[PATCH] D74691: [Attributor] Detect SCCs with unbounded cycles

[Stefanos Baziotis via Phabricator via llvm-commits]

baziotis added a comment.

In D74691#1888491 <https://reviews.llvm.org/D74691#1888491>, @omarahmed wrote:

> In D74691#1888461 <https://reviews.llvm.org/D74691#1888461>, @baziotis wrote:
>
> > > btw, I recommend that you try to answer questions by following the code - it's always a learning experience).
> >
> > Since we took that road, let's have a little more fun. Let's start simple by printing the SCCs we get. A simple way to do that is with sth like:
> >
> >   for (scc_iterator<Function *> It = scc_begin(&F), IE = scc_end(&F); It != IE;
> >        ++It) {
> >  
> >     const std::vector<BasicBlock *> &SCCBBs = *It;
> >     for (BasicBlock *BB : SCCBBs) {
> >       dbgs() << *BB << "\n";
> >     }
> >     dbgs() << "----- END -----\n\n\n\n\n";
> >   }
> >
> >
> > But, we can actually do better if we take a graphical view of the CFG, with `view-cfg`. You can do that with sth like:
> >  `./bin/opt -view-cfg test.ll`
> >  Assuming that you are in the `llvm-project` dir, that you have built `opt` and that your file is `test.ll`. Now, this will generate a `.dot` file.
> >  This is a special "graphics" format for which we should not care about right now. If you do that, you'll probably see something like:
> >
> >   ...
> >   Writing '/tmp/cfgnon_loop_inside_loop-a0c3a3.dot'...  done. 
> >   Trying 'xdg-open' program... Remember to erase graph file: /tmp/cfgnon_loop_inside_loop-a0c3a3.dot
> >   gio: file:///tmp/cfgnon_loop_inside_loop-a0c3a3.dot: No application is registered as handling this file 
> >
> >
> > For me, it created `/tmp/cfgnon_loop_inside_loop-a0c3a3.dot` (you won't necessarily have the same name, that's ok). Then, it will try to find
> >  default program to open it (with the `xdg-open` command), which as you can see, for me it didn't work. Basically, you now have to have a program that understands
> >  that file. The `dot` app will do the job, but you'll probably won't have it by default. Search online for how to install graphviz package. For example, in Ubuntu I think you can do it with `sudo apt-get install graphviz`.
> >  Finally, you should be able to create a PDF out of the `.dot` file as: 
> >  `dot -Tpdf /tmp/cfgnon_loop_inside_loop-a0c3a3.dot -o <pdf_filename>.pdf`
> >  Then, you can open the pdf and expect to see sth like this: https://imgur.com/a/oW2xgNt
> >  I think viewing CFGs like that, at times can be //very// helpful. In this case for example, it becomes instantly apparent that the `for` and the `while` don't form a SCC.
>
>
> wow, that grapical approach is beautiful , thanks


No problem :)

> regarding the old approach i tested it with the if-then-else test i listed before
> 
>   define i32* @external_sink_ret2_nrw(i32* %n0, i32* %r0, i32* %w0) {
>   entry:
>     %tobool = icmp ne i32* %n0, null
>     br i1 %tobool, label %if.end, label %if.then
>   
>   if.then:                                          ; preds = %entry
>     br label %return
>   
>   if.end:                                           ; preds = %entry
>     %0 = load i32, i32* %r0, align 4
>     store i32 %0, i32* %w0, align 4
>     br label %return
>   
>   return:                                           ; preds = %if.end, %if.then
>     ret i32* %w0
>   }
> 
> 
> and it seems to work fine with it after adding the analysis and maxTripCount part but it fails in another tests , I will work on them tmw and submit another diff

Ok, good. I'll check the method again tomorrow.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D74691/new/

https://reviews.llvm.org/D74691