A machine basic block may end in an unconditional branch, however it may have

more than one successor. Normally, these extra successors are dead. However,
some of them may branch to exception handling landing pads. If we remove those
successors, then the landing pads could go away if all predecessors to it are
removed. Before, it was checking if the direct successor was the landing
pad. But it could be the result of jumping through multiple basic blocks to get
to it. If we were to only check for the existence of an EH_LABEL in the basic
block and not remove successors if it's in there, then it could stop actually
dead basic blocks from being removed.

llvm-svn: 91092

include/llvm/CodeGen/MachineBasicBlock.h

@@ -327,6 +327,11 @@ public:
   /// 'Old', change the code and CFG so that it branches to 'New' instead.
   void ReplaceUsesOfBlockWith(MachineBasicBlock *Old, MachineBasicBlock *New);
 
+  /// BranchesToLandingPad - The basic block branches only to a landing pad or
+  /// to another basic block which branches only to a landing pad. No other
+  /// instructions are present other than the unconditional branch.
+  bool BranchesToLandingPad(const MachineBasicBlock *MBB) const;
+
   /// CorrectExtraCFGEdges - Various pieces of code can cause excess edges in
   /// the CFG to be inserted.  If we have proven that MBB can only branch to
   /// DestA and DestB, remove any other MBB successors from the CFG. DestA and

lib/CodeGen/MachineBasicBlock.cpp

@@ -13,15 +13,16 @@
 
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/BasicBlock.h"
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/Assembly/Writer.h"
 #include "llvm/CodeGen/MachineFunction.h"
-#include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/Target/TargetInstrDesc.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetMachine.h"
+#include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/Support/LeakDetector.h"
 #include "llvm/Support/raw_ostream.h"
-#include "llvm/Assembly/Writer.h"
 #include <algorithm>
 using namespace llvm;
 
@@ -448,10 +449,35 @@ void MachineBasicBlock::ReplaceUsesOfBlockWith(MachineBasicBlock *Old,
   addSuccessor(New);
 }
 
+/// BranchesToLandingPad - The basic block branches only to a landing pad or to
+/// another basic block which branches only to a landing pad. No other
+/// instructions are present other than the unconditional branch.
+bool
+MachineBasicBlock::BranchesToLandingPad(const MachineBasicBlock *MBB) const {
+  SmallSet<const MachineBasicBlock*, 32> Visited;
+  const MachineBasicBlock *CurMBB = MBB;
+
+  while (!Visited.count(CurMBB) && !CurMBB->isLandingPad()) {
+    if (CurMBB->size() != 1 || CurMBB->succ_empty() || CurMBB->succ_size() != 1)
+      break;
+
+    const TargetInstrInfo *TII =
+      CurMBB->getParent()->getTarget().getInstrInfo();
+    if (!TII->isUnpredicatedTerminator(CurMBB->begin()))
+      break;
+
+    Visited.insert(CurMBB);
+    CurMBB = *CurMBB->succ_begin();
+  }
+
+  return CurMBB->isLandingPad();
+}
+
 /// CorrectExtraCFGEdges - Various pieces of code can cause excess edges in the
 /// CFG to be inserted.  If we have proven that MBB can only branch to DestA and
 /// DestB, remove any other MBB successors from the CFG.  DestA and DestB can
 /// be null.
+/// 
 /// Besides DestA and DestB, retain other edges leading to LandingPads
 /// (currently there can be only one; we don't check or require that here).
 /// Note it is possible that DestA and/or DestB are LandingPads.
@@ -481,16 +507,17 @@ bool MachineBasicBlock::CorrectExtraCFGEdges(MachineBasicBlock *DestA,
   }
   
   MachineBasicBlock::succ_iterator SI = succ_begin();
-  MachineBasicBlock *OrigDestA = DestA, *OrigDestB = DestB;
+  const MachineBasicBlock *OrigDestA = DestA, *OrigDestB = DestB;
   while (SI != succ_end()) {
-    if (*SI == DestA) {
+    const MachineBasicBlock *MBB = *SI;
+    if (MBB == DestA) {
       DestA = 0;
       ++SI;
-    } else if (*SI == DestB) {
+    } else if (MBB == DestB) {
       DestB = 0;
       ++SI;
-    } else if ((*SI)->isLandingPad() && 
-               *SI!=OrigDestA && *SI!=OrigDestB) {
+    } else if (BranchesToLandingPad(MBB) &&
+               MBB != OrigDestA && MBB != OrigDestB) {
       ++SI;
     } else {
       // Otherwise, this is a superfluous edge, remove it.
@@ -498,12 +525,14 @@ bool MachineBasicBlock::CorrectExtraCFGEdges(MachineBasicBlock *DestA,
       MadeChange = true;
     }
   }
+
   if (!AddedFallThrough) {
     assert(DestA == 0 && DestB == 0 &&
            "MachineCFG is missing edges!");
   } else if (isCond) {
     assert(DestA == 0 && "MachineCFG is missing edges!");
   }
+
   return MadeChange;
 }