Index: llvm/trunk/include/llvm/CodeGen/MachineBasicBlock.h
===================================================================
--- llvm/trunk/include/llvm/CodeGen/MachineBasicBlock.h
+++ llvm/trunk/include/llvm/CodeGen/MachineBasicBlock.h
@@ -425,6 +425,12 @@
   /// Note that duplicate Machine CFG edges are not allowed.
   void addSuccessor(MachineBasicBlock *Succ, uint32_t Weight = 0);
 
+  /// Add Succ as a successor of this MachineBasicBlock.  The Predecessors list
+  /// of Succ is automatically updated. The weight is not provided because BPI
+  /// is not available (e.g. -O0 is used), in which case edge weights won't be
+  /// used. Using this interface can save some space.
+  void addSuccessorWithoutWeight(MachineBasicBlock *Succ);
+
   /// Set successor weight of a given iterator.
   void setSuccWeight(succ_iterator I, uint32_t Weight);
 
Index: llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp
+++ llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp
@@ -506,15 +506,19 @@
 }
 
 void MachineBasicBlock::addSuccessor(MachineBasicBlock *Succ, uint32_t Weight) {
-
-  // If we see non-zero value for the first time it means we actually use Weight
-  // list, so we fill all Weights with 0's.
-  if (Weight != 0 && Weights.empty())
-    Weights.resize(Successors.size());
-
-  if (Weight != 0 || !Weights.empty())
+  // Weight list is either empty (if successor list isn't empty, this means
+  // disabled optimization) or has the same size as successor list.
+  if (!(Weights.empty() && !Successors.empty()))
     Weights.push_back(Weight);
+  Successors.push_back(Succ);
+  Succ->addPredecessor(this);
+}
 
+void MachineBasicBlock::addSuccessorWithoutWeight(MachineBasicBlock *Succ) {
+  // We need to make sure weight list is either empty or has the same size of
+  // successor list. When this function is called, we can safely delete all
+  // weight in the list.
+  Weights.clear();
   Successors.push_back(Succ);
   Succ->addPredecessor(this);
 }
Index: llvm/trunk/lib/CodeGen/SelectionDAG/FastISel.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ llvm/trunk/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1394,25 +1394,28 @@
     TII.InsertBranch(*FuncInfo.MBB, MSucc, nullptr,
                      SmallVector<MachineOperand, 0>(), DbgLoc);
   }
-  uint32_t BranchWeight = 0;
-  if (FuncInfo.BPI)
-    BranchWeight = FuncInfo.BPI->getEdgeWeight(FuncInfo.MBB->getBasicBlock(),
-                                               MSucc->getBasicBlock());
-  FuncInfo.MBB->addSuccessor(MSucc, BranchWeight);
+  if (FuncInfo.BPI) {
+    uint32_t BranchWeight = FuncInfo.BPI->getEdgeWeight(
+        FuncInfo.MBB->getBasicBlock(), MSucc->getBasicBlock());
+    FuncInfo.MBB->addSuccessor(MSucc, BranchWeight);
+  } else
+    FuncInfo.MBB->addSuccessorWithoutWeight(MSucc);
 }
 
 void FastISel::finishCondBranch(const BasicBlock *BranchBB,
                                 MachineBasicBlock *TrueMBB,
                                 MachineBasicBlock *FalseMBB) {
-  uint32_t BranchWeight = 0;
-  if (FuncInfo.BPI)
-    BranchWeight = FuncInfo.BPI->getEdgeWeight(BranchBB,
-                                               TrueMBB->getBasicBlock());
   // Add TrueMBB as successor unless it is equal to the FalseMBB: This can
   // happen in degenerate IR and MachineIR forbids to have a block twice in the
   // successor/predecessor lists.
-  if (TrueMBB != FalseMBB)
-    FuncInfo.MBB->addSuccessor(TrueMBB, BranchWeight);
+  if (TrueMBB != FalseMBB) {
+    if (FuncInfo.BPI) {
+      uint32_t BranchWeight =
+          FuncInfo.BPI->getEdgeWeight(BranchBB, TrueMBB->getBasicBlock());
+      FuncInfo.MBB->addSuccessor(TrueMBB, BranchWeight);
+    } else
+      FuncInfo.MBB->addSuccessorWithoutWeight(TrueMBB);
+  }
 
