diff --git a/include/llvm/Target/TargetInstrInfo.h b/include/llvm/Target/TargetInstrInfo.h
index 90608f1..58d812b 100644
--- a/include/llvm/Target/TargetInstrInfo.h
+++ b/include/llvm/Target/TargetInstrInfo.h
@@ -327,6 +327,14 @@
   /// used by the tail merging pass.
   virtual void ReplaceTailWithBranchTo(MachineBasicBlock::iterator Tail,
                                        MachineBasicBlock *NewDest) const = 0;
+
+  /// isLegalToSplitMBBAt - Return true if it's legal to split the given basic
+  /// block at the specified instruction (i.e. instruction would be the start
+  /// of a new basic block).
+  virtual bool isLegalToSplitMBBAt(MachineBasicBlock &MBB,
+                                   MachineBasicBlock::iterator MBBI) const {
+    return true;
+  }
   
   /// copyRegToReg - Emit instructions to copy between a pair of registers. It
   /// returns false if the target does not how to copy between the specified
diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index e17e47a..7f98df0 100644
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -370,6 +370,9 @@
 /// iterator.  This returns the new MBB.
 MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
                                             MachineBasicBlock::iterator BBI1) {
+  if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
+    return 0;
+
   MachineFunction &MF = *CurMBB.getParent();
 
   // Create the fall-through block.
@@ -614,9 +617,10 @@
 
 /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
 /// only of the common tail.  Create a block that does by splitting one.
-unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
-                                             unsigned maxCommonTailLength) {
-  unsigned commonTailIndex = 0;
+bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
+                                             unsigned maxCommonTailLength,
+                                             unsigned &commonTailIndex) {
+  commonTailIndex = 0;
   unsigned TimeEstimate = ~0U;
   for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
     // Use PredBB if possible; that doesn't require a new branch.
@@ -644,6 +648,11 @@
                << maxCommonTailLength);
 
   MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
+  if (!newMBB) {
+    DEBUG(dbgs() << "... failed!");
+    return false;
+  }
+
   SameTails[commonTailIndex].setBlock(newMBB);
   SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
 
@@ -651,7 +660,7 @@
   if (PredBB == MBB)
     PredBB = newMBB;
 
-  return commonTailIndex;
+  return true;
 }
 
 // See if any of the blocks in MergePotentials (which all have a common single
@@ -746,7 +755,11 @@
          !SameTails[commonTailIndex].tailIsWholeBlock())) {
       // None of the blocks consist entirely of the common tail.
       // Split a block so that one does.
-      commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
+      if (!CreateCommonTailOnlyBlock(PredBB,
+                                     maxCommonTailLength, commonTailIndex)) {
+        RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
+        continue;
+      }
     }
 
     MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
diff --git a/lib/CodeGen/BranchFolding.h b/lib/CodeGen/BranchFolding.h
index b087395..15dfa7f 100644
--- a/lib/CodeGen/BranchFolding.h
+++ b/lib/CodeGen/BranchFolding.h
@@ -102,8 +102,9 @@
                               MachineBasicBlock *PredBB);
     void RemoveBlocksWithHash(unsigned CurHash, MachineBasicBlock* SuccBB,
                                                 MachineBasicBlock* PredBB);
-    unsigned CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
-                                       unsigned maxCommonTailLength);
+    bool CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
+                                   unsigned maxCommonTailLength,
+                                   unsigned &commonTailIndex);
 
     bool OptimizeBranches(MachineFunction &MF);
     bool OptimizeBlock(MachineBasicBlock *MBB);
diff --git a/lib/CodeGen/TargetInstrInfoImpl.cpp b/lib/CodeGen/TargetInstrInfoImpl.cpp
index 53f3ee8..5e145cf 100644
--- a/lib/CodeGen/TargetInstrInfoImpl.cpp
+++ b/lib/CodeGen/TargetInstrInfoImpl.cpp
@@ -28,6 +28,8 @@
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
+/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
+/// after it, replacing it with an unconditional branch to NewDest.
 void
 TargetInstrInfoImpl::ReplaceTailWithBranchTo(MachineBasicBlock::iterator Tail,
                                              MachineBasicBlock *NewDest) const {
diff --git a/lib/Target/ARM/Thumb2ITBlockPass.cpp b/lib/Target/ARM/Thumb2ITBlockPass.cpp
index 52ab71a..57f8eec 100644
--- a/lib/Target/ARM/Thumb2ITBlockPass.cpp
+++ b/lib/Target/ARM/Thumb2ITBlockPass.cpp
@@ -62,13 +62,6 @@
   char Thumb2ITBlockPass::ID = 0;
 }
 
