[LoopInterchange] Add more debug messages to currentLimitations().

[fhahn]

Summary

This makes it easier to find out which limitation prevented this pass from doing its work.

[mcrosier]

LGTM.

lib/Transforms/Scalar/LoopInterchange.cpp

bool LoopInterchangeLegality::currentLimitations() {
   BasicBlock *InnerLoopHeader = InnerLoop->getHeader();
   BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
   BasicBlock *OuterLoopLatch = OuterLoop->getLoopLatch();
   BasicBlock *OuterLoopHeader = OuterLoop->getHeader();
 
   PHINode *InnerInductionVar;
   SmallVector<PHINode *, 8> Inductions;
   SmallVector<PHINode *, 8> Reductions;
-  if (!findInductionAndReductions(InnerLoop, Inductions, Reductions))
+  if (!findInductionAndReductions(InnerLoop, Inductions, Reductions)) {
+    DEBUG(dbgs() << "Only inner loops with induction or reduction PHI nodes "
+                 << "are supported currently.\n");
     return true;
+  }
 
   // TODO: Currently we handle only loops with 1 induction variable.
   if (Inductions.size() != 1) {
     DEBUG(dbgs() << "We currently only support loops with 1 induction variable."
                  << "Failed to interchange due to current limitation\n");
     return true;
   }
   if (Reductions.size() > 0)
     InnerLoopHasReduction = true;
 
   InnerInductionVar = Inductions.pop_back_val();
   Reductions.clear();
-  if (!findInductionAndReductions(OuterLoop, Inductions, Reductions))
+  if (!findInductionAndReductions(OuterLoop, Inductions, Reductions)) {
+    DEBUG(dbgs() << "Only outer loops with induction or reduction PHI nodes "
+                 << "are supported currently.\n");
     return true;
+  }
 
   // Outer loop cannot have reduction because then loops will not be tightly
   // nested.
-  if (!Reductions.empty())
+  if (!Reductions.empty()) {
+    DEBUG(dbgs() << "Outer loops with reductions are not supported "
+                 << "currently.\n");
     return true;
+  }
   // TODO: Currently we handle only loops with 1 induction variable.
-  if (Inductions.size() != 1)
+  if (Inductions.size() != 1) {
+    DEBUG(dbgs() << "Loops with more than 1 induction variables are not "
+                 << "supported currently.\n");
     return true;
+  }
 
   // TODO: Triangular loops are not handled for now.
   if (!isLoopStructureUnderstood(InnerInductionVar)) {
     DEBUG(dbgs() << "Loop structure not understood by pass\n");
     return true;
   }
 
   // TODO: We only handle LCSSA PHI's corresponding to reduction for now.
   BasicBlock *LoopExitBlock =
       getLoopLatchExitBlock(OuterLoopLatch, OuterLoopHeader);
-  if (!LoopExitBlock || !containsSafePHI(LoopExitBlock, true))
+  if (!LoopExitBlock || !containsSafePHI(LoopExitBlock, true)) {
+    DEBUG(dbgs() << "Can only handle LCSSA PHIs in outer loops currently.\n");
     return true;
+  }
 
   LoopExitBlock = getLoopLatchExitBlock(InnerLoopLatch, InnerLoopHeader);
-  if (!LoopExitBlock || !containsSafePHI(LoopExitBlock, false))
+  if (!LoopExitBlock || !containsSafePHI(LoopExitBlock, false)) {
+    DEBUG(dbgs() << "Can only handle LCSSA PHIs in inner loops currently.\n");
     return true;
+  }
 
   // TODO: Current limitation: Since we split the inner loop latch at the point
   // were induction variable is incremented (induction.next); We cannot have
   // more than 1 user of induction.next since it would result in broken code
   // after split.
   // e.g.
   // for(i=0;i<N;i++) {
   //    for(j = 0;j<M;j++) {
   //      A[j+1][i+2] = A[j][i]+k;
   //  }
   // }
   Instruction *InnerIndexVarInc = nullptr;
   if (InnerInductionVar->getIncomingBlock(0) == InnerLoopPreHeader)
     InnerIndexVarInc =
         dyn_cast<Instruction>(InnerInductionVar->getIncomingValue(1));
   else
     InnerIndexVarInc =
         dyn_cast<Instruction>(InnerInductionVar->getIncomingValue(0));
 
-  if (!InnerIndexVarInc)
+  if (!InnerIndexVarInc) {
+    DEBUG(dbgs() << "Did not find an instruction to increment the induction "
+                 << "variable.\n");
     return true;
+  }
 
   // Since we split the inner loop latch on this induction variable. Make sure
   // we do not have any instruction between the induction variable and branch
   // instruction.
 
   bool FoundInduction = false;
   for (const Instruction &I : reverse(*InnerLoopLatch)) {
     if (isa<BranchInst>(I) || isa<CmpInst>(I) || isa<TruncInst>(I))
       continue;
+
     // We found an instruction. If this is not induction variable then it is not
     // safe to split this loop latch.
-    if (!I.isIdenticalTo(InnerIndexVarInc))
+    if (!I.isIdenticalTo(InnerIndexVarInc)) {
+      DEBUG(dbgs() << "Found unsupported instructions between induction "
+                   << "variable increment and branch.\n");
       return true;
+    }
 
     FoundInduction = true;
     break;
   }
   // The loop latch ended and we didn't find the induction variable return as
   // current limitation.
-  if (!FoundInduction)
+  if (!FoundInduction) {
+    DEBUG(dbgs() << "Did not find the induction variable.\n");
     return true;
-
+  }
   return false;
 }
 
 bool LoopInterchangeLegality::canInterchangeLoops(unsigned InnerLoopId,
                                                   unsigned OuterLoopId,
                                                   CharMatrix &DepMatrix) {
 
   if (!isLegalToInterChangeLoops(DepMatrix, InnerLoopId, OuterLoopId)) {