   fastEmitBranch(FalseMBB, DbgLoc);
 }
Index: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -1499,9 +1499,13 @@
 void SelectionDAGBuilder::
 addSuccessorWithWeight(MachineBasicBlock *Src, MachineBasicBlock *Dst,
                        uint32_t Weight /* = 0 */) {
-  if (!Weight)
-    Weight = getEdgeWeight(Src, Dst);
-  Src->addSuccessor(Dst, Weight);
+  if (!FuncInfo.BPI)
+    Src->addSuccessorWithoutWeight(Dst);
+  else {
+    if (!Weight)
+      Weight = getEdgeWeight(Src, Dst);
+    Src->addSuccessor(Dst, Weight);
+  }
 }
 
 
Index: llvm/trunk/lib/Target/AArch64/AArch64FastISel.cpp
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64FastISel.cpp
+++ llvm/trunk/lib/Target/AArch64/AArch64FastISel.cpp
@@ -2376,11 +2376,12 @@
         .addMBB(Target);
 
     // Obtain the branch weight and add the target to the successor list.
-    uint32_t BranchWeight = 0;
-    if (FuncInfo.BPI)
-      BranchWeight = FuncInfo.BPI->getEdgeWeight(BI->getParent(),
-                                                 Target->getBasicBlock());
-    FuncInfo.MBB->addSuccessor(Target, BranchWeight);
+    if (FuncInfo.BPI) {
+      uint32_t BranchWeight =
+          FuncInfo.BPI->getEdgeWeight(BI->getParent(), Target->getBasicBlock());
+      FuncInfo.MBB->addSuccessor(Target, BranchWeight);
+    } else
+      FuncInfo.MBB->addSuccessorWithoutWeight(Target);
     return true;
   } else if (foldXALUIntrinsic(CC, I, BI->getCondition())) {
     // Fake request the condition, otherwise the intrinsic might be completely
Index: llvm/trunk/test/CodeGen/MIR/X86/newline-handling.mir
===================================================================
--- llvm/trunk/test/CodeGen/MIR/X86/newline-handling.mir
+++ llvm/trunk/test/CodeGen/MIR/X86/newline-handling.mir
@@ -35,7 +35,7 @@
 # CHECK-LABEL: name: foo
 # CHECK: body: |
 # CHECK-NEXT: bb.0.entry:
-# CHECK-NEXT: successors: %bb.1.less, %bb.2.exit
+# CHECK-NEXT: successors: %bb.1.less(0), %bb.2.exit(0)
 # CHECK-NEXT: liveins: %edi
 # CHECK:      CMP32ri8 %edi, 10, implicit-def %eflags
 # CHECK-NEXT: JG_1 %bb.2.exit, implicit killed %eflags
@@ -79,7 +79,7 @@
 # CHECK-LABEL: name: bar
 # CHECK: body: |
 # CHECK-NEXT: bb.0.entry:
-# CHECK-NEXT: successors: %bb.1.less, %bb.2.exit
+# CHECK-NEXT: successors: %bb.1.less(0), %bb.2.exit(0)
 # CHECK-NEXT: liveins: %edi
 # CHECK:      CMP32ri8 %edi, 10, implicit-def %eflags
 # CHECK-NEXT: JG_1 %bb.2.exit, implicit killed %eflags
Index: llvm/trunk/test/CodeGen/MIR/X86/successor-basic-blocks.mir
===================================================================
--- llvm/trunk/test/CodeGen/MIR/X86/successor-basic-blocks.mir
+++ llvm/trunk/test/CodeGen/MIR/X86/successor-basic-blocks.mir
@@ -32,7 +32,7 @@
 name:            foo
 body: |
   ; CHECK-LABEL: bb.0.entry:
-  ; CHECK:         successors: %bb.1.less, %bb.2.exit
+  ; CHECK:         successors: %bb.1.less(0), %bb.2.exit(0)
   ; CHECK-LABEL: bb.1.less:
   bb.0.entry:
     successors: %bb.1.less, %bb.2.exit
@@ -58,7 +58,7 @@
   ; Verify that we can have multiple lists of successors that will be merged
   ; into one.
   ; CHECK-LABEL: bb.0.entry:
-  ; CHECK:         successors: %bb.1, %bb.2
+  ; CHECK:         successors: %bb.1(0), %bb.2(0)
   bb.0.entry:
     liveins: %edi
     successors: %bb.1