[Analysis] Print out unreachable loops

[modocache]

Summary

When writing a loop pass I made a mistake and hit the assertion
"Unreachable block in loop". Later, I hit an assertion when I called
BasicBlock::eraseFromParent() incorrectly: "Use still stuck around
after Def is destroyed". This latter assertion, however, printed out
exactly which value is being deleted and what uses remain, which helped
me debug the issue.

To help people debugging their loop passes in the future, print out
exactly which basic block is unreachable in a loop.

[mehdi_amini]

LGTM

[modocache]

Thank you, Mehdi! :)

[davide]

include/llvm/Analysis/LoopInfoImpl.h

272

We prefer llvm_unreachable in this case.

include/llvm/Analysis/LoopInfoImpl.h

#ifndef NDEBUG
   SmallVector<BlockT*, 8> ExitBBs;
   getExitBlocks(ExitBBs);
   df_iterator_default_set<BlockT*> VisitSet;
   VisitSet.insert(ExitBBs.begin(), ExitBBs.end());
   df_ext_iterator<BlockT*, df_iterator_default_set<BlockT*>>
     BI = df_ext_begin(getHeader(), VisitSet),
     BE = df_ext_end(getHeader(), VisitSet);
 
-  // Keep track of the number of BBs visited.
-  unsigned NumVisited = 0;
+  // Keep track of the BBs visited.
+  SmallPtrSet<BlockT*, 8> VisitedBBs;
 
   // Check the individual blocks.
   for ( ; BI != BE; ++BI) {
     BlockT *BB = *BI;
 
     assert(std::any_of(GraphTraits<BlockT*>::child_begin(BB),
                        GraphTraits<BlockT*>::child_end(BB),
                        [&](BlockT *B){return contains(B);}) &&
if (BB == getHeader()) {
       for (BlockT *CB : depth_first(EntryBB))
         for (unsigned i = 0, e = OutsideLoopPreds.size(); i != e; ++i)
           assert(CB != OutsideLoopPreds[i] &&
                  "Loop has multiple entry points!");
     }
     assert(BB != &getHeader()->getParent()->front() &&
            "Loop contains function entry block!");
 
-    NumVisited++;
+    VisitedBBs.insert(BB);
   }
 
-  assert(NumVisited == getNumBlocks() && "Unreachable block in loop");
+  if (VisitedBBs.size() != getNumBlocks()) {
+    dbgs() << "The following blocks are unreachable in the loop: ";
+    for (auto BB : Blocks) {
+      if (!VisitedBBs.count(BB)) {
+        dbgs() << *BB << "\n";
+      }
+    }
+    assert(false && "Unreachable block in loop");
+  }
 
   // Check the subloops.
   for (iterator I = begin(), E = end(); I != E; ++I)
     // Each block in each subloop should be contained within this loop.
     for (block_iterator BI = (*I)->block_begin(), BE = (*I)->block_end();
          BI != BE; ++BI) {
         assert(contains(*BI) &&
                "Loop does not contain all the blocks of a subloop!");