[PATCH] D118808: Loop Strength Reduce - Optimize unused IVs to final values in the exit block with SCEV

[Zaara Syeda via Phabricator via llvm-commits]

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In D118808#3338917 <https://reviews.llvm.org/D118808#3338917>, @Whitney wrote:

> Is it possible to abstract the portion of the code in IndVarSimplify to simplify this code pattern, and call that function instead of creating your own `ReplaceExitPHIsWithFinalVal`?

IndVarSimply calls rewriteLoopExitValues from LoopUtils to do this optimization. I tried to call the same function in LoopStrengthReduce, however rewriteLoopExitValues asserts that it requires: L->isRecursivelyLCSSAForm(*DT, *LI). 
When calling rewriteLoopExitValues from LoopStrengthReduce, this assert is not met.



================
Comment at: llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp:5644
+      continue;
+    for (auto U : IV->users()) {
+        if (U != IVNext) {
----------------
Whitney wrote:
> In what situation, that IV only has a single use (`IV->getNumUses() != 1`) and one of the incoming values of `IV` is `IVNext`, but `U != IVNext`?
There seems to be cases like this in the lit tests. Here an example from llvm/test/CodeGen/PowerPC/common-chain.ll

Exit block includes: 

```
%add23.3.lcssa = phi i64 [ %add23.3, %for.body ]
```
So  %add23.3 is what we are looking to replace.

for.body include these instructions:

```
%inc.addr.050 = phi i64 [ %inc4, %for.body.preheader.new ], [ %add23.3, %for.body ]
%add23 = add nsw i64 %inc.addr.050, %inc4
%add23.3 = add i64 %add23.2, %inc4
```

The IV here is: %inc.addr.050 and it has only one use of:
%add23 = add nsw i64 %inc.addr.050, %inc4
which is not the same as IVNext: %add23.3


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https://reviews.llvm.org/D118808