Index: llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -52,6 +52,7 @@
 #define DEBUG_TYPE "correlated-value-propagation"
 
 STATISTIC(NumPhis,      "Number of phis propagated");
+STATISTIC(NumPhiCommon, "Number of phis deleted via common incoming value");
 STATISTIC(NumSelects,   "Number of selects propagated");
 STATISTIC(NumMemAccess, "Number of memory access targets propagated");
 STATISTIC(NumCmps,      "Number of comparisons propagated");
@@ -123,6 +124,62 @@
   return true;
 }
 
+/// Try to simplify a phi with constant incoming values that match the edge
+/// values of a non-constant value on all other edges:
+/// bb0:
+///   %isnull = icmp eq i8* %x, null
+///   br i1 %isnull, label %bb2, label %bb1
+/// bb1:
+///   br label %bb2
+/// bb2:
+///   %r = phi i8* [ %x, %bb1 ], [ null, %bb0 ]
+/// -->
+///   %r = %x
+static bool simplifyCommonValuePhi(PHINode *P, LazyValueInfo *LVI,
+                                   const SimplifyQuery &SQ) {
+  // Collect incoming constants and initialize possible common value.
+  SmallVector<std::pair<Constant *, unsigned>, 4> IncomingConstants;
+  Value *CommonValue = nullptr;
+  for (unsigned i = 0, e = P->getNumIncomingValues(); i != e; ++i) {
+    Value *Incoming = P->getIncomingValue(i);
+    if (auto *IncomingConstant = dyn_cast<Constant>(Incoming)) {
+      IncomingConstants.push_back(std::make_pair(IncomingConstant, i));
+    } else if (!CommonValue) {
+      // The potential common value is initialized to the first non-constant.
+      CommonValue = Incoming;
+    } else if (Incoming != CommonValue) {
+      // There can be only one non-constant common value.
+      return false;
+    }
+  }
+
+  if (!CommonValue || IncomingConstants.empty())
+    return false;
+
+  // The common value must be valid in all incoming blocks.
+  BasicBlock *ToBB = P->getParent();
+  if (auto *CommonInst = dyn_cast<Instruction>(CommonValue))
+    if (!SQ.DT->dominates(CommonInst, ToBB))
+      return false;
+
+  // We have a phi with exactly 1 variable incoming value and 1 or more constant
+  // incoming values. See if all constant incoming values can be mapped back to
+  // the same incoming variable value.
+  for (auto &IncomingConstant : IncomingConstants) {
+    Constant *C = IncomingConstant.first;
+    BasicBlock *IncomingBB = P->getIncomingBlock(IncomingConstant.second);
+    if (C != LVI->getConstantOnEdge(CommonValue, IncomingBB, ToBB, P))
+      return false;
+  }
+
+  // All constant incoming values map to the same variable along the incoming
+  // edges of the phi. The phi is unnecessary.
+  P->replaceAllUsesWith(CommonValue);
+  P->eraseFromParent();
+  ++NumPhiCommon;
+  return true;
+}
+
 static bool processPHI(PHINode *P, LazyValueInfo *LVI,
                        const SimplifyQuery &SQ) {
   bool Changed = false;
@@ -184,6 +241,9 @@
     Changed = true;
   }
 
+  if (!Changed)
+    Changed = simplifyCommonValuePhi(P, LVI, SQ);
+
   if (Changed)
     ++NumPhis;
 
Index: llvm/trunk/test/Transforms/CorrelatedValuePropagation/phi-common-val.ll
===================================================================
--- llvm/trunk/test/Transforms/CorrelatedValuePropagation/phi-common-val.ll
+++ llvm/trunk/test/Transforms/CorrelatedValuePropagation/phi-common-val.ll
@@ -0,0 +1,94 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -correlated-propagation -S | FileCheck %s
+
+define i8* @simplify_phi_common_value_op0(i8* %ptr, i32* %b) {
+; CHECK-LABEL: @simplify_phi_common_value_op0(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[ISNULL:%.*]] = icmp eq i8* [[PTR:%.*]], null
+; CHECK-NEXT:    br i1 [[ISNULL]], label [[RETURN:%.*]], label [[ELSE:%.*]]
+; CHECK:       else:
+; CHECK-NEXT:    [[LB:%.*]] = load i32, i32* [[B:%.*]]
+; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[LB]], 1
+; CHECK-NEXT:    store i32 [[ADD]], i32* [[B]]
+; CHECK-NEXT:    br label [[RETURN]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i8* [[PTR]]
+;
+entry:
+  %isnull = icmp eq i8* %ptr, null
+  br i1 %isnull, label %return, label %else
+
+else:
+  %lb = load i32, i32* %b
+  %add = add nsw i32 %lb, 1
+  store i32 %add, i32* %b
+  br label %return
+
+return:
+  %r = phi i8* [ %ptr, %else ], [ null, %entry ]
+  ret i8* %r
+}
+
+define i8* @simplify_phi_common_value_op1(i8* %ptr, i32* %b) {
+; CHECK-LABEL: @simplify_phi_common_value_op1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[ISNULL:%.*]] = icmp eq i8* [[PTR:%.*]], null
+; CHECK-NEXT:    br i1 [[ISNULL]], label [[RETURN:%.*]], label [[ELSE:%.*]]
+; CHECK:       else:
+; CHECK-NEXT:    [[LB:%.*]] = load i32, i32* [[B:%.*]]
+; CHECK-NEXT:    [[ADD:%.*]] = add i32 [[LB]], 1
+; CHECK-NEXT:    store i32 [[ADD]], i32* [[B]]
+; CHECK-NEXT:    br label [[RETURN]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i8* [[PTR]]
+;
+entry:
+  %isnull = icmp eq i8* %ptr, null
+  br i1 %isnull, label %return, label %else
+
+else:
+  %lb = load i32, i32* %b
+  %add = add i32 %lb, 1
+  store i32 %add, i32* %b
+  br label %return
+
+return:
+  %r = phi i8* [ null, %entry], [ %ptr, %else ]
+  ret i8* %r
+}
+
+define i8 @simplify_phi_multiple_constants(i8 %x, i32* %b) {
+; CHECK-LABEL: @simplify_phi_multiple_constants(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[IS0:%.*]] = icmp eq i8 [[X:%.*]], 0
+; CHECK-NEXT:    br i1 [[IS0]], label [[RETURN:%.*]], label [[ELSE1:%.*]]
+; CHECK:       else1:
+; CHECK-NEXT:    [[IS42:%.*]] = icmp eq i8 [[X]], 42
+; CHECK-NEXT:    br i1 [[IS42]], label [[RETURN]], label [[ELSE2:%.*]]
+; CHECK:       else2:
+; CHECK-NEXT:    [[LB:%.*]] = load i32, i32* [[B:%.*]]
+; CHECK-NEXT:    [[ADD:%.*]] = add i32 [[LB]], 1
+; CHECK-NEXT:    store i32 [[ADD]], i32* [[B]]
+; CHECK-NEXT:    br label [[RETURN]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i8 [[X]]
+;
+entry:
+  %is0 = icmp eq i8 %x, 0
+  br i1 %is0, label %return, label %else1
+
+else1:
+  %is42 = icmp eq i8 %x, 42
+  br i1 %is42, label %return, label %else2
+
+else2:
+  %lb = load i32, i32* %b
+  %add = add i32 %lb, 1
+  store i32 %add, i32* %b
+  br label %return
+
+return:
+  %r = phi i8 [ 0, %entry], [ %x, %else2 ], [ 42, %else1 ]
+  ret i8 %r
+}
+