[llvm-commits] [PATCH] [msan] Remove unreachable bb's

[Evgeniy Stepanov]

Hi kcc, chandlerc,

In the presence of unreachable blocks, we may see Phi nodes with
incoming nodes from such blocks. Since InstVisitor skips unreachable blocks, such nodes will not have any shadow value associated with them. It's easier to remove unreachable blocks than deal with missing shadow.


http://llvm-reviews.chandlerc.com/D208

Files:
  llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp

Index: llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
===================================================================
--- llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -473,10 +473,46 @@
     DEBUG(dbgs() << "DONE:\n" << F);
   }
 
+  /// \brief Remove unreachable basic blocks from the function.
+  ///
+  /// In the presence of unreachable blocks, we may see Phi nodes with
+  /// incoming nodes from such blocks. Since InstVisitor skips unreachable
+  /// blocks, such nodes will not have any shadow value associated with them.
+  /// It's easier to remove unreachable blocks than deal with missing shadow.
+  void removeUnreachableBlocks() {
+    SmallPtrSet<BasicBlock*, 128> Reachable;
+    SmallVector<BasicBlock*, 128> Worklist;
+    Worklist.push_back(&F.getEntryBlock());
+    do {
+      BasicBlock *BB = Worklist.pop_back_val();
+      if (!Reachable.insert(BB))
+        continue;
+      for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
+        Worklist.push_back(*SI);
+    } while (!Worklist.empty());
+
+    if (Reachable.size() == F.size())
+      return;
+
+    assert(Reachable.size() < F.size());
+    for (Function::iterator BB = ++F.begin(); BB != F.end();)
+      if (!Reachable.count(BB)) {
+        for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
+          if (Reachable.count(*SI))
+            (*SI)->removePredecessor(BB);
+        BB->dropAllReferences();
+        BB = F.getBasicBlockList().erase(BB);
+      } else
+        ++BB;
+  }
+
   /// \brief Add MemorySanitizer instrumentation to a function.
   bool runOnFunction() {
     MS.initializeCallbacks(*F.getParent());
     if (!MS.TD) return false;
+
+    removeUnreachableBlocks();
+
     // Iterate all BBs in depth-first order and create shadow instructions
     // for all instructions (where applicable).
     // For PHI nodes we create dummy shadow PHIs which will be finalized later.
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