[llvm] [LoopInterchange] Check all inner-exit LCSSA PHIs (PR #200860)

[Madhur Amilkanthwar via llvm-commits]

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@@ -1371,8 +1371,10 @@ areInnerLoopExitPHIsSupported(Loop *OuterL, Loop *InnerL,
     // from the loop latch.
     if (PHI.getNumIncomingValues() > 1)
       return false;
+    // The reduction LCSSA PHI's store user is rewritten by reduction2Memory();
+    // skip its user-check but keep validating the remaining LCSSA PHIs.
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madhur13490 wrote:

The PHIs iterated here are in `InnerL->getUniqueExitBlock()`, and as LoopInterchangePass runs as a LoopNestPass via the LoopPassManager, which guarantees `LoopSimplify` (thus the exit block has dedicated exits) and LCSSA. The earlier `getNumIncomingValues() > 1` condition in code further restricts to single-incoming PHIs. Under these preconditions every PHI we continue validating is an LCSSA PHI, so the wording "remaining LCSSA PHIs" is correct.

https://github.com/llvm/llvm-project/pull/200860