[LowerSwitch] Fix a bug when LowerSwitch deletes the default block

Summary: LowerSwitch crashed with the attached test case after deleting the default block. This happened because the current implementation of deleting dead blocks is wrong. After the default block being deleted, it contains no instruction or terminator, and it should no be traversed anymore. However, since the iterator is advanced before processSwitchInst() function is executed, the block advanced to could be deleted inside processSwitchInst(). The deleted block would then be visited next and crash dyn_cast<SwitchInst>(Cur->getTerminator()) because Cur->getTerminator() returns a nullptr. This patch fixes this problem by recording dead default blocks into a list, and delete them after all processSwitchInst() has been done. It still possible to visit dead default blocks and waste time process them. But it is a compile time issue, and I plan to have another patch to add support to skip dead blocks.

Reviewers: kariddi, resistor, hans, reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11852

llvm-svn: 244642

2 files changed, 37 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/LowerSwitch.cpp b/llvm/lib/Transforms/Utils/LowerSwitch.cpp
index 3b447098af1..0e47626ce53 100644
--- a/llvm/lib/Transforms/Utils/LowerSwitch.cpp
+++ b/llvm/lib/Transforms/Utils/LowerSwitch.cpp
@@ -78,7 +78,7 @@ namespace {
     typedef std::vector<CaseRange> CaseVector;
     typedef std::vector<CaseRange>::iterator CaseItr;
   private:
-    void processSwitchInst(SwitchInst *SI);
+    void processSwitchInst(SwitchInst *SI, SmallVectorImpl<BasicBlock*> &DeleteList);
 
     BasicBlock *switchConvert(CaseItr Begin, CaseItr End,
                               ConstantInt *LowerBound, ConstantInt *UpperBound,
@@ -116,16 +116,21 @@ FunctionPass *llvm::createLowerSwitchPass() {
 
 bool LowerSwitch::runOnFunction(Function &F) {
   bool Changed = false;
+  SmallVector<BasicBlock*, 8> DeleteList;
 
   for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
     BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
 
     if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
       Changed = true;
-      processSwitchInst(SI);
+      processSwitchInst(SI, DeleteList);
     }
   }
 
+  for (BasicBlock* BB: DeleteList) {
+    DeleteDeadBlock(BB);
+  }
+
   return Changed;
 }
 
@@ -397,7 +402,7 @@ unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) {
 // processSwitchInst - Replace the specified switch instruction with a sequence
 // of chained if-then insts in a balanced binary search.
 //
-void LowerSwitch::processSwitchInst(SwitchInst *SI) {
+void LowerSwitch::processSwitchInst(SwitchInst *SI, SmallVectorImpl<BasicBlock*> &DeleteList) {
   BasicBlock *CurBlock = SI->getParent();
   BasicBlock *OrigBlock = CurBlock;
   Function *F = CurBlock->getParent();
@@ -518,7 +523,7 @@ void LowerSwitch::processSwitchInst(SwitchInst *SI) {
   BasicBlock *OldDefault = SI->getDefaultDest();
   CurBlock->getInstList().erase(SI);
 
-  // If the Default block has no more predecessors just remove it.
+  // If the Default block has no more predecessors just add it to DeleteList.
   if (pred_begin(OldDefault) == pred_end(OldDefault))
-    DeleteDeadBlock(OldDefault);
+    DeleteList.push_back(OldDefault);
 }
diff --git a/llvm/test/Transforms/LowerSwitch/delete-default-block-crash.ll b/llvm/test/Transforms/LowerSwitch/delete-default-block-crash.ll
new file mode 100644
index 00000000000..23588d56c33
--- /dev/null
+++ b/llvm/test/Transforms/LowerSwitch/delete-default-block-crash.ll
@@ -0,0 +1,27 @@
+; RUN: opt < %s -lowerswitch -disable-output
+
+; This test verify -lowerswitch does not crash after deleting the default block.
+
+declare i32 @f(i32)
+
+define i32 @unreachable(i32 %x) {
+
+entry:
+  switch i32 %x, label %unreachable [
+    i32 5, label %a
+    i32 6, label %a
+    i32 7, label %a
+    i32 10, label %b
+    i32 20, label %b
+    i32 30, label %b
+    i32 40, label %b
+  ]
+unreachable:
+  unreachable
+a:
+  %0 = call i32 @f(i32 0)
+  ret i32 %0
+b:
+  %1 = call i32 @f(i32 1)
+  ret i32 %1
+}