diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 301d5770cf78f..01f2161f27555 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -2297,9 +2297,13 @@ void CodeGenFunction::EmitStoreOfScalar(llvm::Value *Value, Address Addr,
           CGM.getABIInfo().getOptimalVectorMemoryType(VecTy, getLangOpts());
       if (!ClangVecTy->isPackedVectorBoolType(getContext()) &&
           VecTy != NewVecTy) {
-        SmallVector<int, 16> Mask(NewVecTy->getNumElements(), -1);
+        SmallVector<int, 16> Mask(NewVecTy->getNumElements(),
+                                  VecTy->getNumElements());
         std::iota(Mask.begin(), Mask.begin() + VecTy->getNumElements(), 0);
-        Value = Builder.CreateShuffleVector(Value, Mask, "extractVec");
+        // Use undef instead of poison for the padding lanes, to make sure no
+        // padding bits are poisoned, which may break coercion.
+        Value = Builder.CreateShuffleVector(Value, llvm::UndefValue::get(VecTy),
+                                            Mask, "extractVec");
         SrcTy = NewVecTy;
       }
       if (Addr.getElementType() != SrcTy)
diff --git a/clang/test/CodeGen/AArch64/ext-vector-coercion.c b/clang/test/CodeGen/AArch64/ext-vector-coercion.c
new file mode 100644
index 0000000000000..354980afe06d7
--- /dev/null
+++ b/clang/test/CodeGen/AArch64/ext-vector-coercion.c
@@ -0,0 +1,42 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 6
+// RUN: %clang_cc1 -fenable-matrix -triple arm64-apple-macosx %s -emit-llvm -disable-llvm-passes -o - | FileCheck %s
+
+typedef float float3 __attribute__((ext_vector_type(3)));
+struct Vec3 {
+  union {
+    struct {
+      float x;
+      float y;
+      float z;
+    };
+    float vec __attribute__((ext_vector_type(3)));
+  };
+};
+
+// CHECK-LABEL: define i128 @add(
+// CHECK-SAME: i128 [[A_COERCE:%.*]]) #[[ATTR0:[0-9]+]] {
+// CHECK-NEXT:  [[ENTRY:.*:]]
+// CHECK-NEXT:    [[RETVAL:%.*]] = alloca [[STRUCT_VEC3:%.*]], align 16
+// CHECK-NEXT:    [[A:%.*]] = alloca [[STRUCT_VEC3]], align 16
+// CHECK-NEXT:    [[COERCE_DIVE:%.*]] = getelementptr inbounds nuw [[STRUCT_VEC3]], ptr [[A]], i32 0, i32 0
+// CHECK-NEXT:    store i128 [[A_COERCE]], ptr [[COERCE_DIVE]], align 16
+// CHECK-NEXT:    [[TMP0:%.*]] = getelementptr inbounds nuw [[STRUCT_VEC3]], ptr [[A]], i32 0, i32 0
+// CHECK-NEXT:    [[LOADVECN:%.*]] = load <4 x float>, ptr [[TMP0]], align 16
+// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <4 x float> [[LOADVECN]], <4 x float> poison, <3 x i32> <i32 0, i32 1, i32 2>
+// CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds nuw [[STRUCT_VEC3]], ptr [[A]], i32 0, i32 0
+// CHECK-NEXT:    [[LOADVECN1:%.*]] = load <4 x float>, ptr [[TMP1]], align 16
+// CHECK-NEXT:    [[EXTRACTVEC2:%.*]] = shufflevector <4 x float> [[LOADVECN1]], <4 x float> poison, <3 x i32> <i32 0, i32 1, i32 2>
+// CHECK-NEXT:    [[ADD:%.*]] = fadd <3 x float> [[EXTRACTVEC]], [[EXTRACTVEC2]]
+// CHECK-NEXT:    [[TMP2:%.*]] = getelementptr inbounds nuw [[STRUCT_VEC3]], ptr [[RETVAL]], i32 0, i32 0
+// CHECK-NEXT:    [[EXTRACTVEC3:%.*]] = shufflevector <3 x float> [[ADD]], <3 x float> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
+// CHECK-NEXT:    store <4 x float> [[EXTRACTVEC3]], ptr [[TMP2]], align 16
+// CHECK-NEXT:    [[COERCE_DIVE4:%.*]] = getelementptr inbounds nuw [[STRUCT_VEC3]], ptr [[RETVAL]], i32 0, i32 0
+// CHECK-NEXT:    [[TMP3:%.*]] = load i128, ptr [[COERCE_DIVE4]], align 16
