[llvm] [SLP]Support revectorization of the previously vectorized scalars (PR #133091)

[Gaëtan Bossu via llvm-commits]

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@@ -14680,10 +14702,16 @@ BoUpSLP::isGatherShuffledSingleRegisterEntry(
       PHINode *UserPHI = UseEI.UserTE->State != TreeEntry::SplitVectorize
                              ? dyn_cast<PHINode>(UseEI.UserTE->getMainOp())
                              : nullptr;
-      const Instruction *InsertPt =
+      Instruction *InsertPt =
           UserPHI ? UserPHI->getIncomingBlock(UseEI.EdgeIdx)->getTerminator()
                   : &getLastInstructionInBundle(UseEI.UserTE);
       if (TEInsertPt == InsertPt) {
+        // If the schedulable insertion point is used in multiple entries - just
+        // exit, no known ordering at this point, available only after real
+        // scheduling.
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gbossu wrote:

Curious: What is "real scheduling"?

https://github.com/llvm/llvm-project/pull/133091