diff --git a/src/cmd/compile/internal/ssa/check.go b/src/cmd/compile/internal/ssa/check.go
index 398f06053e1..4ea85613040 100644
--- a/src/cmd/compile/internal/ssa/check.go
+++ b/src/cmd/compile/internal/ssa/check.go
@@ -152,7 +152,7 @@ func checkFunc(f *Func) {
 			case auxUInt8:
 				// Cast to int8 due to requirement of AuxInt, check its comment for details.
 				if v.AuxInt != int64(int8(v.AuxInt)) {
-					f.Fatalf("bad uint8 AuxInt value for %v", v)
+					f.Fatalf("bad uint8 AuxInt value for %v, saw %d but need %d", v, v.AuxInt, int64(int8(v.AuxInt)))
 				}
 				canHaveAuxInt = true
 			case auxFloat32:
diff --git a/src/cmd/compile/internal/ssagen/intrinsics.go b/src/cmd/compile/internal/ssagen/intrinsics.go
index ce9a76f6b84..985d899a71e 100644
--- a/src/cmd/compile/internal/ssagen/intrinsics.go
+++ b/src/cmd/compile/internal/ssagen/intrinsics.go
@@ -1614,6 +1614,7 @@ func initIntrinsics(cfg *intrinsicBuildConfig) {
 				return nil
 			},
 			sys.AMD64)
+
 		addF(simdPackage, "Int8x16.IsZero", opLen1(ssa.OpIsZeroVec, types.Types[types.TBOOL]), sys.AMD64)
 		addF(simdPackage, "Int16x8.IsZero", opLen1(ssa.OpIsZeroVec, types.Types[types.TBOOL]), sys.AMD64)
 		addF(simdPackage, "Int32x4.IsZero", opLen1(ssa.OpIsZeroVec, types.Types[types.TBOOL]), sys.AMD64)
@@ -1630,9 +1631,126 @@ func initIntrinsics(cfg *intrinsicBuildConfig) {
 		addF(simdPackage, "Uint16x16.IsZero", opLen1(ssa.OpIsZeroVec, types.Types[types.TBOOL]), sys.AMD64)
 		addF(simdPackage, "Uint32x8.IsZero", opLen1(ssa.OpIsZeroVec, types.Types[types.TBOOL]), sys.AMD64)
 		addF(simdPackage, "Uint64x4.IsZero", opLen1(ssa.OpIsZeroVec, types.Types[types.TBOOL]), sys.AMD64)
+
+		sfp := func(method string, hwop ssa.Op, vectype *types.Type) {
+			addF("simd", method,
+				func(s *state, n *ir.CallExpr, args []*ssa.Value) *ssa.Value {
+					x, a, b, c, d, y := args[0], args[1], args[2], args[3], args[4], args[5]
+					if a.Op == ssa.OpConst8 && b.Op == ssa.OpConst8 && c.Op == ssa.OpConst8 && d.Op == ssa.OpConst8 {
+						return selectFromPair(x, a, b, c, d, y, s, hwop, vectype)
+					} else {
+						return s.callResult(n, callNormal)
+					}
+				},
+				sys.AMD64)
+		}
+
+		sfp("Int32x4.SelectFromPair", ssa.OpconcatSelectedConstantInt32x4, types.TypeVec128)
+		sfp("Uint32x4.SelectFromPair", ssa.OpconcatSelectedConstantUint32x4, types.TypeVec128)
+		sfp("Float32x4.SelectFromPair", ssa.OpconcatSelectedConstantFloat32x4, types.TypeVec128)
+
+		sfp("Int32x8.SelectFromPairGrouped", ssa.OpconcatSelectedConstantGroupedInt32x8, types.TypeVec256)
+		sfp("Uint32x8.SelectFromPairGrouped", ssa.OpconcatSelectedConstantGroupedUint32x8, types.TypeVec256)
+		sfp("Float32x8.SelectFromPairGrouped", ssa.OpconcatSelectedConstantGroupedFloat32x8, types.TypeVec256)
+