-static ARMCC::CondCodes getPredicate(const MachineInstr *MI, unsigned &PredReg){
-  unsigned Opc = MI->getOpcode();
-  if (Opc == ARM::tBcc || Opc == ARM::t2Bcc)
-    return ARMCC::AL;
-  return llvm::getInstrPredicate(MI, PredReg);
-}
-
 bool
 Thumb2ITBlockPass::MoveCPSRUseUp(MachineBasicBlock &MBB,
                                  MachineBasicBlock::iterator MBBI,
@@ -82,7 +75,7 @@
   for (unsigned i = 0; i < 4; ++i) {
     MachineInstr *MI = &*I;
     unsigned MPredReg = 0;
-    ARMCC::CondCodes MCC = getPredicate(MI, MPredReg);
+    ARMCC::CondCodes MCC = llvm::getITInstrPredicate(MI, MPredReg);
     if (MCC != ARMCC::AL) {
       if (MPredReg != PredReg || (MCC != CC && MCC != OCC))
         return false;
@@ -209,7 +202,7 @@
     return false;
 
   unsigned PredReg = 0;
-  ARMCC::CondCodes CC = getPredicate(First, PredReg);
+  ARMCC::CondCodes CC = llvm::getITInstrPredicate(First, PredReg);
   if (CC == ARMCC::AL)
     return Modified;
 
@@ -222,7 +215,7 @@
       return Modified;
     MachineInstr *NMI = &*MBBI;
     unsigned NPredReg = 0;
-    ARMCC::CondCodes NCC = getPredicate(NMI, NPredReg);
+    ARMCC::CondCodes NCC = llvm::getITInstrPredicate(NMI, NPredReg);
     if (NCC != CC && NCC != OCC) {
       if (NCC != ARMCC::AL)
         return Modified;
@@ -321,7 +314,7 @@
       while (I != E && I->isDebugValue())
         ++I;
       unsigned NPredReg = 0;
-      ARMCC::CondCodes NCC = getPredicate(I, NPredReg);
+      ARMCC::CondCodes NCC = llvm::getITInstrPredicate(I, NPredReg);
       if (NCC == CC || NCC == OCC)
         return true;
     }
@@ -339,7 +332,7 @@
     MachineInstr *MI = &*MBBI;
     DebugLoc dl = MI->getDebugLoc();
     unsigned PredReg = 0;
-    ARMCC::CondCodes CC = getPredicate(MI, PredReg);
+    ARMCC::CondCodes CC = llvm::getITInstrPredicate(MI, PredReg);
     if (CC == ARMCC::AL) {
       ++MBBI;
       continue;
@@ -375,7 +368,7 @@
       MI = NMI;
 
       unsigned NPredReg = 0;
-      ARMCC::CondCodes NCC = getPredicate(NMI, NPredReg);
+      ARMCC::CondCodes NCC = llvm::getITInstrPredicate(NMI, NPredReg);
       if (NCC == CC || NCC == OCC) {
         Mask |= (NCC & 1) << Pos;
         // Add implicit use of ITSTATE.
diff --git a/lib/Target/ARM/Thumb2InstrInfo.cpp b/lib/Target/ARM/Thumb2InstrInfo.cpp
index 866ffb1..42fe509 100644
--- a/lib/Target/ARM/Thumb2InstrInfo.cpp
+++ b/lib/Target/ARM/Thumb2InstrInfo.cpp
@@ -88,6 +88,14 @@
 }
 
 bool
+Thumb2InstrInfo::isLegalToSplitMBBAt(MachineBasicBlock &MBB,
+                                     MachineBasicBlock::iterator MBBI) const {
+  unsigned PredReg = 0;
+  return llvm::getITInstrPredicate(MBBI, PredReg) == ARMCC::AL;
+}
+
+
+bool
 Thumb2InstrInfo::copyRegToReg(MachineBasicBlock &MBB,
                               MachineBasicBlock::iterator I,
                               unsigned DestReg, unsigned SrcReg,
@@ -605,3 +613,11 @@
     MBB->insert(++MBBI, SrcMI);
   }
 }
+
+ARMCC::CondCodes
+llvm::getITInstrPredicate(const MachineInstr *MI, unsigned &PredReg) {
+  unsigned Opc = MI->getOpcode();
+  if (Opc == ARM::tBcc || Opc == ARM::t2Bcc)
+    return ARMCC::AL;
+  return llvm::getInstrPredicate(MI, PredReg);
+}
diff --git a/lib/Target/ARM/Thumb2InstrInfo.h b/lib/Target/ARM/Thumb2InstrInfo.h
index d5fc359..8b01bcb 100644
--- a/lib/Target/ARM/Thumb2InstrInfo.h
+++ b/lib/Target/ARM/Thumb2InstrInfo.h
@@ -35,6 +35,9 @@
   void ReplaceTailWithBranchTo(MachineBasicBlock::iterator Tail,
                                MachineBasicBlock *NewDest) const;
 
