diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1142,6 +1142,49 @@
                           CI->getType());
 }
 
+// Optimize a memcmp call CI with constant arrays and nonconstant Size.
+static Value *optimizeMemCmpVarSize(CallInst *CI, Value *LHS, Value *RHS,
+				    Value *Size, IRBuilderBase &B,
+				    const DataLayout &DL) {
+  StringRef LStr, RStr;
+  if (!getConstantStringInfo(LHS, LStr, 0, /*TrimAtNul=*/false) ||
+      !getConstantStringInfo(RHS, RStr, 0, /*TrimAtNul=*/false))
+    return nullptr;
+
+  // If the contents of both constant arrays are known, fold a call to
+  // memcmp(A, B, N) to
+  //   N <= Pos ? 0 : (A < B ? -1 : B < A ? +1 : 0)
+  // where Pos is the first mismatch between A and B, determined below.
+
+  int IRes = 0;
+  uint64_t Pos = 0;
+  for (uint64_t MinSize = std::min(LStr.size(), RStr.size()); ; ++Pos) {
+    if (Pos == MinSize) {
+      // One array is a leading part of the other of equal or greater
+      // size.
+      // Fold the result to zero.  Size is assumed to be in bounds, since
+      // otherwise the call would be undefined.
+      // Setting Pos to MAX ensures that the N <= Pos test inserted below
+      // is eventually folded away.
+      Pos = UINT64_MAX;
+      break;
+    }
+
+    if (LStr[Pos] != RStr[Pos]) {
+      // Normalize the result.
+      IRes = LStr[Pos] < RStr[Pos] ? -1 : 1;
+      break;
+    }
+  }
+
+  Value *Zero = ConstantInt::get(CI->getType(), 0);
+  Value *MaxSize = ConstantInt::get(Size->getType(), Pos);
+  Value *Cmp = B.CreateICmp(ICmpInst::ICMP_ULE, Size, MaxSize);
+  Value *Res = ConstantInt::get(CI->getType(), IRes);
+  return B.CreateSelect(Cmp, Zero, Res);
+}
+
+// Optimize a memcmp call CI with constant size Len.
 static Value *optimizeMemCmpConstantSize(CallInst *CI, Value *LHS, Value *RHS,
                                          uint64_t Len, IRBuilderBase &B,
                                          const DataLayout &DL) {
@@ -1228,19 +1271,19 @@
     return Constant::getNullValue(CI->getType());
 
   annotateNonNullAndDereferenceable(CI, {0, 1}, Size, DL);
-  // Handle constant lengths.
+
   ConstantInt *LenC = dyn_cast<ConstantInt>(Size);
   if (!LenC)
-    return nullptr;
+    // Hande variable Size.
+    return optimizeMemCmpVarSize(CI, LHS, RHS, Size, B, DL);
+
+  // Handle constant Size.
 
   // memcmp(d,s,0) -> 0
   if (LenC->getZExtValue() == 0)
     return Constant::getNullValue(CI->getType());
 
-  if (Value *Res =
-          optimizeMemCmpConstantSize(CI, LHS, RHS, LenC->getZExtValue(), B, DL))
-    return Res;
-  return nullptr;
+  return optimizeMemCmpConstantSize(CI, LHS, RHS, LenC->getZExtValue(), B, DL);
 }
 
 Value *LibCallSimplifier::optimizeMemCmp(CallInst *CI, IRBuilderBase &B) {
diff --git a/llvm/test/Transforms/InstCombine/memcmp-5.ll b/llvm/test/Transforms/InstCombine/memcmp-5.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/memcmp-5.ll
@@ -0,0 +1,249 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+;
+; Exercise folding of memcmp calls with constant arrays and nonconstant
+; sizes.
+
+declare i32 @memcmp(i8*, i8*, i64)
+
+@ax = external constant [8 x i8]
+@a01230123 = constant [8 x i8] c"01230123"
+@b01230123 = constant [8 x i8] c"01230123"
+@c01230129 = constant [8 x i8] c"01230129"
+@d9123012  = constant [7 x i8] c"9123012"
+
+
+; Exercise memcmp(A, B, N) folding of arrays with the same bytes.