+		sfp("Int32x16.SelectFromPairGrouped", ssa.OpconcatSelectedConstantGroupedInt32x16, types.TypeVec512)
+		sfp("Uint32x16.SelectFromPairGrouped", ssa.OpconcatSelectedConstantGroupedUint32x16, types.TypeVec512)
+		sfp("Float32x16.SelectFromPairGrouped", ssa.OpconcatSelectedConstantGroupedFloat32x16, types.TypeVec512)
+
 	}
 }
 
+func cscimm(a, b, c, d uint8) int64 {
+	return se(a + b<<2 + c<<4 + d<<6)
+}
+
+const (
+	_LLLL = iota
+	_HLLL
+	_LHLL
+	_HHLL
+	_LLHL
+	_HLHL
+	_LHHL
+	_HHHL
+	_LLLH
+	_HLLH
+	_LHLH
+	_HHLH
+	_LLHH
+	_HLHH
+	_LHHH
+	_HHHH
+)
+
+func selectFromPair(x, _a, _b, _c, _d, y *ssa.Value, s *state, op ssa.Op, t *types.Type) *ssa.Value {
+	a, b, c, d := uint8(_a.AuxInt8()), uint8(_b.AuxInt8()), uint8(_c.AuxInt8()), uint8(_d.AuxInt8())
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		// TODO DETECT 0,1,2,3, 0,0,0,0
+		return s.newValue2I(op, t, cscimm(a, b, c, d), x, x)
+	case _HHHH:
+		// TODO DETECT 0,1,2,3, 0,0,0,0
+		return s.newValue2I(op, t, cscimm(a, b, c, d), y, y)
+	case _LLHH:
+		return s.newValue2I(op, t, cscimm(a, b, c, d), x, y)
+	case _HHLL:
+		return s.newValue2I(op, t, cscimm(a, b, c, d), y, x)
+
+	case _HLLL:
+		z := s.newValue2I(op, t, cscimm(a, a, b, b), y, x)
+		return s.newValue2I(op, t, cscimm(0, 2, c, d), z, x)
+	case _LHLL:
+		z := s.newValue2I(op, t, cscimm(a, a, b, b), x, y)
+		return s.newValue2I(op, t, cscimm(0, 2, c, d), z, x)
+	case _HLHH:
+		z := s.newValue2I(op, t, cscimm(a, a, b, b), y, x)
+		return s.newValue2I(op, t, cscimm(0, 2, c, d), z, y)
+	case _LHHH:
+		z := s.newValue2I(op, t, cscimm(a, a, b, b), x, y)
+		return s.newValue2I(op, t, cscimm(0, 2, c, d), z, y)
+
+	case _LLLH:
+		z := s.newValue2I(op, t, cscimm(c, c, d, d), x, y)
+		return s.newValue2I(op, t, cscimm(a, b, 0, 2), x, z)
+	case _LLHL:
+		z := s.newValue2I(op, t, cscimm(c, c, d, d), y, x)
+		return s.newValue2I(op, t, cscimm(a, b, 0, 2), x, z)
+
+	case _HHLH:
+		z := s.newValue2I(op, t, cscimm(c, c, d, d), x, y)
+		return s.newValue2I(op, t, cscimm(a, b, 0, 2), y, z)
+
+	case _HHHL:
+		z := s.newValue2I(op, t, cscimm(c, c, d, d), y, x)
+		return s.newValue2I(op, t, cscimm(a, b, 0, 2), y, z)
+
+	case _LHLH:
+		z := s.newValue2I(op, t, cscimm(a, c, b, d), x, y)
+		return s.newValue2I(op, t, se(0b11_01_10_00), z, z)
+	case _HLHL:
+		z := s.newValue2I(op, t, cscimm(b, d, a, c), x, y)
+		return s.newValue2I(op, t, se(0b01_11_00_10), z, z)
+	case _HLLH:
+		z := s.newValue2I(op, t, cscimm(b, c, a, d), x, y)
+		return s.newValue2I(op, t, se(0b11_01_00_10), z, z)
+	case _LHHL:
+		z := s.newValue2I(op, t, cscimm(a, d, b, c), x, y)
+		return s.newValue2I(op, t, se(0b01_11_10_00), z, z)
+	}
+	panic("The preceding switch should have been exhaustive")
+}
+
+// se smears the not-really-a-sign bit of a uint8 to conform to the conventions
+// for representing AuxInt in ssa.
