1 files changed, 40 insertions, 17 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index c3b197e97eb..a5fdfc55487 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2698,32 +2698,55 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
   if (SwitchMBB + 1 != FuncInfo.MF->end())
     NextBlock = SwitchMBB + 1;
 
+
+  // Create a vector of Cases, sorted so that we can efficiently create a binary
+  // search tree from them.
+  CaseVector Cases;
+  Clusterify(Cases, SI);
+
+  // Get the default destination MBB.
   MachineBasicBlock *Default = FuncInfo.MBBMap[SI.getDefaultDest()];
 
-  // If there is only the default destination, branch to it if it is not the
-  // next basic block.  Otherwise, just fall through.
-  if (!SI.getNumCases()) {
+  if (isa<UnreachableInst>(SI.getDefaultDest()->getFirstNonPHIOrDbg()) &&
+      !Cases.empty()) {
+    // Replace an unreachable default destination with the most popular case
+    // destination.
+    DenseMap<const BasicBlock *, uint64_t> Popularity;
+    uint64_t MaxPop = 0;
+    const BasicBlock *MaxBB = nullptr;
+    for (auto I : SI.cases()) {
+      const BasicBlock *BB = I.getCaseSuccessor();
+      if (++Popularity[BB] > MaxPop) {
+        MaxPop = Popularity[BB];
+        MaxBB = BB;
+      }
+    }
+
+    // Set new default.
+    assert(MaxPop > 0);
+    assert(MaxBB);
+    Default = FuncInfo.MBBMap[MaxBB];
+
+    // Remove cases that were pointing to the destination that is now the default.
+    Cases.erase(std::remove_if(Cases.begin(), Cases.end(),
+                               [&](const Case &C) { return C.BB == Default; }),
+                Cases.end());
+  }
+
+  // If there is only the default destination, go there directly.
+  if (Cases.empty()) {
     // Update machine-CFG edges.
     SwitchMBB->addSuccessor(Default);
 
     // If this is not a fall-through branch, emit the branch.
-    if (Default != NextBlock)
-      DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(),
-                              MVT::Other, getControlRoot(),
-                              DAG.getBasicBlock(Default)));
-
+    if (Default != NextBlock) {
+      DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(), MVT::Other,
+                              getControlRoot(), DAG.getBasicBlock(Default)));
+    }
     return;
   }
 
-  // If there are any non-default case statements, create a vector of Cases
-  // representing each one, and sort the vector so that we can efficiently
-  // create a binary search tree from them.
-  CaseVector Cases;
-  Clusterify(Cases, SI);
-
-  // Get the Value to be switched on and default basic blocks, which will be
-  // inserted into CaseBlock records, representing basic blocks in the binary
-  // search tree.
+  // Get the Value to be switched on.
   const Value *SV = SI.getCondition();
 
   // Push the initial CaseRec onto the worklist