[LLVMdev] Why should we have the LoopPass and LoopPassManager? Can we get rid of this complexity?

[Chandler Carruth]

On Wed, Jan 22, 2014 at 12:33 AM, Andrew Trick <atrick at apple.com> wrote:

> > There appear to be two chunks of "functionality" provided by loop passes:
> >
> > 1) A worklist of loops to process. This is very rarely used:
> > 1.1) LoopSimplify and LoopUnswitch add loops to the queue.
>
> I’m making this up without much thought, but we may benefit from iterative
> loop transforms like Rotate -> LICM -> Unswitch -> Rotate -> LICM. We might
> need to come up with a preferred alternative: we can either continue to
> convert transforms into a utilities, or we can invent new pass manager
> tricks. e.g. iterate over a set of function passes or selectively run a
> pass on “dirty” regions.


This is a really good point. Owen pointed it out to me as well in another
guise: when we unroll a loop, we need to re-run a bunch of the passes, but
re-running them when we *haven't* successfully unrolled a loop is a total
waste.

I'd like to think more about this, so a simpler option: what if we *just*
extract LoopSimplify and LCSSA? If all the LoopPasses preserve these, then
them being function passes would be fine. This would allow us to at least
*start* writing function passes over loops (like the LoopVectorizer) rather
than being forced to phrase them as LoopPasses.

I think I could implement this simpler option right away, and that would
unblock a lot of our efforts around unrolling, vectorizing, and PGO.
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