[llvm] r302354 - [Analysis] Print out unreachable loops

[Brian Gesiak via llvm-commits]

Author: modocache
Date: Sat May  6 11:22:53 2017
New Revision: 302354

URL: http://llvm.org/viewvc/llvm-project?rev=302354&view=rev
Log:
[Analysis] Print out unreachable loops

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.

Reviewers: sanjoy, hfinkel, mehdi_amini

Reviewed By: mehdi_amini

Subscribers: mzolotukhin

Differential Revision: https://reviews.llvm.org/D32878

Modified:
    llvm/trunk/include/llvm/Analysis/LoopInfoImpl.h

Modified: llvm/trunk/include/llvm/Analysis/LoopInfoImpl.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/LoopInfoImpl.h?rev=302354&r1=302353&r2=302354&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/LoopInfoImpl.h (original)
+++ llvm/trunk/include/llvm/Analysis/LoopInfoImpl.h Sat May  6 11:22:53 2017
@@ -220,8 +220,8 @@ void LoopBase<BlockT, LoopT>::verifyLoop
     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) {
@@ -259,10 +259,18 @@ void LoopBase<BlockT, LoopT>::verifyLoop
     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)