+// CHECK-NEXT:    ret i128 [[TMP3]]
+//
+struct Vec3 add(struct Vec3 a) {
+  struct Vec3 res;
+  res.vec = a.vec + a.vec;
+  return res;
+}
+
diff --git a/clang/test/CodeGenCXX/matrix-vector-bit-int.cpp b/clang/test/CodeGenCXX/matrix-vector-bit-int.cpp
index 2e7531b334ecb..4be1cb3067c2f 100644
--- a/clang/test/CodeGenCXX/matrix-vector-bit-int.cpp
+++ b/clang/test/CodeGenCXX/matrix-vector-bit-int.cpp
@@ -19,7 +19,7 @@ using i4x3x3 = _BitInt(4) __attribute__((matrix_type(3, 3)));
 // CHECK-NEXT:    store i32 [[A_COERCE]], ptr [[A]], align 4
 // CHECK-NEXT:    [[LOADVECN:%.*]] = load <4 x i8>, ptr [[A]], align 4
 // CHECK-NEXT:    [[A1:%.*]] = shufflevector <4 x i8> [[LOADVECN]], <4 x i8> poison, <3 x i32> <i32 0, i32 1, i32 2>
-// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i8> [[A1]], <3 x i8> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i8> [[A1]], <3 x i8> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
 // CHECK-NEXT:    store <4 x i8> [[EXTRACTVEC]], ptr [[A_ADDR]], align 4
 // CHECK-NEXT:    [[LOADVECN2:%.*]] = load <4 x i8>, ptr [[A_ADDR]], align 4
 // CHECK-NEXT:    [[EXTRACTVEC3:%.*]] = shufflevector <4 x i8> [[LOADVECN2]], <4 x i8> poison, <3 x i32> <i32 0, i32 1, i32 2>
@@ -38,7 +38,7 @@ i8x3 v1(i8x3 a) {
 // CHECK-SAME: <3 x i32> noundef [[A:%.*]]) #[[ATTR1:[0-9]+]] {
 // CHECK-NEXT:  [[ENTRY:.*:]]
 // CHECK-NEXT:    [[A_ADDR:%.*]] = alloca <3 x i32>, align 16
-// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i32> [[A]], <3 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i32> [[A]], <3 x i32> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
 // CHECK-NEXT:    store <4 x i32> [[EXTRACTVEC]], ptr [[A_ADDR]], align 16
 // CHECK-NEXT:    [[LOADVECN:%.*]] = load <4 x i32>, ptr [[A_ADDR]], align 16
 // CHECK-NEXT:    [[EXTRACTVEC1:%.*]] = shufflevector <4 x i32> [[LOADVECN]], <4 x i32> poison, <3 x i32> <i32 0, i32 1, i32 2>
@@ -57,7 +57,7 @@ i32x3 v2(i32x3 a) {
 // CHECK-NEXT:    [[A_ADDR:%.*]] = alloca <3 x i512>, align 256
 // CHECK-NEXT:    [[LOADVECN:%.*]] = load <4 x i512>, ptr [[TMP0]], align 256
 // CHECK-NEXT:    [[A:%.*]] = shufflevector <4 x i512> [[LOADVECN]], <4 x i512> poison, <3 x i32> <i32 0, i32 1, i32 2>
-// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i512> [[A]], <3 x i512> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i512> [[A]], <3 x i512> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
 // CHECK-NEXT:    store <4 x i512> [[EXTRACTVEC]], ptr [[A_ADDR]], align 256
 // CHECK-NEXT:    [[LOADVECN1:%.*]] = load <4 x i512>, ptr [[A_ADDR]], align 256
 // CHECK-NEXT:    [[EXTRACTVEC2:%.*]] = shufflevector <4 x i512> [[LOADVECN1]], <4 x i512> poison, <3 x i32> <i32 0, i32 1, i32 2>
@@ -80,7 +80,7 @@ i512x3 v3(i512x3 a) {
 // CHECK-NEXT:    store i32 [[A_COERCE]], ptr [[A]], align 4
 // CHECK-NEXT:    [[LOADVECN:%.*]] = load <4 x i4>, ptr [[A]], align 4
 // CHECK-NEXT:    [[A1:%.*]] = shufflevector <4 x i4> [[LOADVECN]], <4 x i4> poison, <3 x i32> <i32 0, i32 1, i32 2>
-// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i4> [[A1]], <3 x i4> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i4> [[A1]], <3 x i4> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
 // CHECK-NEXT:    store <4 x i4> [[EXTRACTVEC]], ptr [[A_ADDR]], align 4
 // CHECK-NEXT:    [[LOADVECN2:%.*]] = load <4 x i4>, ptr [[A_ADDR]], align 4
 // CHECK-NEXT:    [[EXTRACTVEC3:%.*]] = shufflevector <4 x i4> [[LOADVECN2]], <4 x i4> poison, <3 x i32> <i32 0, i32 1, i32 2>
