[llvm] [VPlan] Introduce child regions as VPlan transform. (PR #129402)

[Florian Hahn via llvm-commits]

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@@ -376,23 +311,21 @@ void PlainCFGBuilder::buildPlainCFG(
   for (BasicBlock *BB : RPO) {
     // Create or retrieve the VPBasicBlock for this BB.
     VPBasicBlock *VPBB = getOrCreateVPBB(BB);
-    VPRegionBlock *Region = VPBB->getParent();
     Loop *LoopForBB = LI->getLoopFor(BB);
     // Set VPBB predecessors in the same order as they are in the incoming BB.
     if (!isHeaderBB(BB, LoopForBB)) {
       setVPBBPredsFromBB(VPBB, BB);
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fhahn wrote:

For now, the code relies on the header having a specific predecessor order (preheader, then latch). We might be able to remove this restriction, because the dominance based approach could be generalized to handle it. It would require extra logic to determine which block is the preheader/latch.

https://github.com/llvm/llvm-project/pull/129402