[llvm] [VPlan] Refactor VPlan creation, add transform introducing region (NFC). (PR #128419)

[via llvm-commits]

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@@ -353,29 +353,6 @@ void PlainCFGBuilder::createVPInstructionsForVPBB(VPBasicBlock *VPBB,
 // Main interface to build the plain CFG.
 void PlainCFGBuilder::buildPlainCFG(
     DenseMap<VPBlockBase *, BasicBlock *> &VPB2IRBB) {
-  // 0. Reuse the top-level region, vector-preheader and exit VPBBs from the
-  // skeleton. These were created directly rather than via getOrCreateVPBB(),
-  // revisit them now to update BB2VPBB. Note that header/entry and
-  // latch/exiting VPBB's of top-level region have yet to be created.
-  VPRegionBlock *TheRegion = Plan.getVectorLoopRegion();
-  BasicBlock *ThePreheaderBB = TheLoop->getLoopPreheader();
-  assert((ThePreheaderBB->getTerminator()->getNumSuccessors() == 1) &&
-         "Unexpected loop preheader");
-  auto *VectorPreheaderVPBB =
-      cast<VPBasicBlock>(TheRegion->getSinglePredecessor());
-  // ThePreheaderBB conceptually corresponds to both Plan.getPreheader() (which
-  // wraps the original preheader BB) and Plan.getEntry() (which represents the
-  // new vector preheader); here we're interested in setting BB2VPBB to the
-  // latter.
-  BB2VPBB[ThePreheaderBB] = VectorPreheaderVPBB;
-  Loop2Region[LI->getLoopFor(TheLoop->getHeader())] = TheRegion;
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ayalz wrote:

(btw, `LI->getLoopFor(TheLoop->getHeader())` is aka `TheLoop`?)

https://github.com/llvm/llvm-project/pull/128419