diff --git a/clang/test/CodeGenOpenCL/preserve_vec3.cl b/clang/test/CodeGenOpenCL/preserve_vec3.cl
index e76aa81f918cb..0017169b8cf48 100644
--- a/clang/test/CodeGenOpenCL/preserve_vec3.cl
+++ b/clang/test/CodeGenOpenCL/preserve_vec3.cl
@@ -12,7 +12,7 @@ typedef float float4 __attribute__((ext_vector_type(4)));
 // CHECK-SAME: ptr addrspace(1) noundef readonly align 16 captures(none) [[A:%.*]], ptr addrspace(1) noundef writeonly align 16 captures(none) initializes((0, 16)) [[B:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] !kernel_arg_addr_space [[META7:![0-9]+]] !kernel_arg_access_qual [[META8:![0-9]+]] !kernel_arg_type [[META9:![0-9]+]] !kernel_arg_base_type [[META10:![0-9]+]] !kernel_arg_type_qual [[META11:![0-9]+]] {
 // CHECK-NEXT:  [[ENTRY:.*:]]
 // CHECK-NEXT:    [[TMP0:%.*]] = load <3 x float>, ptr addrspace(1) [[A]], align 16
-// CHECK-NEXT:    [[EXTRACTVEC1_I:%.*]] = shufflevector <3 x float> [[TMP0]], <3 x float> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    [[EXTRACTVEC1_I:%.*]] = shufflevector <3 x float> [[TMP0]], <3 x float> <float undef, float poison, float poison>, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
 // CHECK-NEXT:    store <4 x float> [[EXTRACTVEC1_I]], ptr addrspace(1) [[B]], align 16, !tbaa [[CHAR_TBAA12:![0-9]+]]
 // CHECK-NEXT:    ret void
 //
@@ -24,7 +24,7 @@ void kernel foo(global float3 *a, global float3 *b) {
 // CHECK-SAME: ptr addrspace(1) noundef writeonly align 16 captures(none) initializes((0, 16)) [[A:%.*]], ptr addrspace(1) noundef readonly align 16 captures(none) [[B:%.*]]) local_unnamed_addr #[[ATTR0]] !kernel_arg_addr_space [[META7]] !kernel_arg_access_qual [[META8]] !kernel_arg_type [[META13:![0-9]+]] !kernel_arg_base_type [[META14:![0-9]+]] !kernel_arg_type_qual [[META11]] {
 // CHECK-NEXT:  [[ENTRY:.*:]]
 // CHECK-NEXT:    [[TMP0:%.*]] = load <3 x float>, ptr addrspace(1) [[B]], align 16, !tbaa [[CHAR_TBAA12]]
-// CHECK-NEXT:    [[EXTRACTVEC_I:%.*]] = shufflevector <3 x float> [[TMP0]], <3 x float> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    [[EXTRACTVEC_I:%.*]] = shufflevector <3 x float> [[TMP0]], <3 x float> <float undef, float poison, float poison>, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
 // CHECK-NEXT:    store <4 x float> [[EXTRACTVEC_I]], ptr addrspace(1) [[A]], align 16, !tbaa [[CHAR_TBAA12]]
 // CHECK-NEXT:    ret void
 //
@@ -60,7 +60,7 @@ void kernel float3_to_double2(global float3 *a, global double2 *b) {
 // CHECK-SAME: ptr addrspace(1) noundef writeonly align 8 captures(none) initializes((0, 8)) [[A:%.*]], ptr addrspace(1) noundef readonly align 8 captures(none) [[B:%.*]]) local_unnamed_addr #[[ATTR0]] !kernel_arg_addr_space [[META7]] !kernel_arg_access_qual [[META8]] !kernel_arg_type [[META17:![0-9]+]] !kernel_arg_base_type [[META18:![0-9]+]] !kernel_arg_type_qual [[META11]] {
 // CHECK-NEXT:  [[ENTRY:.*:]]
 // CHECK-NEXT:    [[TMP0:%.*]] = load <3 x i16>, ptr addrspace(1) [[B]], align 8, !tbaa [[CHAR_TBAA12]]
-// CHECK-NEXT:    [[EXTRACTVEC_I:%.*]] = shufflevector <3 x i16> [[TMP0]], <3 x i16> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    [[EXTRACTVEC_I:%.*]] = shufflevector <3 x i16> [[TMP0]], <3 x i16> <i16 undef, i16 poison, i16 poison>, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
 // CHECK-NEXT:    store <4 x i16> [[EXTRACTVEC_I]], ptr addrspace(1) [[A]], align 8, !tbaa [[CHAR_TBAA12]]
 // CHECK-NEXT:    ret void
 //
