[LoopUnswitch] Require DominatorTree info.

[mzolotukhin]

Summary

We should either require the DT info to be available, or check if it's
available in every place we use DT (and we already miss such check in
one place, which causes failures in some cases). As other loop passes
preserve DT and it's usually available, it makes sense to just require
it here.

[chandlerc]

LGTM

Mention in the commit log that this is really annoying to test for? =/ Somewhat sad to not have a test but I have no better ideas.

[mzolotukhin]

Thanks! Committed with the corresponding comment.

Michael

llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp

public:
     void getAnalysisUsage(AnalysisUsage &AU) const override {
       AU.addRequired<AssumptionCacheTracker>();
       AU.addRequiredID(LoopSimplifyID);
       AU.addPreservedID(LoopSimplifyID);
       AU.addRequired<LoopInfoWrapperPass>();
       AU.addPreserved<LoopInfoWrapperPass>();
       AU.addRequiredID(LCSSAID);
       AU.addPreservedID(LCSSAID);
+      AU.addRequired<DominatorTreeWrapperPass>();
       AU.addPreserved<DominatorTreeWrapperPass>();
       AU.addPreserved<ScalarEvolutionWrapperPass>();
       AU.addRequired<TargetTransformInfoWrapperPass>();
       AU.addPreserved<GlobalsAAWrapperPass>();
     }
 
   private:
 

 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
   if (skipOptnoneFunction(L))
     return false;
 
   AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
       *L->getHeader()->getParent());
   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
   LPM = &LPM_Ref;
-  DominatorTreeWrapperPass *DTWP =
-      getAnalysisIfAvailable<DominatorTreeWrapperPass>();
-  DT = DTWP ? &DTWP->getDomTree() : nullptr;
+  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
   currentLoop = L;
   Function *F = currentLoop->getHeader()->getParent();
   bool Changed = false;
   do {
-    assert(!DT || currentLoop->isLCSSAForm(*DT));
+    assert(currentLoop->isLCSSAForm(*DT));
     redoLoop = false;
     Changed |= processCurrentLoop();
   } while(redoLoop);
 
-  if (Changed) {
-    // FIXME: Reconstruct dom info, because it is not preserved properly.
-    if (DT)
-      DT->recalculate(*F);
-  }
+  if (Changed)
   return Changed;
 }
 
 /// Do actual work and unswitch loop if possible and profitable.
 bool LoopUnswitch::processCurrentLoop() {
   bool Changed = false;
 
   initLoopData();
for (User *U : LIC->users()) {
     for (BasicBlock::iterator II = NewSISucc->begin();
          PHINode *PN = dyn_cast<PHINode>(II); ++II)
       PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
                            UndefValue::get(PN->getType()));
     // Tell the domtree about the new block. We don't fully update the
     // domtree here -- instead we force it to do a full recomputation
     // after the pass is complete -- but we do need to inform it of
     // new blocks.
-    if (DT)
-      DT->addNewBlock(Abort, NewSISucc);
   }
 
   SimplifyCode(Worklist, L);
 }
 
 /// Now that we have simplified some instructions in the loop, walk over it and
 /// constant prop, dce, and fold control flow where possible. Note that this is
 /// effectively a very simple loop-structure-aware optimizer. During processing