[llvm-dev] CFG SCCs vs Loops and loop breaking transformations

[Tobias Grosser via llvm-dev]

On 01/19/2016 08:17 PM, Philip Reames via llvm-dev wrote:
> I ran across an interesting case and wanted to share it.  I'm not
> proposing any particular changes, but the experience seemed interesting
> to discuss.
>
> First, a bit of background.  An LLVM Loop models a specific type of
> cycle in the CFG.  Not all cycles are Loops.  Many of our optimization
> transforms are phrased over loops, which means that a non-loop cycle
> tends to be less well optimized.
>
> I had some initial IR that had a very complex, oddly written loop. After
> running this through my pass order, I discovered that there was no
> longer a Loop representing the loop.  One of the transforms -
> SimplifyCFG is my guess, but I haven't confirmed - had taken something
> representable as a Loop and converted into a non-Loop SCC.  This seems
> unfortunate and raises a general issue with how we model and
> canonicalize loops.
>
> Long term, I see a couple of options:
> 1) Introduce a new notion for SCCs in the CFG, and rephrase select
> optimizations like LICM over them.  A Loop then becomes a particular
> special case of our more generic SCC concept.
> 2) Avoid breaking loops until some point late in the optimizer.
> Essentially, we designate the Loop representable form as being canonical
> and then lower later.
> 3) Introduce a SCC to Loop conversion pass which tries to take non-loop
> SCCs and make them representable as Loops.  We probably don't want to go
> full out here, but catching the "easy" cases might help a lot in
> practice.  Running this at the start of each LoopPassManager pipeline
> might be an option.  I was slightly surprised to notice we didn't
> already have this.
>
> None of these seem great.  Anyone have another idea on how we could
> approach this?

Hi Philip,

how did the loop look like? Could you provide an example? AFAIU LLVM 
detects all natural loops (cycles with a single entry block that 
dominates all basic blocks in the cycle). The only cycles that are not 
natural loops should be irregular control flow. Is this the case for 
your test case? Or do I miss something?

I would be very surprised if a transformation introduces irregular 
control flow and would like to understand why this would be needed. I 
currently work on the assumption that irregular control flow is rare
and likely not worth optimizing. If it is still performance relevant, I 
would probably just introduce some code duplication to introduce
proper natural loops.

Best,
Tobias

>
> Philip
>
> p.s. Thankfully, the code I noticed this in is not performance sensitive
> so I don't need a near term answer here.
>
>
>
>
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