Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -834,11 +834,12 @@
 static Instruction *simplifyX86MaskedLoad(IntrinsicInst &II, InstCombiner &IC) {
   Value *Ptr = II.getOperand(0);
   Value *Mask = II.getOperand(1);
+  Constant *ZeroVec = Constant::getNullValue(II.getType());
 
   // Special case a zero mask since that's not a ConstantDataVector.
-  // This masked load instruction does nothing, so return an undef.
+  // This masked load instruction creates a zero vector.
   if (isa<ConstantAggregateZero>(Mask))
-    return IC.replaceInstUsesWith(II, UndefValue::get(II.getType()));
+    return IC.replaceInstUsesWith(II, ZeroVec);
 
   auto *ConstMask = dyn_cast<ConstantDataVector>(Mask);
   if (!ConstMask)
@@ -857,7 +858,9 @@
   // on each element's most significant bit (the sign bit).
   Constant *BoolMask = getNegativeIsTrueBoolVec(ConstMask);
 
-  CallInst *NewMaskedLoad = IC.Builder->CreateMaskedLoad(PtrCast, 1, BoolMask);
+  // The pass-through vector for an x86 masked load is a zero vector.
+  CallInst *NewMaskedLoad =
+      IC.Builder->CreateMaskedLoad(PtrCast, 1, BoolMask, ZeroVec);
   return IC.replaceInstUsesWith(II, NewMaskedLoad);
 }
 
Index: test/Transforms/InstCombine/x86-masked-memops.ll
===================================================================
--- test/Transforms/InstCombine/x86-masked-memops.ll
+++ test/Transforms/InstCombine/x86-masked-memops.ll
@@ -13,14 +13,14 @@
 ; CHECK-NEXT:  ret <4 x float> %ld
 }
 
-; Zero mask is a nop.
+; Zero mask returns a zero vector.
 
 define <4 x float> @mload_zeros(i8* %f) {
   %ld = tail call <4 x float> @llvm.x86.avx.maskload.ps(i8* %f, <4 x i32> zeroinitializer)
   ret <4 x float> %ld
 
 ; CHECK-LABEL: @mload_zeros(
-; CHECK-NEXT:  ret <4 x float> undef
+; CHECK-NEXT:  ret <4 x float> zeroinitializer
 }
 
 ; Only the sign bit matters.
@@ -30,7 +30,7 @@
   ret <4 x float> %ld
 
 ; CHECK-LABEL: @mload_fake_ones(
-; CHECK-NEXT:  ret <4 x float> undef
+; CHECK-NEXT:  ret <4 x float> zeroinitializer
 }
 
 ; All mask bits are set, so this is just a vector load.
@@ -53,7 +53,7 @@
 
 ; CHECK-LABEL: @mload_one_one(
 ; CHECK-NEXT:  %castvec = bitcast i8* %f to <4 x float>*
-; CHECK-NEXT:  %1 = call <4 x float> @llvm.masked.load.v4f32(<4 x float>* %castvec, i32 1, <4 x i1> <i1 false, i1 false, i1 false, i1 true>, <4 x float> undef)
+; CHECK-NEXT:  %1 = call <4 x float> @llvm.masked.load.v4f32(<4 x float>* %castvec, i32 1, <4 x i1> <i1 false, i1 false, i1 false, i1 true>, <4 x float> zeroinitializer)
 ; CHECK-NEXT:  ret <4 x float> %1
 }
 
@@ -65,7 +65,7 @@
 
 ; CHECK-LABEL: @mload_one_one_double(
 ; CHECK-NEXT:  %castvec = bitcast i8* %f to <2 x double>*
-; CHECK-NEXT:  %1 = call <2 x double> @llvm.masked.load.v2f64(<2 x double>* %castvec, i32 1, <2 x i1> <i1 true, i1 false>, <2 x double> undef)
+; CHECK-NEXT:  %1 = call <2 x double> @llvm.masked.load.v2f64(<2 x double>* %castvec, i32 1, <2 x i1> <i1 true, i1 false>, <2 x double> zeroinitializer)
 ; CHECK-NEXT:  ret <2 x double> %1
 }
 