+
+define void @fold_memcmp_a_b_n(i32* %pcmp, i64 %n) {
+; CHECK-LABEL: @fold_memcmp_a_b_n(
+; CHECK-NEXT:    store i32 0, i32* [[PCMP:%.*]], align 4
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ne i64 [[N:%.*]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = sext i1 [[TMP1]] to i32
+; CHECK-NEXT:    [[S0_1:%.*]] = getelementptr i32, i32* [[PCMP]], i64 1
+; CHECK-NEXT:    store i32 [[TMP2]], i32* [[S0_1]], align 4
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp ne i64 [[N]], 0
+; CHECK-NEXT:    [[TMP4:%.*]] = sext i1 [[TMP3]] to i32
+; CHECK-NEXT:    [[S0_2:%.*]] = getelementptr i32, i32* [[PCMP]], i64 2
+; CHECK-NEXT:    store i32 [[TMP4]], i32* [[S0_2]], align 4
+; CHECK-NEXT:    [[TMP5:%.*]] = icmp ne i64 [[N]], 0
+; CHECK-NEXT:    [[TMP6:%.*]] = sext i1 [[TMP5]] to i32
+; CHECK-NEXT:    [[S0_3:%.*]] = getelementptr i32, i32* [[PCMP]], i64 3
+; CHECK-NEXT:    store i32 [[TMP6]], i32* [[S0_3]], align 4
+; CHECK-NEXT:    [[S0_4:%.*]] = getelementptr i32, i32* [[PCMP]], i64 4
+; CHECK-NEXT:    store i32 0, i32* [[S0_4]], align 4
+; CHECK-NEXT:    [[TMP7:%.*]] = icmp ne i64 [[N]], 0
+; CHECK-NEXT:    [[TMP8:%.*]] = sext i1 [[TMP7]] to i32
+; CHECK-NEXT:    [[S0_5:%.*]] = getelementptr i32, i32* [[PCMP]], i64 5
+; CHECK-NEXT:    store i32 [[TMP8]], i32* [[S0_5]], align 4
+; CHECK-NEXT:    ret void
+;
+
+  %p0 = getelementptr [8 x i8], [8 x i8]* @a01230123, i64 0, i64 0
+
+  %q0 = getelementptr [8 x i8], [8 x i8]* @b01230123, i64 0, i64 0
+  %q1 = getelementptr [8 x i8], [8 x i8]* @b01230123, i64 0, i64 1
+  %q2 = getelementptr [8 x i8], [8 x i8]* @b01230123, i64 0, i64 2
+  %q3 = getelementptr [8 x i8], [8 x i8]* @b01230123, i64 0, i64 3
+  %q4 = getelementptr [8 x i8], [8 x i8]* @b01230123, i64 0, i64 4
+  %q5 = getelementptr [8 x i8], [8 x i8]* @b01230123, i64 0, i64 5
+
+  ; Fold memcmp(a, b, n) to 0.
+  %c0_0 = call i32 @memcmp(i8* %p0, i8* %q0, i64 %n)
+  %s0_0 = getelementptr i32, i32* %pcmp, i64 0
+  store i32 %c0_0, i32* %s0_0
+
+  ; Fold memcmp(a, b + 1, n) to N != 0 ? -1 : 0.
+  %c0_1 = call i32 @memcmp(i8* %p0, i8* %q1, i64 %n)
+  %s0_1 = getelementptr i32, i32* %pcmp, i64 1
+  store i32 %c0_1, i32* %s0_1
+
+  ; Fold memcmp(a, b + 2, n) to N != 0 ? -1 : 0.
+  %c0_2 = call i32 @memcmp(i8* %p0, i8* %q2, i64 %n)
+  %s0_2 = getelementptr i32, i32* %pcmp, i64 2
+  store i32 %c0_2, i32* %s0_2
+
+  ; Fold memcmp(a, b + 3, n) to N != 0 ? -1 : 0.
