Index: lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyCFG.cpp
+++ lib/Transforms/Utils/SimplifyCFG.cpp
@@ -73,6 +73,7 @@
 STATISTIC(NumLinearMaps, "Number of switch instructions turned into linear mapping");
 STATISTIC(NumLookupTables, "Number of switch instructions turned into lookup tables");
 STATISTIC(NumLookupTablesHoles, "Number of switch instructions turned into lookup tables (holes checked)");
+STATISTIC(NumTableCmpReuses, "Number of reused switch table lookup compares");
 STATISTIC(NumSinkCommons, "Number of common instructions sunk down to the end block");
 STATISTIC(NumSpeculations, "Number of speculative executed instructions");
 
@@ -3982,6 +3983,78 @@
   return SI->getNumCases() * 10 >= TableSize * 4;
 }
 
+/// Try to reuse the switch table index compare. Following pattern:
+/// \code
+///     if (idx < tablesize)
+///        r = table[idx]; // table does not contain default_value
+///     else
+///        r = default_value;
+///     if (r != default_value)
+///        ...
+/// \endcode
+/// Is optimized to:
+/// \code
+///     cond = idx < tablesize;
+///     if (cond)
+///        r = table[idx];
+///     else
+///        r = default_value;
+///     if (cond)
+///        ...
+/// \endcode
+/// Jump threading will then eliminate the second if(cond).
+void reuseTableCompare(User *PhiUser, BasicBlock *PhiBlock,
+          BranchInst *RangeCheckBranch, Constant *DefaultValue,
+          const SmallVectorImpl<std::pair<ConstantInt*, Constant*> >& Values) {
+
+  ICmpInst *CmpInst = dyn_cast<ICmpInst>(PhiUser);
+  if (!CmpInst)
+    return;
+
+  // We require that the compare is in the same block as the phi so that jump
+  // threading can do its work afterwards.
+  if (CmpInst->getParent() != PhiBlock)
+    return;
+
+  Constant *CmpOp1 = dyn_cast<Constant>(CmpInst->getOperand(1));
+  if (!CmpOp1)
+    return;
+
+  Value *RangeCmp = RangeCheckBranch->getCondition();
+  Constant *TrueConst = ConstantInt::getTrue(RangeCmp->getType());
+  Constant *FalseConst = ConstantInt::getFalse(RangeCmp->getType());
+
+  // Check if the compare with the default value is constant true or false.
+  Constant *DefaultConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
+                                                 DefaultValue, CmpOp1, true);
+  if (DefaultConst != TrueConst && DefaultConst != FalseConst)
+    return;
+
+  // Check if the compare with the case values is distinct from the default
+  // compare result.
+  for (auto ValuePair : Values) {
+    Constant *CaseConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
+                              ValuePair.second, CmpOp1, true);
+    if (!CaseConst || CaseConst == DefaultConst)
+      return;
+    assert((CaseConst == TrueConst || CaseConst == FalseConst) &&
+           "expect true or false as compare result");
+  }
+
+  if (DefaultConst == FalseConst) {
+    // The compare yields the same result. We can replace it.
+    CmpInst->replaceAllUsesWith(RangeCmp);
+    ++NumTableCmpReuses;
+  } else {
+    // The compare yields the same result, just inverted. We can replace it.
+    Value *InvertedTableCmp = BinaryOperator::CreateXor(RangeCmp,
+                ConstantInt::get(RangeCmp->getType(), 1), "inverted.cmp",
+                RangeCheckBranch);
+    CmpInst->replaceAllUsesWith(InvertedTableCmp);
+    ++NumTableCmpReuses;
+  }
+}
+
 /// SwitchToLookupTable - If the switch is only used to initialize one or more
 /// phi nodes in a common successor block with different constant values,
 /// replace the switch with lookup tables.
@@ -4058,11 +4131,8 @@
   // If the table has holes, we need a constant result for the default case
   // or a bitmask that fits in a register.
   SmallVector<std::pair<PHINode*, Constant*>, 4> DefaultResultsList;
-  bool HasDefaultResults = false;
-  if (TableHasHoles) {
-    HasDefaultResults = GetCaseResults(SI, nullptr, SI->getDefaultDest(),
+  bool HasDefaultResults = GetCaseResults(SI, nullptr, SI->getDefaultDest(),
                                        &CommonDest, DefaultResultsList, DL);
-  }
 
   bool NeedMask = (TableHasHoles && !HasDefaultResults);
   if (NeedMask) {
@@ -4106,6 +4176,8 @@
   // lookup table BB. Otherwise, check if the condition value is within the case
   // range. If it is so, branch to the new BB. Otherwise branch to SI's default
   // destination.
+  BranchInst *RangeCheckBranch = nullptr;
+
   const bool GeneratingCoveredLookupTable = MaxTableSize == TableSize;
   if (GeneratingCoveredLookupTable) {
     Builder.CreateBr(LookupBB);
@@ -4116,7 +4188,7 @@
   } else {
     Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
                                        MinCaseVal->getType(), TableSize));
-    Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
+    RangeCheckBranch = Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
   }
 
   // Populate the BB that does the lookups.
@@ -4167,11 +4239,11 @@
   bool ReturnedEarly = false;
   for (size_t I = 0, E = PHIs.size(); I != E; ++I) {
     PHINode *PHI = PHIs[I];
+    const ResultListTy &ResultList = ResultLists[PHI];
 
     // If using a bitmask, use any value to fill the lookup table holes.
     Constant *DV = NeedMask ? ResultLists[PHI][0].second : DefaultResults[PHI];
-    SwitchLookupTable Table(Mod, TableSize, MinCaseVal, ResultLists[PHI],
-                            DV, DL);
+    SwitchLookupTable Table(Mod, TableSize, MinCaseVal, ResultList, DV, DL);
 
