[llvm] [LoopUtils] Fix metadata generated by makeFollowupLoopID (PR #131985)

[Ryotaro Kasuga via llvm-commits]

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@@ -317,6 +317,35 @@ std::optional<MDNode *> llvm::makeFollowupLoopID(
 
     HasAnyFollowup = true;
     for (const MDOperand &Option : drop_begin(FollowupNode->operands())) {
+      // The followup metadata typically forms as follows:
+      //
+      //   !0 = distinct !{!0, !1, !2}
+      //   !1 = !{!"llvm.loop.distribute.enable", i1 true}
+      //   !2 = !{!"llvm.loop.distribute.followup_all", !3}
+      //   !3 = distinct !{!3, !4}
+      //   !4 = !{!"llvm.loop.vectorize.enable", i1 true}
+      //
+      // If we push Option (!3 in this case) in MDs, the new metadata looks
+      // something like:
+      //
+      //   !5 = distinct !{!5, !3}
+      //
+      // This doesn't contain !4, so the vectorization pass doesn't recognize
+      // this loop as vectorization enabled. To make the new metadata contain !4
+      // instead of !3, traverse all of Option's operands and push them into
+      // MDs if Option seems to be a LoopID.
+      if (auto *MDN = dyn_cast<MDNode>(Option)) {
+        // TODO: Is there a proper way to detect LoopID?
+        if (MDN->getNumOperands() > 1 && MDN->getOperand(0) == MDN) {
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kasuga-fj wrote:

I don't think this is a very good implementation, but I couldn't think of a better way.

It may be better to force the followup metadata to have exactly one LoopID. As far as I've investigated, some tests (e.g., `Transforms/LoopDistribute/followup.ll`) contain followup metadata with non-LoopID data, but I believe we can fix them easily. Also, if my understanding is correct, those which actually generated by the frontend seem to have exactly one LoopID. Would it make sense to set such a restriction?

https://github.com/llvm/llvm-project/pull/131985