@@ -71,8 +71,8 @@ void kernel char8_to_short3(global short3 *a, global char8 *b) {
 // CHECK-LABEL: define dso_local spir_func void @from_char3(
 // CHECK-SAME: <3 x i8> noundef [[A:%.*]], ptr addrspace(1) noundef writeonly captures(none) initializes((0, 4)) [[OUT:%.*]]) local_unnamed_addr #[[ATTR2:[0-9]+]] {
 // CHECK-NEXT:  [[ENTRY:.*:]]
-// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i8> [[A]], <3 x i8> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
-// CHECK-NEXT:    store <4 x i8> [[EXTRACTVEC]], ptr addrspace(1) [[OUT]], align 4, !tbaa [[INT_TBAA3:![0-9]+]]
+// CHECK-NEXT:    [[TMP0:%.*]] = shufflevector <3 x i8> [[A]], <3 x i8> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    store <4 x i8> [[TMP0]], ptr addrspace(1) [[OUT]], align 4, !tbaa [[INT_TBAA3:![0-9]+]]
 // CHECK-NEXT:    ret void
 //
 void from_char3(char3 a, global int *out) {
@@ -82,8 +82,8 @@ void from_char3(char3 a, global int *out) {
 // CHECK-LABEL: define dso_local spir_func void @from_short3(
 // CHECK-SAME: <3 x i16> noundef [[A:%.*]], ptr addrspace(1) noundef writeonly captures(none) initializes((0, 8)) [[OUT:%.*]]) local_unnamed_addr #[[ATTR2]] {
 // CHECK-NEXT:  [[ENTRY:.*:]]
-// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i16> [[A]], <3 x i16> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
-// CHECK-NEXT:    store <4 x i16> [[EXTRACTVEC]], ptr addrspace(1) [[OUT]], align 8, !tbaa [[LONG_TBAA19:![0-9]+]]
+// CHECK-NEXT:    [[TMP0:%.*]] = shufflevector <3 x i16> [[A]], <3 x i16> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    store <4 x i16> [[TMP0]], ptr addrspace(1) [[OUT]], align 8, !tbaa [[LONG_TBAA19:![0-9]+]]
 // CHECK-NEXT:    ret void
 //
 void from_short3(short3 a, global long *out) {
@@ -94,7 +94,8 @@ void from_short3(short3 a, global long *out) {
 // CHECK-SAME: i32 noundef [[A:%.*]], ptr addrspace(1) noundef writeonly captures(none) initializes((0, 4)) [[OUT:%.*]]) local_unnamed_addr #[[ATTR2]] {
 // CHECK-NEXT:  [[ENTRY:.*:]]
 // CHECK-NEXT:    [[TMP0:%.*]] = bitcast i32 [[A]] to <4 x i8>
-// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <4 x i8> [[TMP0]], <4 x i8> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    [[ASTYPE:%.*]] = shufflevector <4 x i8> [[TMP0]], <4 x i8> poison, <3 x i32> <i32 0, i32 1, i32 2>
+// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i8> [[ASTYPE]], <3 x i8> <i8 undef, i8 poison, i8 poison>, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
 // CHECK-NEXT:    store <4 x i8> [[EXTRACTVEC]], ptr addrspace(1) [[OUT]], align 4, !tbaa [[CHAR_TBAA12]]
 // CHECK-NEXT:    ret void
 //
@@ -106,7 +107,8 @@ void scalar_to_char3(int a, global char3 *out) {
 // CHECK-SAME: i64 noundef [[A:%.*]], ptr addrspace(1) noundef writeonly captures(none) initializes((0, 8)) [[OUT:%.*]]) local_unnamed_addr #[[ATTR2]] {
 // CHECK-NEXT:  [[ENTRY:.*:]]
 // CHECK-NEXT:    [[TMP0:%.*]] = bitcast i64 [[A]] to <4 x i16>
-// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <4 x i16> [[TMP0]], <4 x i16> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
+// CHECK-NEXT:    [[ASTYPE:%.*]] = shufflevector <4 x i16> [[TMP0]], <4 x i16> poison, <3 x i32> <i32 0, i32 1, i32 2>
+// CHECK-NEXT:    [[EXTRACTVEC:%.*]] = shufflevector <3 x i16> [[ASTYPE]], <3 x i16> <i16 undef, i16 poison, i16 poison>, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
 // CHECK-NEXT:    store <4 x i16> [[EXTRACTVEC]], ptr addrspace(1) [[OUT]], align 8, !tbaa [[CHAR_TBAA12]]
 // CHECK-NEXT:    ret void
 //