+  bool isLegalToSplitMBBAt(MachineBasicBlock &MBB,
+                           MachineBasicBlock::iterator MBBI) const;
+
   bool copyRegToReg(MachineBasicBlock &MBB,
                     MachineBasicBlock::iterator I,
                     unsigned DestReg, unsigned SrcReg,
@@ -68,6 +71,13 @@
   ScheduleHazardRecognizer *
   CreateTargetPostRAHazardRecognizer(const InstrItineraryData &II) const;
 };
+
+/// getITInstrPredicate - Valid only in Thumb2 mode. This function is identical
+/// to llvm::getInstrPredicate except it returns AL for conditional branch
+/// instructions which are "predicated", but are not in IT blocks.
+ARMCC::CondCodes getITInstrPredicate(const MachineInstr *MI, unsigned &PredReg);
+
+
 }
 
 #endif // THUMB2INSTRUCTIONINFO_H
diff --git a/test/CodeGen/Thumb2/2010-06-21-TailMergeBug.ll b/test/CodeGen/Thumb2/2010-06-21-TailMergeBug.ll
new file mode 100644
index 0000000..c5fc509
--- /dev/null
+++ b/test/CodeGen/Thumb2/2010-06-21-TailMergeBug.ll
@@ -0,0 +1,127 @@
+; RUN: llc < %s -mtriple=thumbv7-apple-darwin -O3 -relocation-model=pic -mcpu=cortex-a8 | FileCheck %s
+; rdar://8115404
+; Tail merging must not split an IT block.
+
+%struct.FILE = type { i8*, i32, i32, i16, i16, %struct.__sbuf, i32, i8*, i32 (i8*)*, i32 (i8*, i8*, i32)*, i64 (i8*, i64, i32)*, i32 (i8*, i8*, i32)*, %struct.__sbuf, %struct.__sFILEX*, i32, [3 x i8], [1 x i8], %struct.__sbuf, i32, i64 }
+%struct._RuneCharClass = type { [14 x i8], i32 }
+%struct._RuneEntry = type { i32, i32, i32, i32* }
+%struct._RuneLocale = type { [8 x i8], [32 x i8], i32 (i8*, i32, i8**)*, i32 (i32, i8*, i32, i8**)*, i32, [256 x i32], [256 x i32], [256 x i32], %struct._RuneRange, %struct._RuneRange, %struct._RuneRange, i8*, i32, i32, %struct._RuneCharClass* }
+%struct._RuneRange = type { i32, %struct._RuneEntry* }
+%struct.__sFILEX = type opaque
+%struct.__sbuf = type { i8*, i32 }
+
+@finput = external global %struct.FILE*           ; <%struct.FILE**> [#uses=1]
+@_DefaultRuneLocale = external global %struct._RuneLocale ; <%struct._RuneLocale*> [#uses=0]
+@token_buffer = external global [1025 x i8], align 4 ; <[1025 x i8]*> [#uses=1]
+@.str73 = external constant [6 x i8], align 4     ; <[6 x i8]*> [#uses=0]
+@.str174 = external constant [5 x i8], align 4    ; <[5 x i8]*> [#uses=0]
+@.str275 = external constant [6 x i8], align 4    ; <[6 x i8]*> [#uses=0]
+@.str376 = external constant [5 x i8], align 4    ; <[5 x i8]*> [#uses=0]
+@.str477 = external constant [6 x i8], align 4    ; <[6 x i8]*> [#uses=0]
+@.str578 = external constant [6 x i8], align 4    ; <[6 x i8]*> [#uses=0]
+@.str679 = external constant [7 x i8], align 4    ; <[7 x i8]*> [#uses=0]
+@.str780 = external constant [6 x i8], align 4    ; <[6 x i8]*> [#uses=0]
+@.str881 = external constant [5 x i8], align 4    ; <[5 x i8]*> [#uses=0]
+@.str982 = external constant [6 x i8], align 4    ; <[6 x i8]*> [#uses=0]
+@.str1083 = external constant [9 x i8], align 4   ; <[9 x i8]*> [#uses=0]
+@.str1184 = external constant [7 x i8], align 4   ; <[7 x i8]*> [#uses=0]
+@.str1285 = external constant [16 x i8], align 4  ; <[16 x i8]*> [#uses=0]
+@.str1386 = external constant [12 x i8], align 4  ; <[12 x i8]*> [#uses=0]
+@.str1487 = external constant [5 x i8], align 4   ; <[5 x i8]*> [#uses=0]