@@ -77,7 +77,7 @@
 
 ; CHECK-LABEL: @mload_v8f32(
 ; CHECK-NEXT:  %castvec = bitcast i8* %f to <8 x float>*
-; CHECK-NEXT:  %1 = call <8 x float> @llvm.masked.load.v8f32(<8 x float>* %castvec, i32 1, <8 x i1> <i1 false, i1 false, i1 false, i1 true, i1 false, i1 false, i1 false, i1 false>, <8 x float> undef)
+; CHECK-NEXT:  %1 = call <8 x float> @llvm.masked.load.v8f32(<8 x float>* %castvec, i32 1, <8 x i1> <i1 false, i1 false, i1 false, i1 true, i1 false, i1 false, i1 false, i1 false>, <8 x float> zeroinitializer)
 ; CHECK-NEXT:  ret <8 x float> %1
 }
 
@@ -87,7 +87,7 @@
 
 ; CHECK-LABEL: @mload_v4f64(
 ; CHECK-NEXT:  %castvec = bitcast i8* %f to <4 x double>*
-; CHECK-NEXT:  %1 = call <4 x double> @llvm.masked.load.v4f64(<4 x double>* %castvec, i32 1, <4 x i1> <i1 true, i1 false, i1 false, i1 false>, <4 x double> undef)
+; CHECK-NEXT:  %1 = call <4 x double> @llvm.masked.load.v4f64(<4 x double>* %castvec, i32 1, <4 x i1> <i1 true, i1 false, i1 false, i1 false>, <4 x double> zeroinitializer)
 ; CHECK-NEXT:  ret <4 x double> %1
 }
 
@@ -99,7 +99,7 @@
 
 ; CHECK-LABEL: @mload_v4i32(
 ; CHECK-NEXT:  %castvec = bitcast i8* %f to <4 x i32>*
-; CHECK-NEXT:  %1 = call <4 x i32> @llvm.masked.load.v4i32(<4 x i32>* %castvec, i32 1, <4 x i1> <i1 false, i1 false, i1 false, i1 true>, <4 x i32> undef)
+; CHECK-NEXT:  %1 = call <4 x i32> @llvm.masked.load.v4i32(<4 x i32>* %castvec, i32 1, <4 x i1> <i1 false, i1 false, i1 false, i1 true>, <4 x i32> zeroinitializer)
 ; CHECK-NEXT:  ret <4 x i32> %1
 }
 
@@ -109,7 +109,7 @@
 
 ; CHECK-LABEL: @mload_v2i64(
 ; CHECK-NEXT:  %castvec = bitcast i8* %f to <2 x i64>*
-; CHECK-NEXT:  %1 = call <2 x i64> @llvm.masked.load.v2i64(<2 x i64>* %castvec, i32 1, <2 x i1> <i1 true, i1 false>, <2 x i64> undef)
+; CHECK-NEXT:  %1 = call <2 x i64> @llvm.masked.load.v2i64(<2 x i64>* %castvec, i32 1, <2 x i1> <i1 true, i1 false>, <2 x i64> zeroinitializer)
 ; CHECK-NEXT:  ret <2 x i64> %1
 }
 
@@ -119,7 +119,7 @@
 
 ; CHECK-LABEL: @mload_v8i32(
 ; CHECK-NEXT:  %castvec = bitcast i8* %f to <8 x i32>*
-; CHECK-NEXT:  %1 = call <8 x i32> @llvm.masked.load.v8i32(<8 x i32>* %castvec, i32 1, <8 x i1> <i1 false, i1 false, i1 false, i1 true, i1 false, i1 false, i1 false, i1 false>, <8 x i32> undef)
+; CHECK-NEXT:  %1 = call <8 x i32> @llvm.masked.load.v8i32(<8 x i32>* %castvec, i32 1, <8 x i1> <i1 false, i1 false, i1 false, i1 true, i1 false, i1 false, i1 false, i1 false>, <8 x i32> zeroinitializer)
 ; CHECK-NEXT:  ret <8 x i32> %1
 }
 
@@ -129,7 +129,7 @@
 
 ; CHECK-LABEL: @mload_v4i64(
 ; CHECK-NEXT:  %castvec = bitcast i8* %f to <4 x i64>*
-; CHECK-NEXT:  %1 = call <4 x i64> @llvm.masked.load.v4i64(<4 x i64>* %castvec, i32 1, <4 x i1> <i1 true, i1 false, i1 false, i1 false>, <4 x i64> undef)
+; CHECK-NEXT:  %1 = call <4 x i64> @llvm.masked.load.v4i64(<4 x i64>* %castvec, i32 1, <4 x i1> <i1 true, i1 false, i1 false, i1 false>, <4 x i64> zeroinitializer)
 ; CHECK-NEXT:  ret <4 x i64> %1
 }