+func se(x uint8) int64 {
+	return int64(int8(x))
+}
+
 func opLen1(op ssa.Op, t *types.Type) func(s *state, n *ir.CallExpr, args []*ssa.Value) *ssa.Value {
 	return func(s *state, n *ir.CallExpr, args []*ssa.Value) *ssa.Value {
 		return s.newValue1(op, t, args[0])
diff --git a/src/simd/internal/simd_test/simd_test.go b/src/simd/internal/simd_test/simd_test.go
index f05c6d6f66e..6deadde45e6 100644
--- a/src/simd/internal/simd_test/simd_test.go
+++ b/src/simd/internal/simd_test/simd_test.go
@@ -594,3 +594,224 @@ func TestIsZero(t *testing.T) {
 		t.Errorf("Result incorrect, want true, got false")
 	}
 }
+
+func TestSelectFromPairConst(t *testing.T) {
+	x := simd.LoadInt32x4Slice([]int32{0, 1, 2, 3})
+	y := simd.LoadInt32x4Slice([]int32{4, 5, 6, 7})
+
+	llll := x.SelectFromPair(0, 1, 2, 3, y)
+	hhhh := x.SelectFromPair(4, 5, 6, 7, y)
+	llhh := x.SelectFromPair(0, 1, 6, 7, y)
+	hhll := x.SelectFromPair(6, 7, 0, 1, y)
+
+	lllh := x.SelectFromPair(0, 1, 2, 7, y)
+	llhl := x.SelectFromPair(0, 1, 7, 2, y)
+	lhll := x.SelectFromPair(0, 7, 1, 2, y)
+	hlll := x.SelectFromPair(7, 0, 1, 2, y)
+
+	hhhl := x.SelectFromPair(4, 5, 6, 0, y)
+	hhlh := x.SelectFromPair(4, 5, 0, 6, y)
+	hlhh := x.SelectFromPair(4, 0, 5, 6, y)
+	lhhh := x.SelectFromPair(0, 4, 5, 6, y)
+
+	lhlh := x.SelectFromPair(0, 4, 1, 5, y)
+	hlhl := x.SelectFromPair(4, 0, 5, 1, y)
+	lhhl := x.SelectFromPair(0, 4, 5, 1, y)
+	hllh := x.SelectFromPair(4, 0, 1, 5, y)
+
+	r := make([]int32, 4, 4)
+
+	foo := func(v simd.Int32x4, a, b, c, d int32) {
+		v.StoreSlice(r)
+		checkSlices[int32](t, r, []int32{a, b, c, d})
+	}
+
+	foo(llll, 0, 1, 2, 3)
+	foo(hhhh, 4, 5, 6, 7)
+	foo(llhh, 0, 1, 6, 7)
+	foo(hhll, 6, 7, 0, 1)
+
+	foo(lllh, 0, 1, 2, 7)
+	foo(llhl, 0, 1, 7, 2)
+	foo(lhll, 0, 7, 1, 2)
+	foo(hlll, 7, 0, 1, 2)
+
+	foo(hhhl, 4, 5, 6, 0)
+	foo(hhlh, 4, 5, 0, 6)
+	foo(hlhh, 4, 0, 5, 6)
+	foo(lhhh, 0, 4, 5, 6)
+
+	foo(lhlh, 0, 4, 1, 5)
+	foo(hlhl, 4, 0, 5, 1)
+	foo(lhhl, 0, 4, 5, 1)
+	foo(hllh, 4, 0, 1, 5)
+}
+
+//go:noinline
+func selectFromPairInt32x4(x simd.Int32x4, a, b, c, d uint8, y simd.Int32x4) simd.Int32x4 {
+	return x.SelectFromPair(a, b, c, d, y)
+}
+
+func TestSelectFromPairVar(t *testing.T) {
+	x := simd.LoadInt32x4Slice([]int32{0, 1, 2, 3})
+	y := simd.LoadInt32x4Slice([]int32{4, 5, 6, 7})
+
+	llll := selectFromPairInt32x4(x, 0, 1, 2, 3, y)
+	hhhh := selectFromPairInt32x4(x, 4, 5, 6, 7, y)
+	llhh := selectFromPairInt32x4(x, 0, 1, 6, 7, y)
+	hhll := selectFromPairInt32x4(x, 6, 7, 0, 1, y)
+
+	lllh := selectFromPairInt32x4(x, 0, 1, 2, 7, y)
+	llhl := selectFromPairInt32x4(x, 0, 1, 7, 2, y)
+	lhll := selectFromPairInt32x4(x, 0, 7, 1, 2, y)
+	hlll := selectFromPairInt32x4(x, 7, 0, 1, 2, y)
+
+	hhhl := selectFromPairInt32x4(x, 4, 5, 6, 0, y)
+	hhlh := selectFromPairInt32x4(x, 4, 5, 0, 6, y)
+	hlhh := selectFromPairInt32x4(x, 4, 0, 5, 6, y)
+	lhhh := selectFromPairInt32x4(x, 0, 4, 5, 6, y)
+
+	lhlh := selectFromPairInt32x4(x, 0, 4, 1, 5, y)
+	hlhl := selectFromPairInt32x4(x, 4, 0, 5, 1, y)
+	lhhl := selectFromPairInt32x4(x, 0, 4, 5, 1, y)
+	hllh := selectFromPairInt32x4(x, 4, 0, 1, 5, y)
+
+	r := make([]int32, 4, 4)
+
+	foo := func(v simd.Int32x4, a, b, c, d int32) {
+		v.StoreSlice(r)
+		checkSlices[int32](t, r, []int32{a, b, c, d})
+	}
+
+	foo(llll, 0, 1, 2, 3)
+	foo(hhhh, 4, 5, 6, 7)
+	foo(llhh, 0, 1, 6, 7)
+	foo(hhll, 6, 7, 0, 1)
+
+	foo(lllh, 0, 1, 2, 7)
+	foo(llhl, 0, 1, 7, 2)
+	foo(lhll, 0, 7, 1, 2)
+	foo(hlll, 7, 0, 1, 2)
+
+	foo(hhhl, 4, 5, 6, 0)