+  %c0_3 = call i32 @memcmp(i8* %p0, i8* %q3, i64 %n)
+  %s0_3 = getelementptr i32, i32* %pcmp, i64 3
+  store i32 %c0_3, i32* %s0_3
+
+  ; Fold memcmp(a, b + 4, n) to 0.
+  %c0_4 = call i32 @memcmp(i8* %p0, i8* %q4, i64 %n)
+  %s0_4 = getelementptr i32, i32* %pcmp, i64 4
+  store i32 %c0_4, i32* %s0_4
+
+  ; Fold memcmp(a, b + 5, n) to N != 0 ? -1 : 0.
+  %c0_5 = call i32 @memcmp(i8* %p0, i8* %q5, i64 %n)
+  %s0_5 = getelementptr i32, i32* %pcmp, i64 5
+  store i32 %c0_5, i32* %s0_5
+
+  ret void
+}
+
+; Vefify that a memcmp() call involving a constant array with unknown
+; contents is not folded.
+
+define void @call_memcmp_a_ax_n(i32* %pcmp, i64 %n) {
+; CHECK-LABEL: @call_memcmp_a_ax_n(
+; CHECK-NEXT:    [[C0_0:%.*]] = call i32 @memcmp(i8* getelementptr inbounds ([8 x i8], [8 x i8]* @a01230123, i64 0, i64 0), i8* getelementptr inbounds ([8 x i8], [8 x i8]* @ax, i64 0, i64 0), i64 [[N:%.*]])
+; CHECK-NEXT:    store i32 [[C0_0]], i32* [[PCMP:%.*]], align 4
+; CHECK-NEXT:    ret void
+;
+
+  %p0 = getelementptr [8 x i8], [8 x i8]* @a01230123, i64 0, i64 0
+  %q0 = getelementptr [8 x i8], [8 x i8]* @ax, i64 0, i64 0
+
+  ; Do not fold memcmp(a, ax, n).
+  %c0_0 = call i32 @memcmp(i8* %p0, i8* %q0, i64 %n)
+  %s0_0 = getelementptr i32, i32* %pcmp, i64 0
+  store i32 %c0_0, i32* %s0_0
+
+  ret void
+}
+
+
+; Exercise memcmp(A, C, N) folding of arrays with the same leading bytes
+; but a difference in the trailing byte.
+
+define void @fold_memcmp_a_c_n(i32* %pcmp, i64 %n) {
+; CHECK-LABEL: @fold_memcmp_a_c_n(
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ugt i64 [[N:%.*]], 7
+; CHECK-NEXT:    [[TMP2:%.*]] = sext i1 [[TMP1]] to i32
+; CHECK-NEXT:    store i32 [[TMP2]], i32* [[PCMP:%.*]], align 4
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp ne i64 [[N]], 0
+; CHECK-NEXT:    [[TMP4:%.*]] = sext i1 [[TMP3]] to i32
+; CHECK-NEXT:    [[S0_1:%.*]] = getelementptr i32, i32* [[PCMP]], i64 1
+; CHECK-NEXT:    store i32 [[TMP4]], i32* [[S0_1]], align 4
+; CHECK-NEXT:    [[TMP5:%.*]] = icmp ne i64 [[N]], 0
+; CHECK-NEXT:    [[TMP6:%.*]] = sext i1 [[TMP5]] to i32
+; CHECK-NEXT:    [[S0_2:%.*]] = getelementptr i32, i32* [[PCMP]], i64 2
+; CHECK-NEXT:    store i32 [[TMP6]], i32* [[S0_2]], align 4
+; CHECK-NEXT:    [[TMP7:%.*]] = icmp ne i64 [[N]], 0
+; CHECK-NEXT:    [[TMP8:%.*]] = sext i1 [[TMP7]] to i32
