[PATCH] D97290: [LoopInterchange] Replace tightly-nesting-ness check with the one from `LoopNest`

[Whitney Tsang via Phabricator via llvm-commits]

Whitney added inline comments.


================
Comment at: llvm/lib/Transforms/Scalar/LoopInterchange.cpp:453
   bool run(LoopNest &LN) {
-    const auto &LoopList = LN.getLoops();
-    for (unsigned I = 1; I < LoopList.size(); ++I)
-      if (LoopList[I]->getParentLoop() != LoopList[I - 1])
-        return false;
-    return processLoopList(LoopList);
+    if (!LN.isTotallyNested())
+      return false;
----------------
TaWeiTu wrote:
> Whitney wrote:
> > As `checkLoopsStructure` is going to check `the inner loop should be the outer loop's only child`, do we really need to check `isTotallyNested` here?
> > 
> I think the logic would be different if we remove `isTotallyNested`. For example, consider the following loop-nest:
> 
> ```
> for (i)
>   for (j1)
>     for (k)
>       for (l)
>   for (j2)
> ```
> 
> Before removing `isTotallyNested`, `k` and `l` can never be interchanged. But since we only check whether the two loops that are currently being interchanged are tightly-nested or not, `k` and `l` might get interchanged before realizing that `i` has two subloops, in which case we simply return from `processLoopList` without unrolling the changes.
> 
> I'm not sure whether `LoopInterchange` is intended to operate **only** on "totally nested" loops or not, because in the previous example swapping `k` and `l` does seem feasible.
> Anyhow, if such improvement is what we want, I think it will be better to have a separate patch for that.
> 
> What do you think? 
> Thanks!
If we only want to operate on perfect loop nest, then we can check `LN.getMaxPerfectDepth() == LN.getNestDepth()` here.
Then we don't need to add function `isTotallyNested`.


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https://reviews.llvm.org/D97290