[llvm] [LoopInterchange] Reject inner preheader PHIs with non-identical incoming values (PR #203842)

[Madhur Amilkanthwar via llvm-commits]

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@@ -1541,7 +1531,26 @@ bool LoopInterchangeLegality::canInterchangeLoops(unsigned InnerLoopId,
     return false;
   }
 
-  if (!areInnerLoopLatchPHIsSupported(InnerLoop)) {
+  BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
+  // Inner-preheader PHIs are removed by substituting each with its first
+  // incoming value. That is only correct when all incoming values are the
+  // same; a PHI with different incoming values would produce the wrong result.
+  if (InnerLoopPreHeader != OuterLoop->getHeader() &&
+      any_of(InnerLoopPreHeader->phis(),
+             [](PHINode &PHI) { return !all_equal(PHI.incoming_values()); })) {
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madhur13490 wrote:

yes, I think I was over-thinking. Made the changes.

https://github.com/llvm/llvm-project/pull/203842