+; CHECK-NEXT:    [[S0_3:%.*]] = getelementptr i32, i32* [[PCMP]], i64 3
+; CHECK-NEXT:    store i32 [[TMP8]], i32* [[S0_3]], align 4
+; CHECK-NEXT:    [[TMP9:%.*]] = icmp ugt i64 [[N]], 3
+; CHECK-NEXT:    [[TMP10:%.*]] = sext i1 [[TMP9]] to i32
+; CHECK-NEXT:    [[S0_4:%.*]] = getelementptr i32, i32* [[PCMP]], i64 4
+; CHECK-NEXT:    store i32 [[TMP10]], i32* [[S0_4]], align 4
+; CHECK-NEXT:    [[TMP11:%.*]] = icmp ugt i64 [[N]], 3
+; CHECK-NEXT:    [[TMP12:%.*]] = sext i1 [[TMP11]] to i32
+; CHECK-NEXT:    [[S0_5:%.*]] = getelementptr i32, i32* [[PCMP]], i64 5
+; CHECK-NEXT:    store i32 [[TMP12]], i32* [[S0_5]], align 4
+; CHECK-NEXT:    ret void
+;
+
+  %p0 = getelementptr [8 x i8], [8 x i8]* @a01230123, i64 0, i64 0
+
+  %q0 = getelementptr [8 x i8], [8 x i8]* @c01230129, i64 0, i64 0
+  %q1 = getelementptr [8 x i8], [8 x i8]* @c01230129, i64 0, i64 1
+  %q2 = getelementptr [8 x i8], [8 x i8]* @c01230129, i64 0, i64 2
+  %q3 = getelementptr [8 x i8], [8 x i8]* @c01230129, i64 0, i64 3
+  %q4 = getelementptr [8 x i8], [8 x i8]* @c01230129, i64 0, i64 4
+  %q5 = getelementptr [8 x i8], [8 x i8]* @c01230129, i64 0, i64 5
+
+  ; Fold memcmp(a, c, n) to N > 7 ? -1 : 0.
+  %c0_0 = call i32 @memcmp(i8* %p0, i8* %q0, i64 %n)
+  %s0_0 = getelementptr i32, i32* %pcmp, i64 0
+  store i32 %c0_0, i32* %s0_0
+
+  ; Fold memcmp(a, c + 1, n) to N != 0 ? -1 : 0.
+  %c0_1 = call i32 @memcmp(i8* %p0, i8* %q1, i64 %n)
+  %s0_1 = getelementptr i32, i32* %pcmp, i64 1
+  store i32 %c0_1, i32* %s0_1
+
+  ; Fold memcmp(a, c + 2, n) to N != 0 ? -1 : 0.
+  %c0_2 = call i32 @memcmp(i8* %p0, i8* %q2, i64 %n)
+  %s0_2 = getelementptr i32, i32* %pcmp, i64 2
+  store i32 %c0_2, i32* %s0_2
+
+  ; Fold memcmp(a, c + 3, n) to N != 0 ? -1 : 0.
+  %c0_3 = call i32 @memcmp(i8* %p0, i8* %q3, i64 %n)
+  %s0_3 = getelementptr i32, i32* %pcmp, i64 3
+  store i32 %c0_3, i32* %s0_3
+
+  ; Fold memcmp(a, c + 4, n) to N > 3 ? -1 : 0.
+  %c0_4 = call i32 @memcmp(i8* %p0, i8* %q4, i64 %n)
+  %s0_4 = getelementptr i32, i32* %pcmp, i64 4
+  store i32 %c0_4, i32* %s0_4
+
+  ; Fold memcmp(a, c + 5, n) to N != 0 ? -1 : 0.
+  %c0_5 = call i32 @memcmp(i8* %p0, i8* %q4, i64 %n)
+  %s0_5 = getelementptr i32, i32* %pcmp, i64 5
+  store i32 %c0_5, i32* %s0_5
+
+  ret void
+}
+
+
+; Exercise memcmp(A, D, N) folding of arrays of different sizes and
+; a difference in the leading byte.