+	foo(hhlh, 4, 5, 0, 6)
+	foo(hlhh, 4, 0, 5, 6)
+	foo(lhhh, 0, 4, 5, 6)
+
+	foo(lhlh, 0, 4, 1, 5)
+	foo(hlhl, 4, 0, 5, 1)
+	foo(lhhl, 0, 4, 5, 1)
+	foo(hllh, 4, 0, 1, 5)
+}
+
+func TestSelectFromPairConstGroupedFloat32x8(t *testing.T) {
+	x := simd.LoadFloat32x8Slice([]float32{0, 1, 2, 3, 10, 11, 12, 13})
+	y := simd.LoadFloat32x8Slice([]float32{4, 5, 6, 7, 14, 15, 16, 17})
+
+	llll := x.SelectFromPairGrouped(0, 1, 2, 3, y)
+	hhhh := x.SelectFromPairGrouped(4, 5, 6, 7, y)
+	llhh := x.SelectFromPairGrouped(0, 1, 6, 7, y)
+	hhll := x.SelectFromPairGrouped(6, 7, 0, 1, y)
+
+	lllh := x.SelectFromPairGrouped(0, 1, 2, 7, y)
+	llhl := x.SelectFromPairGrouped(0, 1, 7, 2, y)
+	lhll := x.SelectFromPairGrouped(0, 7, 1, 2, y)
+	hlll := x.SelectFromPairGrouped(7, 0, 1, 2, y)
+
+	hhhl := x.SelectFromPairGrouped(4, 5, 6, 0, y)
+	hhlh := x.SelectFromPairGrouped(4, 5, 0, 6, y)
+	hlhh := x.SelectFromPairGrouped(4, 0, 5, 6, y)
+	lhhh := x.SelectFromPairGrouped(0, 4, 5, 6, y)
+
+	lhlh := x.SelectFromPairGrouped(0, 4, 1, 5, y)
+	hlhl := x.SelectFromPairGrouped(4, 0, 5, 1, y)
+	lhhl := x.SelectFromPairGrouped(0, 4, 5, 1, y)
+	hllh := x.SelectFromPairGrouped(4, 0, 1, 5, y)
+
+	r := make([]float32, 8, 8)
+
+	foo := func(v simd.Float32x8, a, b, c, d float32) {
+		v.StoreSlice(r)
+		checkSlices[float32](t, r, []float32{a, b, c, d, 10 + a, 10 + b, 10 + c, 10 + d})
+	}
+
+	foo(llll, 0, 1, 2, 3)
+	foo(hhhh, 4, 5, 6, 7)
+	foo(llhh, 0, 1, 6, 7)
+	foo(hhll, 6, 7, 0, 1)
+
+	foo(lllh, 0, 1, 2, 7)
+	foo(llhl, 0, 1, 7, 2)
+	foo(lhll, 0, 7, 1, 2)
+	foo(hlll, 7, 0, 1, 2)
+
+	foo(hhhl, 4, 5, 6, 0)
+	foo(hhlh, 4, 5, 0, 6)
+	foo(hlhh, 4, 0, 5, 6)
+	foo(lhhh, 0, 4, 5, 6)
+
+	foo(lhlh, 0, 4, 1, 5)
+	foo(hlhl, 4, 0, 5, 1)
+	foo(lhhl, 0, 4, 5, 1)
+	foo(hllh, 4, 0, 1, 5)
+}
+
+func TestSelectFromPairConstGroupedUint32x16(t *testing.T) {
+	if !simd.HasAVX512() {
+		t.Skip("Test requires HasAVX512, not available on this hardware")
+		return
+	}
+	x := simd.LoadUint32x16Slice([]uint32{0, 1, 2, 3, 10, 11, 12, 13, 20, 21, 22, 23, 30, 31, 32, 33})
+	y := simd.LoadUint32x16Slice([]uint32{4, 5, 6, 7, 14, 15, 16, 17, 24, 25, 26, 27, 34, 35, 36, 37})
+
+	llll := x.SelectFromPairGrouped(0, 1, 2, 3, y)
+	hhhh := x.SelectFromPairGrouped(4, 5, 6, 7, y)
+	llhh := x.SelectFromPairGrouped(0, 1, 6, 7, y)
+	hhll := x.SelectFromPairGrouped(6, 7, 0, 1, y)
+
+	lllh := x.SelectFromPairGrouped(0, 1, 2, 7, y)
+	llhl := x.SelectFromPairGrouped(0, 1, 7, 2, y)
+	lhll := x.SelectFromPairGrouped(0, 7, 1, 2, y)
+	hlll := x.SelectFromPairGrouped(7, 0, 1, 2, y)
+
+	hhhl := x.SelectFromPairGrouped(4, 5, 6, 0, y)
+	hhlh := x.SelectFromPairGrouped(4, 5, 0, 6, y)
+	hlhh := x.SelectFromPairGrouped(4, 0, 5, 6, y)
+	lhhh := x.SelectFromPairGrouped(0, 4, 5, 6, y)
+
+	lhlh := x.SelectFromPairGrouped(0, 4, 1, 5, y)
+	hlhl := x.SelectFromPairGrouped(4, 0, 5, 1, y)
+	lhhl := x.SelectFromPairGrouped(0, 4, 5, 1, y)
+	hllh := x.SelectFromPairGrouped(4, 0, 1, 5, y)
+
+	r := make([]uint32, 16, 16)
+
+	foo := func(v simd.Uint32x16, a, b, c, d uint32) {
+		v.StoreSlice(r)
+		checkSlices[uint32](t, r, []uint32{a, b, c, d,
+			10 + a, 10 + b, 10 + c, 10 + d,
+			20 + a, 20 + b, 20 + c, 20 + d,
+			30 + a, 30 + b, 30 + c, 30 + d,
+		})
+	}
+
+	foo(llll, 0, 1, 2, 3)
+	foo(hhhh, 4, 5, 6, 7)
+	foo(llhh, 0, 1, 6, 7)
+	foo(hhll, 6, 7, 0, 1)
+
+	foo(lllh, 0, 1, 2, 7)
+	foo(llhl, 0, 1, 7, 2)
+	foo(lhll, 0, 7, 1, 2)
+	foo(hlll, 7, 0, 1, 2)
+
+	foo(hhhl, 4, 5, 6, 0)
+	foo(hhlh, 4, 5, 0, 6)
