[PATCH] D131646: [clang][dataflow] Restructure loops to call widen on back edges

[Eric Li via Phabricator via cfe-commits]

li.zhe.hua marked an inline comment as done.
li.zhe.hua added inline comments.


================
Comment at: clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp:168-169
+// back edge block. (That is, all paths from the entry block to the back edge
+// block must go through `Block`.) It also means that there are only two
+// predecessors; the other is along the path from the entry block to `Block`.
+static const CFGBlock *findBackEdge(const CFGBlock *Block) {
----------------
xazax.hun wrote:
> Is this also true when we have multiple `continue` statements in the loop?
Yes. The end of the loop, and each of the `continue` statements, target the back edge block. They all get funneled through that back edge to `Block`, such that `Block` only has two predecessors. However, I haven't verified this in the CFG code, only by not finding a counterexample.


================
Comment at: clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp:228-234
+  // If we are at the start of a loop, we will have two precessors, but we don't
+  // want to join these two predecessors. Instead, we want to take the back edge
+  // block (i.e. the result of the previous iteration) and use that directly as
+  // the input block.
+  //
+  // For the first iteration of loop, the "zeroth" iteration state is set up by
+  // `prepareBackEdges`.
----------------
xazax.hun wrote:
> Could you elaborate on this? Let's consider this loop:
> 
> ```
>  Pred
>    |
>    v
>  LoopHeader <---BackEdge
> ```
> 
> Do we ignore the state coming from `Pred` on purpose? Is that sound?
> 
> I would expect the analysis to always compute `join(PredState, BackEdgeState)`, and I would expect the widening to happen between the previous iteration of `BackEdgeState` and the current iteration of `BackEdgeState`. So, I wonder if we already invoke the widening operator along back edges, wouldn't the regular logic work just fine? Do I miss something?
> 
> Do we ignore the state coming from `Pred` on purpose? Is that sound?

We don't, and this is what the comment

```
  // For the first iteration of loop, the "zeroth" iteration state is set up by
  // `prepareBackEdges`.
```
failed to explain. After transferring `PredState`, we copy `PredState` into `BackEdgeState`, which is done in `prepareBackEdges`.

> I would expect the analysis to always compute `join(PredState, BackEdgeState)`

I'm not sure I see that we should always join `PredState` and `BackEdgeState`. Execution doesn't go from `Pred` into the Nth iteration of the loop, it only goes from `Pred` into the first iteration of the loop, e.g. the predecessor for the 4th iteration of the loop is only the back-edge from the 3rd iteration of the loop, not `Pred`.

Let me know if this makes sense.


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https://reviews.llvm.org/D131646