[PATCH] Reimplement heuristic for estimating complete-unroll optimization effects.

[Michael Zolotukhin]

Hi Chandler,

Thanks for the comments! I believe I've addressed in the source all of them, except this one:

> Is there a reason you don't make visit() return a bool indicating whether it's cost should be counted or not, and localize all the counting in this function? It would be much easier to understand IMO.

> 

> I think I would also find it easier to read this as counting the number of instructions that will actually result from unrolling (essentially, the *un*optimized instructions) and the optimized instructions. You could still sum them and divide to compute the percentage, but it would make the threshold check not require subtraction. That could be done in a follow-up patch though.


I kept counting the cost inside the `visit` function, because we might have three cases there:

1. instruction was simplified to a constant (e.g. x = a[0] * y = 0 * y = 0)
2. instruction was simplified, but not to a constant (e.g. x = a[0] + y = 0 + y = y)
3. instruction wasn't simplified

In case we want to distinguish (1) and (2) outside the `visit` function, `bool` won't be enough. Currently we won't lose much by merging them though, but I didn't want to limit ourselves here from the very beginning.


http://reviews.llvm.org/D8816

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