+@llvm.used = external global [1 x i8*]            ; <[1 x i8*]*> [#uses=0]
+
+define fastcc i32 @parse_percent_token() nounwind {
+entry:
+; CHECK: ittt eq
+; CHECK: ittt eq
+; CHECK: ittt eq
+; CHECK: ittt eq
+; CHECK: ittt eq
+; CHECK: moveq r0
+; CHECK-NOT: LBB0_
+; CHECK: ldreq
+; CHECK: popeq
+  switch i32 undef, label %bb7 [
+    i32 37, label %bb43
+    i32 48, label %bb5
+    i32 50, label %bb4
+    i32 60, label %bb2
+    i32 61, label %bb6
+    i32 62, label %bb3
+    i32 123, label %bb1
+  ]
+
+bb1:                                              ; preds = %entry
+  ret i32 8
+
+bb2:                                              ; preds = %entry
+  ret i32 15
+
+bb3:                                              ; preds = %entry
+  ret i32 16
+
+bb4:                                              ; preds = %entry
+  ret i32 17
+
+bb5:                                              ; preds = %entry
+  ret i32 9
+
+bb6:                                              ; preds = %entry
+  ret i32 18
+
+bb7:                                              ; preds = %entry
+  br i1 undef, label %bb.i.i, label %bb1.i.i
+
+bb.i.i:                                           ; preds = %bb7
+  br i1 undef, label %bb43, label %bb12
+
+bb1.i.i:                                          ; preds = %bb7
+  unreachable
+
+bb9:                                              ; preds = %bb.i.i2
+  br i1 undef, label %bb10, label %bb11
+
+bb10:                                             ; preds = %bb9
+  br label %bb11
+
+bb11:                                             ; preds = %bb10, %bb9
+  %p.0 = phi i8* [ undef, %bb10 ], [ %p.1, %bb9 ] ; <i8*> [#uses=1]
+  %0 = load %struct.FILE** @finput, align 4       ; <%struct.FILE*> [#uses=1]
+  %1 = tail call i32 @getc(%struct.FILE* %0) nounwind ; <i32> [#uses=0]
+  br label %bb12
+
+bb12:                                             ; preds = %bb11, %bb.i.i
+  %p.1 = phi i8* [ %p.0, %bb11 ], [ getelementptr inbounds ([1025 x i8]* @token_buffer, i32 0, i32 0), %bb.i.i ] ; <i8*> [#uses=2]
+  %2 = icmp ult i32 undef, 128                    ; <i1> [#uses=1]
+  br i1 %2, label %bb.i.i2, label %bb1.i.i3
+
+bb.i.i2:                                          ; preds = %bb12
+  %3 = load i32* null, align 4                    ; <i32> [#uses=1]
+  %4 = lshr i32 %3, 8                             ; <i32> [#uses=1]
+  %.lobit.i1 = and i32 %4, 1                      ; <i32> [#uses=1]
+  %.not = icmp ne i32 %.lobit.i1, 0               ; <i1> [#uses=1]
+  %or.cond = or i1 %.not, undef                   ; <i1> [#uses=1]
+  br i1 %or.cond, label %bb9, label %bb14
+
+bb1.i.i3:                                         ; preds = %bb12
+  unreachable
+
+bb14:                                             ; preds = %bb.i.i2
+  store i8 0, i8* %p.1, align 1
+  br i1 undef, label %bb43, label %bb15
+
+bb15:                                             ; preds = %bb14
+  unreachable
+
+bb43:                                             ; preds = %bb14, %bb.i.i, %entry
+  %.0 = phi i32 [ 7, %entry ], [ 24, %bb.i.i ], [ 9, %bb14 ] ; <i32> [#uses=1]
+  ret i32 %.0
+}
+
+declare i32 @getc(%struct.FILE* nocapture) nounwind
+
+declare i32 @strcmp(i8* nocapture, i8* nocapture) nounwind readonly
+
+declare i32 @__maskrune(i32, i32)
+
+declare i32 @ungetc(i32, %struct.FILE* nocapture) nounwind