diff --git a/include/llvm/CodeGen/MachineBranchProbabilityInfo.h b/include/llvm/CodeGen/MachineBranchProbabilityInfo.h
index 39d2f32982b..943901f59ff 100644
--- a/include/llvm/CodeGen/MachineBranchProbabilityInfo.h
+++ b/include/llvm/CodeGen/MachineBranchProbabilityInfo.h
@@ -34,8 +34,10 @@ class MachineBranchProbabilityInfo : public ImmutablePass {
   // weight to just "inherit" the non-zero weight of an adjacent successor.
   static const uint32_t DEFAULT_WEIGHT = 16;
 
-  // Get sum of the block successors' weights.
-  uint32_t getSumForBlock(MachineBasicBlock *MBB) const;
+  // Get sum of the block successors' weights, potentially scaling them to fit
+  // within 32-bits. If scaling is required, sets Scale based on the necessary
+  // adjustment. Any edge weights used with the sum should be divided by Scale.
+  uint32_t getSumForBlock(MachineBasicBlock *MBB, uint32_t &Scale) const;
 
 public:
   static char ID;
diff --git a/lib/CodeGen/MachineBranchProbabilityInfo.cpp b/lib/CodeGen/MachineBranchProbabilityInfo.cpp
index 987403796f4..0037d525151 100644
--- a/lib/CodeGen/MachineBranchProbabilityInfo.cpp
+++ b/lib/CodeGen/MachineBranchProbabilityInfo.cpp
@@ -27,19 +27,34 @@ INITIALIZE_PASS_END(MachineBranchProbabilityInfo, "machine-branch-prob",
 char MachineBranchProbabilityInfo::ID = 0;
 
 uint32_t MachineBranchProbabilityInfo::
-getSumForBlock(MachineBasicBlock *MBB) const {
-  uint32_t Sum = 0;
-
+getSumForBlock(MachineBasicBlock *MBB, uint32_t &Scale) const {
+  // First we compute the sum with 64-bits of precision, ensuring that cannot
+  // overflow by bounding the number of weights considered. Hopefully no one
+  // actually needs 2^32 successors.
+  assert(MBB->succ_size() < UINT32_MAX);
+  uint64_t Sum = 0;
+  Scale = 1;
   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
        E = MBB->succ_end(); I != E; ++I) {
-    MachineBasicBlock *Succ = *I;
-    uint32_t Weight = getEdgeWeight(MBB, Succ);
-    uint32_t PrevSum = Sum;
-
+    uint32_t Weight = getEdgeWeight(MBB, *I);
     Sum += Weight;
-    assert(Sum > PrevSum); (void) PrevSum;
   }
 
+  // If the computed sum fits in 32-bits, we're done.
+  if (Sum <= UINT32_MAX)
+    return Sum;
+
+  // Otherwise, compute the scale necessary to cause the weights to fit, and
+  // re-sum with that scale applied.
+  assert((Sum / UINT32_MAX) < UINT32_MAX);
+  Scale = (Sum / UINT32_MAX) + 1;
+  Sum = 0;
+  for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
+       E = MBB->succ_end(); I != E; ++I) {
+    uint32_t Weight = getEdgeWeight(MBB, *I);
+    Sum += Weight / Scale;
+  }
+  assert(Sum <= UINT32_MAX);
   return Sum;
 }
 
@@ -89,8 +104,9 @@ MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
 BranchProbability
 MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
                                                  MachineBasicBlock *Dst) const {
-  uint32_t N = getEdgeWeight(Src, Dst);
-  uint32_t D = getSumForBlock(Src);
+  uint32_t Scale = 1;
+  uint32_t D = getSumForBlock(Src, Scale);
+  uint32_t N = getEdgeWeight(Src, Dst) / Scale;
 
   return BranchProbability(N, D);
 }
diff --git a/test/CodeGen/X86/block-placement.ll b/test/CodeGen/X86/block-placement.ll
index fed27c63531..e41d52c4183 100644
--- a/test/CodeGen/X86/block-placement.ll
+++ b/test/CodeGen/X86/block-placement.ll
@@ -271,3 +271,54 @@ loop.body5:
   %ptr2 = load i32** undef, align 4
   br label %loop.body3
 }
+
+define i32 @problematic_switch() {
+; This function's CFG caused overlow in the machine branch probability
+; calculation, triggering asserts. Make sure we don't crash on it.
+; CHECK: problematic_switch
+
+entry:
+  switch i32 undef, label %exit [
+    i32 879, label %bogus
+    i32 877, label %step
+    i32 876, label %step
+    i32 875, label %step
+    i32 874, label %step
+    i32 873, label %step
+    i32 872, label %step
+    i32 868, label %step
+    i32 867, label %step
+    i32 866, label %step
+    i32 861, label %step
+    i32 860, label %step
+    i32 856, label %step
+    i32 855, label %step
+    i32 854, label %step
+    i32 831, label %step
+    i32 830, label %step
+    i32 829, label %step
+    i32 828, label %step
+    i32 815, label %step
+    i32 814, label %step
+    i32 811, label %step
+    i32 806, label %step
+    i32 805, label %step
+    i32 804, label %step
+    i32 803, label %step
+    i32 802, label %step
+    i32 801, label %step
+    i32 800, label %step
+    i32 799, label %step
+    i32 798, label %step
+    i32 797, label %step
+    i32 796, label %step
+    i32 795, label %step
+  ]
+bogus:
+  unreachable
+step:
+  br label %exit
+exit:
+  %merge = phi i32 [ 3, %step ], [ 6, %entry ]
+  ret i32 %merge
+}