+	foo(hlhh, 4, 0, 5, 6)
+	foo(lhhh, 0, 4, 5, 6)
+
+	foo(lhlh, 0, 4, 1, 5)
+	foo(hlhl, 4, 0, 5, 1)
+	foo(lhhl, 0, 4, 5, 1)
+	foo(hllh, 4, 0, 1, 5)
+}
diff --git a/src/simd/pkginternal_test.go b/src/simd/pkginternal_test.go
index 801cd0d17af..557a0537b4e 100644
--- a/src/simd/pkginternal_test.go
+++ b/src/simd/pkginternal_test.go
@@ -46,3 +46,187 @@ func TestConcatSelectedConstantGrouped32(t *testing.T) {
 	z.StoreSlice(a)
 	test_helpers.CheckSlices[uint32](t, a, []uint32{2, 0, 5, 7, 10, 8, 13, 15})
 }
+
+func TestSelect2x4x32(t *testing.T) {
+	for a := range uint8(8) {
+		for b := range uint8(8) {
+			for c := range uint8(8) {
+				for d := range uint8(8) {
+					x := LoadInt32x4Slice([]int32{0, 1, 2, 3})
+					y := LoadInt32x4Slice([]int32{4, 5, 6, 7})
+					z := select2x4x32(x, a, b, c, d, y)
+					w := make([]int32, 4, 4)
+					z.StoreSlice(w)
+					if w[0] != int32(a) || w[1] != int32(b) ||
+						w[2] != int32(c) || w[3] != int32(d) {
+						t.Errorf("Expected [%d %d %d %d] got %v", a, b, c, d, w)
+					}
+				}
+			}
+		}
+	}
+}
+
+func TestSelect2x8x32Grouped(t *testing.T) {
+	for a := range uint8(8) {
+		for b := range uint8(8) {
+			for c := range uint8(8) {
+				for d := range uint8(8) {
+					x := LoadInt32x8Slice([]int32{0, 1, 2, 3, 10, 11, 12, 13})
+					y := LoadInt32x8Slice([]int32{4, 5, 6, 7, 14, 15, 16, 17})
+					z := select2x8x32Grouped(x, a, b, c, d, y)
+					w := make([]int32, 8, 8)
+					z.StoreSlice(w)
+					if w[0] != int32(a) || w[1] != int32(b) ||
+						w[2] != int32(c) || w[3] != int32(d) ||
+						w[4] != int32(10+a) || w[5] != int32(10+b) ||
+						w[6] != int32(10+c) || w[7] != int32(10+d) {
+						t.Errorf("Expected [%d %d %d %d %d %d %d %d] got %v", a, b, c, d, 10+a, 10+b, 10+c, 10+d, w)
+					}
+				}
+			}
+		}
+	}
+}
+
+// select2x4x32 returns a selection of 4 elements in x and y, numbered
+// 0-7, where 0-3 are the four elements of x and 4-7 are the four elements
+// of y.
+func select2x4x32(x Int32x4, a, b, c, d uint8, y Int32x4) Int32x4 {
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstant(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstant(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstant(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstant(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstant(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstant(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstant(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstant(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstant(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstant(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstant(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstant(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstant(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstant(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstant(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstant(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstant(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstant(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstant(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstant(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// select2x8x32Grouped returns a pair of selection of 4 elements in x and y,
+// numbered 0-7, where 0-3 are the four elements of x's two groups (lower and
+// upper 128 bits) and 4-7 are the four elements of y's two groups.
