[LoopPeel] Use reference instead of pointer for DT argument

Cleanup code in peelLoop API. We already have usage of DT without guarding
against a null DT, so this change constant folds the remaining null DT
checks.
Also make the argument a reference so that it is clear the argument is
a nonnull DT.
Extracted from D118472.

llvm/include/llvm/Transforms/Utils/LoopPeel.h

@@ -21,7 +21,7 @@ namespace llvm {
 bool canPeel(Loop *L);
 
 bool peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI, ScalarEvolution *SE,
-              DominatorTree *DT, AssumptionCache *AC, bool PreserveLCSSA);
+              DominatorTree &DT, AssumptionCache *AC, bool PreserveLCSSA);
 
 TargetTransformInfo::PeelingPreferences
 gatherPeelingPreferences(Loop *L, ScalarEvolution &SE,

llvm/lib/Transforms/Scalar/LoopFuse.cpp

@@ -767,7 +767,7 @@ struct LoopFuser {
     LLVM_DEBUG(dbgs() << "Attempting to peel first " << PeelCount
                       << " iterations of the first loop. \n");
 
-    FC0.Peeled = peelLoop(FC0.L, PeelCount, &LI, &SE, &DT, &AC, true);
+    FC0.Peeled = peelLoop(FC0.L, PeelCount, &LI, &SE, DT, &AC, true);
     if (FC0.Peeled) {
       LLVM_DEBUG(dbgs() << "Done Peeling\n");
 

llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp

@@ -1281,7 +1281,7 @@ static LoopUnrollResult tryToUnrollLoop(
              << " iterations";
     });
 
-    if (peelLoop(L, PP.PeelCount, LI, &SE, &DT, &AC, PreserveLCSSA)) {
+    if (peelLoop(L, PP.PeelCount, LI, &SE, DT, &AC, PreserveLCSSA)) {
       simplifyLoopAfterUnroll(L, true, LI, &SE, &DT, &AC, &TTI);
       // If the loop was peeled, we already "used up" the profile information
       // we had, so we don't want to unroll or peel again.

llvm/lib/Transforms/Utils/LoopPeel.cpp

@@ -737,7 +737,7 @@ TargetTransformInfo::PeelingPreferences llvm::gatherPeelingPreferences(
 /// for the bulk of dynamic execution, can be further simplified by scalar
 /// optimizations.
 bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
-                    ScalarEvolution *SE, DominatorTree *DT, AssumptionCache *AC,
+                    ScalarEvolution *SE, DominatorTree &DT, AssumptionCache *AC,
                     bool PreserveLCSSA) {
   assert(PeelCount > 0 && "Attempt to peel out zero iterations?");
   assert(canPeel(L) && "Attempt to peel a loop which is not peelable?");
@@ -756,23 +756,21 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
   // routes which can lead to the exit: we can reach it from the peeled
   // iterations too.
   DenseMap<BasicBlock *, BasicBlock *> NonLoopBlocksIDom;
-  if (DT) {
-    for (auto *BB : L->blocks()) {
-      auto *BBDomNode = DT->getNode(BB);
-      SmallVector<BasicBlock *, 16> ChildrenToUpdate;
-      for (auto *ChildDomNode : BBDomNode->children()) {
-        auto *ChildBB = ChildDomNode->getBlock();
-        if (!L->contains(ChildBB))
-          ChildrenToUpdate.push_back(ChildBB);
-      }
-      // The new idom of the block will be the nearest common dominator
-      // of all copies of the previous idom. This is equivalent to the
-      // nearest common dominator of the previous idom and the first latch,
-      // which dominates all copies of the previous idom.
-      BasicBlock *NewIDom = DT->findNearestCommonDominator(BB, Latch);
-      for (auto *ChildBB : ChildrenToUpdate)
-        NonLoopBlocksIDom[ChildBB] = NewIDom;
+  for (auto *BB : L->blocks()) {
+    auto *BBDomNode = DT.getNode(BB);
+    SmallVector<BasicBlock *, 16> ChildrenToUpdate;
+    for (auto *ChildDomNode : BBDomNode->children()) {
+      auto *ChildBB = ChildDomNode->getBlock();
+      if (!L->contains(ChildBB))
+        ChildrenToUpdate.push_back(ChildBB);
     }
+    // The new idom of the block will be the nearest common dominator
+    // of all copies of the previous idom. This is equivalent to the
+    // nearest common dominator of the previous idom and the first latch,
+    // which dominates all copies of the previous idom.
+    BasicBlock *NewIDom = DT.findNearestCommonDominator(BB, Latch);
+    for (auto *ChildBB : ChildrenToUpdate)
+      NonLoopBlocksIDom[ChildBB] = NewIDom;
   }
 
