[PATCH] D87293: [GVN] Fix undef incoming value for phi node when new loop exit block created.

[weiwei via Phabricator via llvm-commits]

wwei added inline comments.


================
Comment at: llvm/lib/Transforms/Scalar/GVN.cpp:2751
     }
 
+    for (BasicBlock *C : Candidates) {
----------------
asbirlea wrote:
> Could you please explain in more details the mechanics here? 
> When added into Candidates, the llvm::any_of condition is true. What makes the condition become false below?
> What's the purpose of the intersection of non-dead succs and preds?
I have updated the code, and some comments to descibe the details for it.

```
    //     - - - - - - - -
    //    |    ... ...    |
    //    |[D1]           |   =>  A simplified loop<L>
    //    | |  \          |
    //    | |   [D2] [B1] |
    //     -|- - |- - |- -
    //      |    |    |
    //      \    |    /
    //       \   |   /
    //        [ DF1 ]        => exit block for loop<L>
    //
    // Consider the above scenario, there're two dead blocks [D1] + [D2], and
// a live block[B1]([D1][D2][B1] in one loop). [DF1] will a common dominance
    // frontier block for [D1] and [D2], so Candidates will contain two elements
    // [D1, D2]. [D1] has two succs: [D2] and [DF1], for [D1],first llvm::any_of
    // will return true, so splitCriticalEdges will be called.
    //
    //     - - - - - - - -
    //    |    ... ...    |
    //    |[D1]           |   =>  A simplified loop<L>
    //    | |  \          |
    //    | |   [D2] [B1] |
    //     -|- - |- - |- -
    //      |    |    |
    //      |    \    /
    //     [D3]   [DF2]      => New Exit block[DF2] for loop<L>
    //       \      |
    //        [ DF1 ]
    //
    // After splitCriticalEdges, a new crit_edge block[D3] will be created, and
    // it is added into dead blocks. Also, a new loop exit block[DF2] is created
    // to preserve LoopSimplify. After that, llvm::any_of will return false for
    // [D2], and we need to add [DF2] as a new dominance frontier block for [D2]
    // since it has a pred edge to non-dead block[B1], and then, we need to
    // update the undef incoming value for phi in [DF2].
```

About the purpose of the intersection of non-dead succs and preds, we should consider another case like below:

```
//     - - - - - - - -
//    |    ... ...    |
//    |    ... ...    |  =>  A simplified loop<L>
//    | [  D1 ]  [B1] |
//     -|- |- |- -|- - 
//      |  |  |   |
//      |  |  |   |
//      \  |  |   |
//       \ |  /  /
//        [ DF1 ]        => exit block for loop<L>
//
// After split edeges,
//     - - - - - - - -
//    |    ... ...    |
//    |    ... ...    |  =>  A simplified loop<L>
//    | [  D1  ]  [B1]|
//     -|-  |- |- -|-- 
//      |   |  |   |
//      |   |  \   /
//     [D2][D3][DF2]     => New Exit block[DF2] for loop<L>
//       \  |    /
//        [ DF1 ]
//
//    switch-case: [  D1  ] will have multi edges to  [ DF1 ], after split,  [D2][D3] are new created,
//    both will be dead blocks, so we need to use  the intersection of non-dead succs and preds to get [DF2]
```


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D87293/new/

https://reviews.llvm.org/D87293