+
+func select2x8x32Grouped(x Int32x8, a, b, c, d uint8, y Int32x8) Int32x8 {
+	// selections as being expressible in the concatSelectedConstant pattern,
+	// or not. Classification is by H and L, where H is a selection from 4-7
+	// and L is a selection from 0-3.
+	// _LLHH -> CSC(x,y, a, b, c&3, d&3)
+	// _HHLL -> CSC(y,x, a&3, b&3, c, d)
+	// _LLLL -> CSC(x,x, a, b, c, d)
+	// _HHHH -> CSC(y,y, a&3, b&3, c&3, d&3)
+
+	// _LLLH -> z = CSC(x, y, c, c, d&3, d&3); CSC(x, z, a, b, 0, 2)
+	// _LLHL -> z = CSC(x, y, c&3, c&3, d, d); CSC(x, z, a, b, 0, 2)
+	// _HHLH -> z = CSC(x, y, c, c, d&3, d&3); CSC(y, z, a&3, b&3, 0, 2)
+	// _HHHL -> z = CSC(x, y, c&3, c&3, d, d); CSC(y, z, a&3, b&3, 0, 2)
+
+	// _LHLL -> z = CSC(x, y, a, a, b&3, b&3); CSC(z, x, 0, 2, c, d)
+	// etc
+
+	// _LHLH -> z = CSC(x, y, a, c, b&3, d&3); CSC(z, z, 0, 2, 1, 3)
+	// _HLHL -> z = CSC(x, y, b, d, a&3, c&3); CSC(z, z, 2, 0, 3, 1)
+
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstantGrouped(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
diff --git a/src/simd/shuffles_amd64.go b/src/simd/shuffles_amd64.go
index 4445a88f31c..68c840730b3 100644
--- a/src/simd/shuffles_amd64.go
+++ b/src/simd/shuffles_amd64.go
@@ -13,3 +13,697 @@ package simd
 func (x Int32x4) FlattenedTranspose(y Int32x4) (a, b Int32x4) {
 	return x.InterleaveLo(y), x.InterleaveHi(y)
 }
+
+// These constants represent the source pattern for the four parameters
+// (a, b, c, d) passed to SelectFromPair and SelectFromPairGrouped.
+// L means the element comes from the 'x' vector (Low), and
+// H means it comes from the 'y' vector (High).
+// The order of the letters corresponds to elements a, b, c, d.
+// The underlying integer value is a bitmask where:
+// Bit 0: Source of element 'a' (0 for x, 1 for y)
+// Bit 1: Source of element 'b' (0 for x, 1 for y)
+// Bit 2: Source of element 'c' (0 for x, 1 for y)
+// Bit 3: Source of element 'd' (0 for x, 1 for y)
+// Note that the least-significant bit is on the LEFT in this encoding.
+const (
+	_LLLL = iota // a:x, b:x, c:x, d:x
+	_HLLL        // a:y, b:x, c:x, d:x
+	_LHLL        // a:x, b:y, c:x, d:x
+	_HHLL        // a:y, b:y, c:x, d:x
+	_LLHL        // a:x, b:x, c:y, d:x
+	_HLHL        // a:y, b:x, c:y, d:x
+	_LHHL        // a:x, b:y, c:y, d:x
+	_HHHL        // a:y, b:y, c:y, d:x
+	_LLLH        // a:x, b:x, c:x, d:y
+	_HLLH        // a:y, b:x, c:x, d:y
+	_LHLH        // a:x, b:y, c:x, d:y
+	_HHLH        // a:y, b:y, c:x, d:y
+	_LLHH        // a:x, b:x, c:y, d:y
+	_HLHH        // a:y, b:x, c:y, d:y
+	_LHHH        // a:x, b:y, c:y, d:y
+	_HHHH        // a:y, b:y, c:y, d:y
+)
+
+// SelectFromPair returns the selection of four elements from the two
+// vectors x and y, where selector values in the range 0-3 specify
+// elements from x and values in the range 4-7 specify the 0-3 elements
+// of y.  When the selectors are constants and the selection can be
+// implemented in a single instruction, it will be, otherwise it
+// requires two.  a is the source index of the least element in the
+// output, and b, c, and d are the indices of the 2nd, 3rd, and 4th
+// elements in the output.  For example,
+// {1,2,4,8}.SelectFromPair(2,3,5,7,{9,25,49,81}) returns {4,8,25,81}
+//
+// If the selectors are not constant this will translate to a function
+// call.
+//
+// Asm: VSHUFPS, CPU Feature: AVX
+func (x Int32x4) SelectFromPair(a, b, c, d uint8, y Int32x4) Int32x4 {
+	// pattern gets the concatenation of "x or y?" bits
+	// (0 == x, 1 == y)
+	// This will determine operand choice/order and whether a second
+	// instruction is needed.