   Function *F = Header->getParent();
@@ -822,11 +820,11 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
   //  If (cond) goto Header
   // Exit:
 
-  BasicBlock *InsertTop = SplitEdge(PreHeader, Header, DT, LI);
+  BasicBlock *InsertTop = SplitEdge(PreHeader, Header, &DT, LI);
   BasicBlock *InsertBot =
-      SplitBlock(InsertTop, InsertTop->getTerminator(), DT, LI);
+      SplitBlock(InsertTop, InsertTop->getTerminator(), &DT, LI);
   BasicBlock *NewPreHeader =
-      SplitBlock(InsertBot, InsertBot->getTerminator(), DT, LI);
+      SplitBlock(InsertBot, InsertBot->getTerminator(), &DT, LI);
 
   InsertTop->setName(Header->getName() + ".peel.begin");
   InsertBot->setName(Header->getName() + ".peel.next");
@@ -852,23 +850,21 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
     ValueToValueMapTy VMap;
 
     cloneLoopBlocks(L, Iter, InsertTop, InsertBot, ExitEdges, NewBlocks,
-                    LoopBlocks, VMap, LVMap, DT, LI,
+                    LoopBlocks, VMap, LVMap, &DT, LI,
                     LoopLocalNoAliasDeclScopes);
 
     // Remap to use values from the current iteration instead of the
     // previous one.
     remapInstructionsInBlocks(NewBlocks, VMap);
 
-    if (DT) {
-      // Update IDoms of the blocks reachable through exits.
-      if (Iter == 0)
-        for (auto BBIDom : NonLoopBlocksIDom)
-          DT->changeImmediateDominator(BBIDom.first,
-                                       cast<BasicBlock>(LVMap[BBIDom.second]));
+    // Update IDoms of the blocks reachable through exits.
+    if (Iter == 0)
+      for (auto BBIDom : NonLoopBlocksIDom)
+        DT.changeImmediateDominator(BBIDom.first,
+                                     cast<BasicBlock>(LVMap[BBIDom.second]));
 #ifdef EXPENSIVE_CHECKS
-      assert(DT->verify(DominatorTree::VerificationLevel::Fast));
+    assert(DT.verify(DominatorTree::VerificationLevel::Fast));
 #endif
-    }
 
     auto *LatchBRCopy = cast<BranchInst>(VMap[LatchBR]);
     updateBranchWeights(InsertBot, LatchBRCopy, ExitWeight, FallThroughWeight);
@@ -877,7 +873,7 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
     LatchBRCopy->setMetadata(LLVMContext::MD_loop, nullptr);
 
     InsertTop = InsertBot;
-    InsertBot = SplitBlock(InsertBot, InsertBot->getTerminator(), DT, LI);
+    InsertBot = SplitBlock(InsertBot, InsertBot->getTerminator(), &DT, LI);
     InsertBot->setName(Header->getName() + ".peel.next");
 
     F->getBasicBlockList().splice(InsertTop->getIterator(),
@@ -912,10 +908,10 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
   SE->forgetTopmostLoop(L);
 
   // Finally DomtTree must be correct.
-  assert(DT->verify(DominatorTree::VerificationLevel::Fast));
+  assert(DT.verify(DominatorTree::VerificationLevel::Fast));
 
   // FIXME: Incrementally update loop-simplify
-  simplifyLoop(L, DT, LI, SE, AC, nullptr, PreserveLCSSA);
+  simplifyLoop(L, &DT, LI, SE, AC, nullptr, PreserveLCSSA);
 
   NumPeeled++;