diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1048,7 +1048,9 @@
   BasicBlock *NonConstBB = nullptr;
   for (unsigned i = 0; i != NumPHIValues; ++i) {
     Value *InVal = PN->getIncomingValue(i);
-    if (isa<Constant>(InVal) && !isa<ConstantExpr>(InVal))
+    // If I is a freeze instruction, count undef as a non-constant.
+    if (isa<Constant>(InVal) && !isa<ConstantExpr>(InVal) &&
+        (!isa<FreezeInst>(I) || isGuaranteedNotToBeUndefOrPoison(InVal)))
       continue;
 
     if (isa<PHINode>(InVal)) return nullptr;  // Itself a phi.
@@ -1141,6 +1143,15 @@
                                              Builder);
       NewPN->addIncoming(InV, PN->getIncomingBlock(i));
     }
+  } else if (auto *FI = dyn_cast<FreezeInst>(&I)) {
+    for (unsigned i = 0; i != NumPHIValues; ++i) {
+      Value *InV;
+      if (NonConstBB == PN->getIncomingBlock(i))
+        InV = Builder.CreateFreeze(PN->getIncomingValue(i), "phi.fr");
+      else
+        InV = PN->getIncomingValue(i);
+      NewPN->addIncoming(InV, PN->getIncomingBlock(i));
+    }
   } else {
     CastInst *CI = cast<CastInst>(&I);
     Type *RetTy = CI->getType();
@@ -3370,6 +3381,12 @@
   if (Value *V = SimplifyFreezeInst(Op0, SQ.getWithInstruction(&I)))
     return replaceInstUsesWith(I, V);
 
+  // freeze (phi const, x) --> phi const, (freeze x)
+  if (auto *PN = dyn_cast<PHINode>(Op0)) {
+    if (Instruction *NV = foldOpIntoPhi(I, PN))
+      return NV;
+  }
+
   return nullptr;
 }
 
diff --git a/llvm/test/Transforms/InstCombine/freeze-phi.ll b/llvm/test/Transforms/InstCombine/freeze-phi.ll
--- a/llvm/test/Transforms/InstCombine/freeze-phi.ll
+++ b/llvm/test/Transforms/InstCombine/freeze-phi.ll
@@ -51,11 +51,11 @@
 ; CHECK:       A:
 ; CHECK-NEXT:    br label [[C:%.*]]
 ; CHECK:       B:
+; CHECK-NEXT:    [[PHI_FR:%.*]] = freeze <2 x i32> <i32 2, i32 undef>
 ; CHECK-NEXT:    br label [[C]]
 ; CHECK:       C:
-; CHECK-NEXT:    [[Y:%.*]] = phi <2 x i32> [ <i32 0, i32 1>, [[A]] ], [ <i32 2, i32 undef>, [[B]] ]
-; CHECK-NEXT:    [[Y_FR:%.*]] = freeze <2 x i32> [[Y]]
-; CHECK-NEXT:    ret <2 x i32> [[Y_FR]]
+; CHECK-NEXT:    [[Y:%.*]] = phi <2 x i32> [ <i32 0, i32 1>, [[A]] ], [ [[PHI_FR]], [[B]] ]
+; CHECK-NEXT:    ret <2 x i32> [[Y]]
 ;
   br i1 %cond, label %A, label %B
 A:
@@ -74,11 +74,11 @@
 ; CHECK:       A:
 ; CHECK-NEXT:    br label [[C:%.*]]
 ; CHECK:       B:
+; CHECK-NEXT:    [[PHI_FR:%.*]] = freeze i32 [[X:%.*]]
 ; CHECK-NEXT:    br label [[C]]
 ; CHECK:       C:
-; CHECK-NEXT:    [[Y:%.*]] = phi i32 [ 0, [[A]] ], [ [[X:%.*]], [[B]] ]
-; CHECK-NEXT:    [[Y_FR:%.*]] = freeze i32 [[Y]]
-; CHECK-NEXT:    ret i32 [[Y_FR]]
+; CHECK-NEXT:    [[Y:%.*]] = phi i32 [ 0, [[A]] ], [ [[PHI_FR]], [[B]] ]
+; CHECK-NEXT:    ret i32 [[Y]]
 ;
   br i1 %cond, label %A, label %B
 A:
@@ -154,15 +154,15 @@
 ; CHECK-NEXT:    i8 1, label [[C:%.*]]
 ; CHECK-NEXT:    ]
 ; CHECK:       A:
+; CHECK-NEXT:    [[PHI_FR:%.*]] = freeze i32 undef
 ; CHECK-NEXT:    br label [[D:%.*]]
 ; CHECK:       B:
 ; CHECK-NEXT:    br label [[D]]
 ; CHECK:       C:
 ; CHECK-NEXT:    br label [[D]]
 ; CHECK:       D:
-; CHECK-NEXT:    [[Y:%.*]] = phi i32 [ undef, [[A]] ], [ 32, [[B]] ], [ 0, [[C]] ]
-; CHECK-NEXT:    [[Y_FR:%.*]] = freeze i32 [[Y]]
-; CHECK-NEXT:    ret i32 [[Y_FR]]
+; CHECK-NEXT:    [[Y:%.*]] = phi i32 [ [[PHI_FR]], [[A]] ], [ 32, [[B]] ], [ 0, [[C]] ]
+; CHECK-NEXT:    ret i32 [[Y]]
 ;
   switch i8 %cond, label %A [
   i8 0, label %B
@@ -189,15 +189,15 @@
 ; CHECK-NEXT:    i8 1, label [[C:%.*]]
 ; CHECK-NEXT:    ]
 ; CHECK:       A:
+; CHECK-NEXT:    [[PHI_FR:%.*]] = freeze i32 ptrtoint (i8* getelementptr inbounds (i8, i8* @glb, i64 2) to i32)
 ; CHECK-NEXT:    br label [[D:%.*]]
 ; CHECK:       B:
 ; CHECK-NEXT:    br label [[D]]
 ; CHECK:       C:
 ; CHECK-NEXT:    br label [[D]]
 ; CHECK:       D:
-; CHECK-NEXT:    [[Y:%.*]] = phi i32 [ ptrtoint (i8* getelementptr inbounds (i8, i8* @glb, i64 2) to i32), [[A]] ], [ 32, [[B]] ], [ 0, [[C]] ]
-; CHECK-NEXT:    [[Y_FR:%.*]] = freeze i32 [[Y]]
-; CHECK-NEXT:    ret i32 [[Y_FR]]
+; CHECK-NEXT:    [[Y:%.*]] = phi i32 [ [[PHI_FR]], [[A]] ], [ 32, [[B]] ], [ 0, [[C]] ]
+; CHECK-NEXT:    ret i32 [[Y]]
 ;
   switch i8 %cond, label %A [
   i8 0, label %B