Index: lib/CodeGen/BranchRelaxation.cpp
===================================================================
--- lib/CodeGen/BranchRelaxation.cpp
+++ lib/CodeGen/BranchRelaxation.cpp
@@ -78,7 +78,8 @@
 
   MachineBasicBlock *createNewBlockAfter(MachineBasicBlock &BB);
 
-  MachineBasicBlock *splitBlockBeforeInstr(MachineInstr &MI);
+  MachineBasicBlock *splitBlockBeforeInstr(MachineInstr &MI,
+                                           MachineBasicBlock *DestBB);
   void adjustBlockOffsets(MachineBasicBlock &MBB);
   bool isBlockInRange(const MachineInstr &MI, const MachineBasicBlock &BB) const;
 
@@ -116,6 +117,7 @@
     unsigned Num = MBB.getNumber();
     assert(BlockInfo[Num].Offset % (1u << Align) == 0);
     assert(!Num || BlockInfo[PrevNum].postOffset(MBB) <= BlockInfo[Num].Offset);
+    assert(BlockInfo[Num].Size == computeBlockSize(MBB));
     PrevNum = Num;
   }
 #endif
@@ -205,10 +207,8 @@
 /// Split the basic block containing MI into two blocks, which are joined by
 /// an unconditional branch.  Update data structures and renumber blocks to
 /// account for this change and returns the newly created block.
-/// NOTE: Successor list of the original BB is out of date after this function,
-/// and must be updated by the caller! Other transforms follow using this
-/// utility function, so no point updating now rather than waiting.
-MachineBasicBlock *BranchRelaxation::splitBlockBeforeInstr(MachineInstr &MI) {
+MachineBasicBlock *BranchRelaxation::splitBlockBeforeInstr(MachineInstr &MI,
+                                                           MachineBasicBlock *DestBB) {
   MachineBasicBlock *OrigBB = MI.getParent();
 
   // Create a new MBB for the code after the OrigBB.
@@ -228,6 +228,16 @@
   // Insert an entry into BlockInfo to align it properly with the block numbers.
   BlockInfo.insert(BlockInfo.begin() + NewBB->getNumber(), BasicBlockInfo());
 
+
+  NewBB->transferSuccessors(OrigBB);
+  OrigBB->addSuccessor(NewBB);
+  OrigBB->addSuccessor(DestBB);
+
+  // Cleanup potential unconditional branch to successor block.
+  // Note that updateTerminator may change the size of the blocks.
+  NewBB->updateTerminator();
+  OrigBB->updateTerminator();
+
   // Figure out how large the OrigBB is.  As the first half of the original
   // block, it cannot contain a tablejump.  The size includes
   // the new jump we added.  (It should be possible to do this without
@@ -386,12 +396,9 @@
 
   DebugLoc DL = MI.getDebugLoc();
   MI.eraseFromParent();
-
-  // insertUnconditonalBranch may have inserted a new block.
-  BlockInfo[MBB->getNumber()].Size += TII->insertIndirectBranch(
+  BlockInfo[BranchBB->getNumber()].Size += TII->insertIndirectBranch(
     *BranchBB, *DestBB, DL, DestOffset - SrcOffset, RS.get());
 
-  computeBlockSize(*BranchBB);
   adjustBlockOffsets(*MBB);
   return true;
 }
@@ -440,14 +447,7 @@
             // analyzable block. Split later terminators into a new block so
             // each one will be analyzable.
 
-            MachineBasicBlock *NewBB = splitBlockBeforeInstr(*Next);
-            NewBB->transferSuccessors(&MBB);
-            MBB.addSuccessor(NewBB);
-            MBB.addSuccessor(DestBB);
-
-            // Cleanup potential unconditional branch to successor block.
-            NewBB->updateTerminator();
-            MBB.updateTerminator();
+            splitBlockBeforeInstr(*Next, DestBB);
           } else {
             fixupConditionalBranch(MI);
             ++NumConditionalRelaxed;
Index: test/CodeGen/AMDGPU/branch-relaxation.ll
===================================================================
--- test/CodeGen/AMDGPU/branch-relaxation.ll
+++ test/CodeGen/AMDGPU/branch-relaxation.ll
@@ -475,5 +475,59 @@
   ret void
 }
 
+; GCN-LABEL: {{^}}long_branch_hang:
+; GCN: s_cmp_lt_i32 s{{[0-9]+}}, 6
+; GCN-NEXT: s_cbranch_scc1 [[LONG_BR_0:BB[0-9]+_[0-9]+]]
+
+; GCN: s_add_u32 vcc_lo, vcc_lo, [[LONG_BR_DEST0:BB[0-9]+_[0-9]+]]-(
+; GCN: s_setpc_b64
+
+; GCN-NEXT: [[LONG_BR_0]]:
+; GCN: s_setpc_b64
+
+; GCN-NEXT: [[LONG_BR_DEST0]]:
+; GCN-DAG: v_cmp_lt_i32
+; GCN-DAG: v_cmp_gt_i32
+; GCN: s_cbranch_vccnz
+
+; GCN: s_setpc_b64
+; GCN: s_setpc_b64
+
+; GCN: s_cmp_eq_u32
+; GCN-NEXT: s_cbranch_scc0
+; GCN: s_setpc_b64
+
+; GCN: s_endpgm
+define amdgpu_kernel void @long_branch_hang(i32 addrspace(1)* nocapture %arg, i32 %arg1, i32 %arg2, i32 %arg3, i32 %arg4, i64 %arg5) #0 {
+bb:
+  %tmp = icmp slt i32 %arg2, 9
+  %tmp6 = icmp eq i32 %arg1, 0
+  %tmp7 = icmp sgt i32 %arg4, 0
+  %tmp8 = icmp sgt i32 %arg4, 5
+  br i1 %tmp8, label %bb9, label %bb13
+
+bb9:                                              ; preds = %bb
+  %tmp10 = and i1 %tmp7, %tmp
+  %tmp11 = icmp slt i32 %arg3, %arg4
+  %tmp12 = or i1 %tmp11, %tmp7
+  br i1 %tmp12, label %bb19, label %bb14
+
+bb13:                                             ; preds = %bb
+  br i1 %tmp6, label %bb19, label %bb14
+
+bb14:                                             ; preds = %bb13, %bb9
+  %tmp15 = icmp slt i32 %arg3, %arg4
+  %tmp16 = or i1 %tmp15, %tmp
+  %tmp17 = and i1 %tmp6, %tmp16
+  %tmp18 = zext i1 %tmp17 to i32
+  br label %bb19
+
+bb19:                                             ; preds = %bb14, %bb13, %bb9
+  %tmp20 = phi i32 [ undef, %bb9 ], [ undef, %bb13 ], [ %tmp18, %bb14 ]
+  %tmp21 = getelementptr inbounds i32, i32 addrspace(1)* %arg, i64 %arg5
+  store i32 %tmp20, i32 addrspace(1)* %tmp21, align 4
+  ret void
+}
+
 attributes #0 = { nounwind }
 attributes #1 = { nounwind readnone }