[llvm-dev] Inner Loop extraction in LLVM

[Johannes Doerfert via llvm-dev]

In the case below you probably don't need to split much.

It might be more complicated if the two loops share edges, i.a.,


```

for (...) {

   for (...) {

     if (...)

       return;

   }

}

```


I guess the trick is to isolate the inner loop properly so that 
extraction becomes trivial.

If you for example make sure there is a single "entering" edge (from 
outside the loop to the header),

and a single "exiting" edge (from the loop to a block not in the loop), 
outlining can be done with

the code extractor for blocks we already have. (Unclear about updating 
analyses though).





On 7/1/20 5:36 PM, iulian brumar wrote:
>   Hi Johannes,
>
> This is very helpful and thank you for the quick response.
>
> I imagine that the usual nested loop structure is something like:
>
> loop1_header_bb
> bbs_between_loop1_header_and_loop2_header
> loop2_header_bb
> loop_body_bbs
> br loop2_header_bb
> instructions_before_loop1_end_and_after_loop2_end
> br loop1_header_bb
>
> In this case you mean that loop2_header_bb might need to be split before extracting loop 2 into its own function? I'm not sure yet about how to use bb splitting to enable inner-loop extraction and what splitting edges means.
>
> Sincerely,
> Iulian
>       On Wednesday, July 1, 2020, 06:06:01 PM EDT, Johannes Doerfert <johannesdoerfert at gmail.com> wrote:
>   
>    
> Hi Iulian,
>   
>
>   
>   
> On first thought I would think that extracting top-level loops is only minimally easier than extracting any other loop level,
>   
> if you consider arbitrary input, e.g,. potential irreducible "outer-outer-most" loops. For inner ones you might want to split
>   
>   some blocks and edges to make sure they are not shared with the outer loop but other than that, I imagine it to be pretty similar.
>   
>   (This assumes you do not want to preserve things like LoopInfo, ScalarEvolution, ...).
>   
>   
> Extracting a single basic block is probably easier still. Again, assuming you do not want to update analyses.
>   
>
>   
>   
> If you want more details or think I haven't grasped the essence of your questions, feel free to say so :)
>   
>   
>
>   
>   
> ~ Johannes
>   
>
>   
>   
>
>   
>   On 7/1/20 4:23 PM, iulian brumar via llvm-dev wrote:
>    
>   Hello everyone,Quick question about loop extraction in llvm. I've been using the LoopExtractor pass in llvm/lib/Transforms/IPO/LoopExtractor.cpp to extract top level loops from programs.I'm wondering if extracting inner-most loops is any more complex than using the BlockExtractor pass in llvm/lib/Transforms/IPO/BlockExtractor.cpp and making sure that the basic blocks in the extracted region are single entry-single exit. Would there be any fundamental differences wrt what LoopExtractor is doing for top-level loops?Thanks,Iulian_______________________________________________LLVM Developers mailing listllvm-dev at lists.llvm.orghttps://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>     
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