we can simplify the condition to a constant and only traverse one of the successors.

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