[PATCH] SimplifyCFG: don't remove unreachable defaults from switch instructions; exploit them instead!

[Hans Wennborg]

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D6471

Files:
  llvm/trunk/lib/Transforms/Utils/Local.cpp

Index: llvm/trunk/lib/Transforms/Utils/Local.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Utils/Local.cpp
+++ llvm/trunk/lib/Transforms/Utils/Local.cpp
@@ -110,11 +110,17 @@
   }
 
   if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
-    // If we are switching on a constant, we can convert the switch into a
-    // single branch instruction!
+    // If we are switching on a constant, we can convert the switch to an
+    // unconditional branch.
     ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
-    BasicBlock *TheOnlyDest = SI->getDefaultDest();
-    BasicBlock *DefaultDest = TheOnlyDest;
+    BasicBlock *DefaultDest = SI->getDefaultDest();
+    BasicBlock *TheOnlyDest = DefaultDest;
+
+    // If the default is unreachable, ignore it when searching for TheOnlyDest.
+    if (isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg()) &&
+        SI->getNumCases() > 0) {
+      TheOnlyDest = SI->case_begin().getCaseSuccessor();
+    }
 
     // Figure out which case it goes to.
     for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();

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