[LV] Remove implicit single basic block assumption

[gilr]

Summary

This patch is part of D28975's breakdown - no change in output intended.

LV's code currently assumes the vectorized loop is a single basic block up until predicateInstructions() is called. This patch removes two manifestations of this assumption (loop phi incoming values, dominator tree update) by replacing the use of vectorLoopBody with the vectorized loop's latch/header.

[mkuper]

LGTM

lib/Transforms/Vectorize/LoopVectorize.cpp

5054

Should this be

DT->addNewBlock(LI->getLoopFor(LoopVectorBody)->getHeader(), LoopVectorPreHeader);

as long as we're at it?

[gilr]

Applied Michael's comment and removed the comment explaining how using the body as header/latch is safe at this point.

lib/Transforms/Vectorize/LoopVectorize.cpp

void InnerLoopVectorizer::fixReduction(PHINode *Phi) {
   const VectorParts &Val = getVectorValue(LoopVal);
   for (unsigned part = 0; part < UF; ++part) {
     // Make sure to add the reduction stat value only to the
     // first unroll part.
     Value *StartVal = (part == 0) ? VectorStart : Identity;
     cast<PHINode>(VecRdxPhi[part])
       ->addIncoming(StartVal, LoopVectorPreHeader);
     cast<PHINode>(VecRdxPhi[part])
-      ->addIncoming(Val[part], LoopVectorBody);
+      ->addIncoming(Val[part], LI->getLoopFor(LoopVectorBody)->getLoopLatch());
   }
 
   // Before each round, move the insertion point right between
   // the PHIs and the values we are going to write.
   // This allows us to write both PHINodes and the extractelement
   // instructions.
   Builder.SetInsertPoint(&*LoopMiddleBlock->getFirstInsertionPt());
 
void InnerLoopVectorizer::updateAnalysis() {
   PSE.getSE()->forgetLoop(OrigLoop);
 
   // Update the dominator tree information.
   assert(DT->properlyDominates(LoopBypassBlocks.front(), LoopExitBlock) &&
          "Entry does not dominate exit.");
 
   // We don't predicate stores by this point, so the vector body should be a
   // single loop.
   DT->addNewBlock(LoopVectorBody, LoopVectorPreHeader);
 
-  DT->addNewBlock(LoopMiddleBlock, LoopVectorBody);
+  DT->addNewBlock(LoopMiddleBlock,
+                  LI->getLoopFor(LoopVectorBody)->getLoopLatch());
   DT->addNewBlock(LoopScalarPreHeader, LoopBypassBlocks[0]);
   DT->changeImmediateDominator(LoopScalarBody, LoopScalarPreHeader);
   DT->changeImmediateDominator(LoopExitBlock, LoopBypassBlocks[0]);
 
   DEBUG(DT->verifyDomTree());
 }
 
 /// \brief Check whether it is safe to if-convert this phi node.