+
+define void @fold_memcmp_a_d_n(i32* %pcmp, i64 %n) {
+; CHECK-LABEL: @fold_memcmp_a_d_n(
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ne i64 [[N:%.*]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = sext i1 [[TMP1]] to i32
+; CHECK-NEXT:    store i32 [[TMP2]], i32* [[PCMP:%.*]], align 4
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp ne i64 [[N]], 0
+; CHECK-NEXT:    [[TMP4:%.*]] = sext i1 [[TMP3]] to i32
+; CHECK-NEXT:    [[S0_1:%.*]] = getelementptr i32, i32* [[PCMP]], i64 1
+; CHECK-NEXT:    store i32 [[TMP4]], i32* [[S0_1]], align 4
+; CHECK-NEXT:    [[S1_1:%.*]] = getelementptr i32, i32* [[PCMP]], i64 2
+; CHECK-NEXT:    store i32 0, i32* [[S1_1]], align 4
+; CHECK-NEXT:    [[S6_6:%.*]] = getelementptr i32, i32* [[PCMP]], i64 3
+; CHECK-NEXT:    store i32 0, i32* [[S6_6]], align 4
+; CHECK-NEXT:    ret void
+;
+
+  %p0 = getelementptr [8 x i8], [8 x i8]* @a01230123, i64 0, i64 0
+  %p1 = getelementptr [8 x i8], [8 x i8]* @a01230123, i64 0, i64 1
+  %p6 = getelementptr [8 x i8], [8 x i8]* @a01230123, i64 0, i64 6
+
+  %q0 = getelementptr [7 x i8], [7 x i8]* @d9123012, i64 0, i64 0
+  %q1 = getelementptr [7 x i8], [7 x i8]* @d9123012, i64 0, i64 1
+  %q6 = getelementptr [7 x i8], [7 x i8]* @d9123012, i64 0, i64 6
+
+  ; Fold memcmp(a, d, n) to N != 0 ? -1 : 0.
+  %c0_0 = call i32 @memcmp(i8* %p0, i8* %q0, i64 %n)
+  %s0_0 = getelementptr i32, i32* %pcmp, i64 0
+  store i32 %c0_0, i32* %s0_0
+
+  ; Fold memcmp(a, d + 1, n) to N != 0 -1 : 0.
+  %c0_1 = call i32 @memcmp(i8* %p0, i8* %q1, i64 %n)
+  %s0_1 = getelementptr i32, i32* %pcmp, i64 1
+  store i32 %c0_1, i32* %s0_1
+
+  ; Fold memcmp(a + 1, d + 1, n) to 0.
+  %c1_1 = call i32 @memcmp(i8* %p1, i8* %q1, i64 %n)
+  %s1_1 = getelementptr i32, i32* %pcmp, i64 2
+  store i32 %c1_1, i32* %s1_1
+
+  ; Fold memcmp(a + 6, d + 6, n) to 0.
+  %c6_6 = call i32 @memcmp(i8* %p6, i8* %q6, i64 %n)
+  %s6_6 = getelementptr i32, i32* %pcmp, i64 3
+  store i32 %c6_6, i32* %s6_6
+
+  ret void
+}
+
+
+; Exercise memcmp(A, D, N) folding of arrays with the same bytes and
+; a nonzero size.
+
+define void @fold_memcmp_a_d_nz(i32* %pcmp, i64 %n) {
+; CHECK-LABEL: @fold_memcmp_a_d_nz(
+; CHECK-NEXT:    store i32 -1, i32* [[PCMP:%.*]], align 4
+; CHECK-NEXT:    ret void
+;
+
+  %p0 = getelementptr [8 x i8], [8 x i8]* @a01230123, i64 0, i64 0
+  %q0 = getelementptr [7 x i8], [7 x i8]* @d9123012, i64 0, i64 0
+  %nz = or i64 %n, 1
+
+  %c0_0 = call i32 @memcmp(i8* %p0, i8* %q0, i64 %nz)
+  %s0_0 = getelementptr i32, i32* %pcmp, i64 0
+  store i32 %c0_0, i32* %s0_0
+
+  ret void
+}