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	// a-d are masked down to their offsets within x or y
+	// this is not necessary for x, but this is easier on the
+	// eyes and reduces the risk of an error now or later.
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstant(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstant(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstant(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstant(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstant(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstant(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstant(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstant(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstant(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstant(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstant(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstant(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstant(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstant(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstant(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstant(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstant(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstant(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstant(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstant(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// SelectFromPair returns the selection of four elements from the two
+// vectors x and y, where selector values in the range 0-3 specify
+// elements from x and values in the range 4-7 specify the 0-3 elements
+// of y.  When the selectors are constants and can be the selection
+// can be implemented in a single instruction, it will be, otherwise
+// it requires two. a is the source index of the least element in the
+// output, and b, c, and d are the indices of the 2nd, 3rd, and 4th
+// elements in the output.  For example,
+// {1,2,4,8}.SelectFromPair(2,3,5,7,{9,25,49,81}) returns {4,8,25,81}
+//
+// If the selectors are not constant this will translate to a function
+// call.
+//
+// Asm: VSHUFPS, CPU Feature: AVX
+func (x Uint32x4) SelectFromPair(a, b, c, d uint8, y Uint32x4) Uint32x4 {
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstant(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstant(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstant(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstant(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstant(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstant(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstant(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstant(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstant(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstant(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstant(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstant(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstant(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstant(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstant(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstant(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstant(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstant(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstant(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstant(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// SelectFromPair returns the selection of four elements from the two
+// vectors x and y, where selector values in the range 0-3 specify
+// elements from x and values in the range 4-7 specify the 0-3 elements
+// of y.  When the selectors are constants and can be the selection
+// can be implemented in a single instruction, it will be, otherwise
+// it requires two. a is the source index of the least element in the
+// output, and b, c, and d are the indices of the 2nd, 3rd, and 4th
+// elements in the output.  For example,
+// {1,2,4,8}.SelectFromPair(2,3,5,7,{9,25,49,81}) returns {4,8,25,81}
+//
+// If the selectors are not constant this will translate to a function
+// call.
+//
+// Asm: VSHUFPS, CPU Feature: AVX
+func (x Float32x4) SelectFromPair(a, b, c, d uint8, y Float32x4) Float32x4 {
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstant(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstant(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstant(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstant(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstant(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstant(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstant(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstant(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstant(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstant(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstant(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstant(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstant(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstant(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstant(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstant(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstant(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstant(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstant(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstant(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstant(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstant(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// SelectFromPairGrouped returns, for each of the two 128-bit halves of
+// the vectors x and y, the selection of four elements from  x and y,
+// where selector values in the range 0-3 specify elements from x and
+// values in the range 4-7 specify the 0-3 elements of y.
+// When the selectors are constants and can be the selection
+// can be implemented in a single instruction, it will be, otherwise
+// it requires two. a is the source index of the least element in the
+// output, and b, c, and d are the indices of the 2nd, 3rd, and 4th
+// elements in the output.  For example,
+// {1,2,4,8,16,32,64,128}.SelectFromPair(2,3,5,7,{9,25,49,81,121,169,225,289})
+//
+//	returns {4,8,25,81,64,128,169,289}
+//
+// If the selectors are not constant this will translate to a function
+// call.
+//
+// Asm: VSHUFPS, CPU Feature: AVX
+func (x Int32x8) SelectFromPairGrouped(a, b, c, d uint8, y Int32x8) Int32x8 {
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstantGrouped(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// SelectFromPairGrouped returns, for each of the two 128-bit halves of
+// the vectors x and y, the selection of four elements from  x and y,
+// where selector values in the range 0-3 specify elements from x and
+// values in the range 4-7 specify the 0-3 elements of y.
+// When the selectors are constants and can be the selection
+// can be implemented in a single instruction, it will be, otherwise
+// it requires two. a is the source index of the least element in the
+// output, and b, c, and d are the indices of the 2nd, 3rd, and 4th
+// elements in the output.  For example,
+// {1,2,4,8,16,32,64,128}.SelectFromPair(2,3,5,7,{9,25,49,81,121,169,225,289})
+//
+//	returns {4,8,25,81,64,128,169,289}
+//
+// If the selectors are not constant this will translate to a function
+// call.
+//
+// Asm: VSHUFPS, CPU Feature: AVX
+func (x Uint32x8) SelectFromPairGrouped(a, b, c, d uint8, y Uint32x8) Uint32x8 {
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstantGrouped(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// SelectFromPairGrouped returns, for each of the two 128-bit halves of
+// the vectors x and y, the selection of four elements from  x and y,
+// where selector values in the range 0-3 specify elements from x and
+// values in the range 4-7 specify the 0-3 elements of y.
