[PATCH] D35918: [GVNHoist] Factor out reachability to search for anticipable instructions quickly

[Jakub Kuderski via Phabricator via llvm-commits]

kuhar added a comment.

In https://reviews.llvm.org/D35918#831102, @hiraditya wrote:

>




> Try  `./bin/opt -S -adce < llvm/test/Transforms/GVNHoist/infinite-loop-direct.ll`
> 
> with the following patch. I added the code here for easy reproducibility.
> 
>   diff --git a/lib/Transforms/Scalar/ADCE.cpp b/lib/Transforms/Scalar/ADCE.cpp
>   index 5b467dc..eb711b9 100644
>   --- a/lib/Transforms/Scalar/ADCE.cpp
>   +++ b/lib/Transforms/Scalar/ADCE.cpp
>   @@ -164,6 +164,10 @@ public:
>    }
>   
>    bool AggressiveDeadCodeElimination::performDeadCodeElimination() {
>   +  auto PrevBB = PDT.getNode(nullptr);
>   +  for (auto it = df_begin(PrevBB), E = df_end(PrevBB); it != E; ++it) {
>   +      dbgs() << "\nTest\n";
>   +  }
>    
> 
> Here is the reduced test case:
> 
>   $ cat a.ll
>    ; ModuleID = 'bugpoint-reduced-simplified.bc'
>    source_filename = "bugpoint-output-f4ca947.bc"
>    target triple = "x86_64-unknown-linux-gnu"
>    
>    define void @bazv1() local_unnamed_addr {
>    entry:
>      br label %while.cond
>    
>    while.cond:                                       ; preds = %while.cond, %entry
>      br label %while.cond
>    }
>    

Oh, yes, that happens because the entire function is reverse-unreachable, which causes the tree to be completely empty. https://reviews.llvm.org/D35851 puts reverse-unreachable CFG nodes in the tree and solves this problem.
In the meanwhile, you can check if `PDT.getNode(nullptr)` returns a nullptr and not iterate over it that situation.


https://reviews.llvm.org/D35918