     Value *Result = Table.BuildLookup(TableIndex, Builder);
 
@@ -4184,6 +4256,16 @@
       break;
     }
 
+    // Do a small peephole optimization: re-use the switch table compare if
+    // possible.
+    if (!TableHasHoles && HasDefaultResults && RangeCheckBranch) {
+      BasicBlock *PhiBlock = PHI->getParent();
+      // Search for compare instructions which use the phi.
+      for (auto *User : PHI->users()) {
+        reuseTableCompare(User, PhiBlock, RangeCheckBranch, DV, ResultList);
+      }
+    }
+
     PHI->addIncoming(Result, LookupBB);
   }
 
Index: test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
===================================================================
--- test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
+++ test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
@@ -1077,3 +1077,97 @@
 ; CHECK-NEXT: ret i8 %switch.idx.cast
 }
 
+; Reuse the inverted table range compare.
+define i32 @reuse_cmp1(i32 %x) {
+entry:
+  switch i32 %x, label %sw.default [
+    i32 0, label %sw.bb
+    i32 1, label %sw.bb1
+    i32 2, label %sw.bb2
+    i32 3, label %sw.bb3
+  ]
+sw.bb: br label %sw.epilog
+sw.bb1: br label %sw.epilog
+sw.bb2: br label %sw.epilog
+sw.bb3: br label %sw.epilog
+sw.default: br label %sw.epilog
+sw.epilog:
+  %r.0 = phi i32 [ 0, %sw.default ], [ 13, %sw.bb3 ], [ 12, %sw.bb2 ], [ 11, %sw.bb1 ], [ 10, %sw.bb ]
+  %cmp = icmp eq i32 %r.0, 0       ; This compare can be "replaced".
+  br i1 %cmp, label %if.then, label %if.end
+if.then: br label %return
+if.end: br label %return
+return:
+  %retval.0 = phi i32 [ 100, %if.then ], [ %r.0, %if.end ]
+  ret i32 %retval.0
+; CHECK-LABEL: @reuse_cmp1(
+; CHECK: entry:
+; CHECK-NEXT: %switch.tableidx = sub i32 %x, 0
+; CHECK-NEXT: [[C:%.+]] = icmp ult i32 %switch.tableidx, 4
+; CHECK-NEXT: %inverted.cmp = xor i1 [[C]], true
+; CHECK:      [[R:%.+]] = select i1 %inverted.cmp, i32 100, i32 {{.*}}
+; CHECK-NEXT: ret i32 [[R]]
+}
+
+; Reuse the table range compare.
+define i32 @reuse_cmp2(i32 %x) {
+entry:
+  switch i32 %x, label %sw.default [
+    i32 0, label %sw.bb
+    i32 1, label %sw.bb1
+    i32 2, label %sw.bb2
+    i32 3, label %sw.bb3
+  ]
+sw.bb: br label %sw.epilog
+sw.bb1: br label %sw.epilog
+sw.bb2: br label %sw.epilog
+sw.bb3: br label %sw.epilog
+sw.default: br label %sw.epilog
+sw.epilog:
+  %r.0 = phi i32 [ 4, %sw.default ], [ 3, %sw.bb3 ], [ 2, %sw.bb2 ], [ 1, %sw.bb1 ], [ 0, %sw.bb ]
+  %cmp = icmp ne i32 %r.0, 4       ; This compare can be "replaced".
+  br i1 %cmp, label %if.then, label %if.end
+if.then: br label %return
+if.end: br label %return
+return:
+  %retval.0 = phi i32 [ %r.0, %if.then ], [ 100, %if.end ]
+  ret i32 %retval.0
+; CHECK-LABEL: @reuse_cmp2(
+; CHECK: entry:
+; CHECK-NEXT: %switch.tableidx = sub i32 %x, 0
+; CHECK-NEXT: [[C:%.+]] = icmp ult i32 %switch.tableidx, 4
+; CHECK:      [[R:%.+]] = select i1 [[C]], i32 {{.*}}, i32 100
+; CHECK-NEXT: ret i32 [[R]]
+}
+
+; Cannot reuse the table range compare, because the default value is the same
+; as one of the case values.
+define i32 @no_reuse_cmp(i32 %x) {
+entry:
+  switch i32 %x, label %sw.default [
+    i32 0, label %sw.bb
+    i32 1, label %sw.bb1
+    i32 2, label %sw.bb2
+    i32 3, label %sw.bb3
+  ]
+sw.bb: br label %sw.epilog
+sw.bb1: br label %sw.epilog
+sw.bb2: br label %sw.epilog
+sw.bb3: br label %sw.epilog
+sw.default: br label %sw.epilog
+sw.epilog:
+  %r.0 = phi i32 [ 12, %sw.default ], [ 13, %sw.bb3 ], [ 12, %sw.bb2 ], [ 11, %sw.bb1 ], [ 10, %sw.bb ]
+  %cmp = icmp ne i32 %r.0, 0
+  br i1 %cmp, label %if.then, label %if.end
+if.then: br label %return
+if.end: br label %return
+return:
+  %retval.0 = phi i32 [ %r.0, %if.then ], [ 100, %if.end ]
+  ret i32 %retval.0
+; CHECK-LABEL: @no_reuse_cmp(
+; CHECK:  [[S:%.+]] = select 
+; CHECK-NEXT:  %cmp = icmp ne i32 [[S]], 0
+; CHECK-NEXT:  [[R:%.+]] = select i1 %cmp, i32 [[S]], i32 100
+; CHECK-NEXT:  ret i32 [[R]]
+}
+