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

[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);
-    } else if (Region) {
-      // BB is a loop header and there's a corresponding region, set the
-      // predecessor for it.
-      setRegionPredsFromBB(Region, BB);
+    } else {
+      VPBB->setPredecessors({getOrCreateVPBB(LoopForBB->getLoopPredecessor()),
+                             getOrCreateVPBB(LoopForBB->getLoopLatch())});
     }
 
     // Create VPInstructions for BB.
     createVPInstructionsForVPBB(VPBB, BB);
 
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ayalz wrote:

Assumptions about successors and successor order should be consistent with terminating recipes that target them, including the removal of early-exit branches/successors. Perhaps similar to [header]phisToFix/fixHeaderPhis() that should arguably post-process predecessors to align with phi recipes as they are fixed, after first mirroring the IR predecessors as-is, a post-processing of VPIRInstructions that first wrap IR terminals should take care of both converting them into BranchOnCond/Switch VP[nonIR]Instructions along with possible swapping of successors, plus removal of early-exit branches? Can be done separately if preferred.

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