[dev.simd] simd: initialize directory to make it suitable for testing SIMD

this is a multistep operation between two repos to coordinate
this move.  First copy internal/simd top simd (and adjust so
that it works with future generated SIMD), after this lands,
update golang/arch/internal/simdgen to target this directory
and add it to the end-to-end test (which will also be added
once it works and is truly end-to-end), finally remove internal/simd
once the updated generator has been submitted.

Change-Id: If372baadc0c02e47cc32bc55b39ac19d551b2b21
Reviewed-on: https://go-review.googlesource.com/c/go/+/676955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

src/simd/dummy.s

@@ -0,0 +1,7 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build amd64
+
+// Empty file to allow bodyless functions.

src/simd/testdata/sample.go

@@ -0,0 +1,154 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"simd"
+	"unsafe"
+)
+
+func load(s []float64) simd.Float64x4 {
+	return simd.LoadFloat64x4((*[4]float64)(s[:4]))
+}
+
+type S1 = simd.Float64x4
+
+type S2 simd.Float64x4
+
+func (s S2) Len() int {
+	return simd.Float64x4(s).Len()
+}
+
+func (s S2) Load(a []float64) S2 {
+	return S2(load(a))
+}
+
+func (s S2) Store(a *[4]float64) {
+	simd.Float64x4(s).Store(a)
+}
+
+func (s S2) Add(a S2) S2 {
+	return S2(simd.Float64x4(s).Add(simd.Float64x4(a)))
+}
+
+func (s S2) Mul(a S2) S2 {
+	return S2(simd.Float64x4(s).Mul(simd.Float64x4(a)))
+}
+
+type S3 struct {
+	simd.Float64x4
+}
+
+func ip64_0(a, b []float64) float64 {
+	s := 0.0
+	for i := range a {
+		s += a[i] * b[i]
+	}
+	return s
+}
+
+func ip64_1(a, b []float64) float64 {
+	var z S1
+	sum := z
+	var i int
+	stride := z.Len()
+	for ; i <= len(a)-stride; i += stride {
+		va := load(a[i:])
+		vb := load(b[i:])
+		sum = sum.Add(va.Mul(vb))
+	}
+	var tmp [4]float64
+	sum.Store(&tmp)
+	return tmp[0] + tmp[1] + tmp[2] + tmp[3]
+}
+
+func ip64_1a(a, b []float64) float64 {
+	var z S1
+	sum := z
+	var i int
+	stride := z.Len()
+	for ; i <= len(a)-stride; i += stride {
+		va := load(a[i:])
+		vb := load(b[i:])
+		sum = FMA(sum, va, vb)
+	}
+	var tmp [4]float64
+	sum.Store(&tmp)
+	return tmp[0] + tmp[1] + tmp[2] + tmp[3]
+}
+
+//go:noinline
+func FMA(a, b, c simd.Float64x4) simd.Float64x4 {
+	return a.Add(b.Mul(c))
+}
+
+func ip64_2(a, b []float64) float64 {
+	var z S2
+	sum := z
+	var i int
+	stride := z.Len()
+	for ; i <= len(a)-stride; i += stride {
+		va := z.Load(a[i:])
+		vb := z.Load(b[i:])
+		sum = sum.Add(va.Mul(vb))
+	}
+	var tmp [4]float64
+	sum.Store(&tmp)
+	return tmp[0] + tmp[1] + tmp[2] + tmp[3]
+}
+
+func ip64_3(a, b []float64) float64 {
+	var z S3
+	sum := z
+	var i int
+	stride := z.Len()
+	for ; i <= len(a)-stride; i += stride {
+		va := load(a[i:])
+		vb := load(b[i:])
+		sum = S3{sum.Add(va.Mul(vb))}
+	}
+	var tmp [4]float64
+	sum.Store(&tmp)
+	return tmp[0] + tmp[1] + tmp[2] + tmp[3]
+}
+
+func main() {
+	a := []float64{1, 2, 3, 4, 5, 6, 7, 8}
+	ip0 := ip64_0(a, a)
+	ip1 := ip64_1(a, a)
+	ip1a := ip64_1a(a, a)
+	ip2 := ip64_2(a, a)
+	ip3 := ip64_3(a, a)
+	fmt.Printf("Test IP    = %f\n", ip0)
+	fmt.Printf("SIMD IP 1  = %f\n", ip1)
+	fmt.Printf("SIMD IP 1a = %f\n", ip1a)
+	fmt.Printf("SIMD IP 2  = %f\n", ip2)
+	fmt.Printf("SIMD IP 3 = %f\n", ip3)
+	var z1 S1
+	var z2 S2
+	var z3 S2
+
+	s1, s2, s3 := unsafe.Sizeof(z1), unsafe.Sizeof(z2), unsafe.Sizeof(z3)
+
+	fmt.Printf("unsafe.Sizeof(z1, z2, z3)=%d, %d, %d\n", s1, s2, s3)
+
+	fail := false
+
+	if s1 != 32 || s2 != 32 || s3 != 32 {
+		fmt.Println("Failed a sizeof check, should all be 32")
+		fail = true
+	}
+
+	if ip1 != ip0 || ip1a != ip0 || ip2 != ip0 || ip3 != ip0 {
+		fmt.Println("Failed an inner product check, should all be", ip0)
+		fail = true
+	}
+
+	if fail {
+		os.Exit(1)
+	}
+}