+// When the selectors are constants and can be the selection
+// can be implemented in a single instruction, it will be, otherwise
+// it requires two. a is the source index of the least element in the
+// output, and b, c, and d are the indices of the 2nd, 3rd, and 4th
+// elements in the output.  For example,
+// {1,2,4,8,16,32,64,128}.SelectFromPair(2,3,5,7,{9,25,49,81,121,169,225,289})
+//
+//	returns {4,8,25,81,64,128,169,289}
+//
+// If the selectors are not constant this will translate to a function
+// call.
+//
+// Asm: VSHUFPS, CPU Feature: AVX
+func (x Float32x8) SelectFromPairGrouped(a, b, c, d uint8, y Float32x8) Float32x8 {
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstantGrouped(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// SelectFromPairGrouped returns, for each of the four 128-bit subvectors
+// of the vectors x and y, the selection of four elements from  x and y,
+// where selector values in the range 0-3 specify elements from x and
+// values in the range 4-7 specify the 0-3 elements of y.
+// When the selectors are constants and can be the selection
+// can be implemented in a single instruction, it will be, otherwise
+// it requires two.
+//
+// If the selectors are not constant this will translate to a function
+// call.
+//
+// Asm: VSHUFPS, CPU Feature: AVX512
+func (x Int32x16) SelectFromPairGrouped(a, b, c, d uint8, y Int32x16) Int32x16 {
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstantGrouped(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// SelectFromPairGrouped returns, for each of the four 128-bit subvectors
+// of the vectors x and y, the selection of four elements from  x and y,
+// where selector values in the range 0-3 specify elements from x and
+// values in the range 4-7 specify the 0-3 elements of y.
+// When the selectors are constants and can be the selection
+// can be implemented in a single instruction, it will be, otherwise
+// it requires two.
+//
+// If the selectors are not constant this will translate to a function
+// call.
+//
+// Asm: VSHUFPS, CPU Feature: AVX512
+func (x Uint32x16) SelectFromPairGrouped(a, b, c, d uint8, y Uint32x16) Uint32x16 {
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstantGrouped(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// SelectFromPairGrouped returns, for each of the four 128-bit subvectors
+// of the vectors x and y, the selection of four elements from  x and y,
+// where selector values in the range 0-3 specify elements from x and
+// values in the range 4-7 specify the 0-3 elements of y.
+// When the selectors are constants and can be the selection
+// can be implemented in a single instruction, it will be, otherwise
+// it requires two.
+//
+// If the selectors are not constant this will translate to a function
+// call.
+//
+// Asm: VSHUFPS, CPU Feature: AVX512
+func (x Float32x16) SelectFromPairGrouped(a, b, c, d uint8, y Float32x16) Float32x16 {
+	pattern := a>>2 + (b&4)>>1 + (c & 4) + (d&4)<<1
+
+	a, b, c, d = a&3, b&3, c&3, d&3
+
+	switch pattern {
+	case _LLLL:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+	case _HHHH:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _LLHH:
+		return x.concatSelectedConstantGrouped(cscimm(a, b, c, d), y)
+	case _HHLL:
+		return y.concatSelectedConstantGrouped(cscimm(a, b, c, d), x)
+
+	case _HLLL:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+	case _LHLL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), x)
+
+	case _HLHH:
+		z := y.concatSelectedConstantGrouped(cscimm(a, a, b, b), x)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+	case _LHHH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, a, b, b), y)
+		return z.concatSelectedConstantGrouped(cscimm(0, 2, c, d), y)
+
+	case _LLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _LLHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return x.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(c, c, d, d), y)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+	case _HHHL:
+		z := y.concatSelectedConstantGrouped(cscimm(c, c, d, d), x)
+		return y.concatSelectedConstantGrouped(cscimm(a, b, 0, 2), z)
+
+	case _LHLH:
+		z := x.concatSelectedConstantGrouped(cscimm(a, c, b, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_10_00 /* =cscimm(0, 2, 1, 3) */, z)
+	case _HLHL:
+		z := x.concatSelectedConstantGrouped(cscimm(b, d, a, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_00_10 /* =cscimm(2, 0, 3, 1) */, z)
+	case _HLLH:
+		z := x.concatSelectedConstantGrouped(cscimm(b, c, a, d), y)
+		return z.concatSelectedConstantGrouped(0b11_01_00_10 /* =cscimm(2, 0, 1, 3) */, z)
+	case _LHHL:
+		z := x.concatSelectedConstantGrouped(cscimm(a, d, b, c), y)
+		return z.concatSelectedConstantGrouped(0b01_11_10_00 /* =cscimm(0, 2, 3, 1) */, z)
+	}
+	panic("missing case, switch should be exhaustive")
+}
+
+// cscimm converts the 4 vector element indices into a single
+// uint8 for use as an immediate.
+func cscimm(a, b, c, d uint8) uint8 {
+	return uint8(a + b<<2 + c<<4 + d<<6)
+}