simd/archsimd: add package doc

Cherry-pick CL 728560 from the dev.simd branch, for Go 1.26.

Change-Id: I9d353769861b35cc808458aa66d243240c235c9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/729221
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: David Chase <drchase@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/simd/archsimd/doc.go

@@ -0,0 +1,63 @@
+// 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 goexperiment.simd
+
+// Package archsimd provides access to architecture-specific SIMD operations.
+//
+// This is a low-level package that exposes hardware-specific functionality.
+// It currently supports AMD64.
+//
+// This package is experimental, and not subject to the Go 1 compatibility promise.
+// It only exists when building with the GOEXPERIMENT=simd environment variable set.
+//
+// # Vector types and operations
+//
+// Vector types are defined as structs, such as Int8x16 and Float64x8, corresponding
+// to the hardware's vector registers. On AMD64, 128-, 256-, and 512-bit vectors are
+// supported.
+//
+// Mask types are defined similarly, such as Mask8x16, and are represented as
+// opaque types, handling the differences in the underlying representations.
+// A mask can be converted to/from the corresponding integer vector type, or
+// to/from a bitmask.
+//
+// Operations are mostly defined as methods on the vector types. Most of them
+// are compiler intrinsics and correspond directly to hardware instructions.
+//
+// Common operations include:
+//   - Load/Store: Load a vector from memory or store a vector to memory.
+//   - Arithmetic: Add, Sub, Mul, etc.
+//   - Bitwise: And, Or, Xor, etc.
+//   - Comparison: Equal, Greater, etc., which produce a mask.
+//   - Conversion: Convert between different vector types.
+//   - Field selection and rearrangement: GetElem, Permute, etc.
+//   - Masking: Masked, Merge.
+//
+// The compiler recognizes certain patterns of operations and may optimize
+// them to more performant instructions. For example, on AVX512, an Add operation
+// followed by Masked may be optimized to a masked add instruction.
+// For this reason, not all hardware instructions are available as APIs.
+//
+// # CPU feature checks
+//
+// The package provides global variables to check for CPU features available
+// at runtime. For example, on AMD64, the [X86] variable provides methods to
+// check for AVX2, AVX512, etc.
+// It is recommended to check for CPU features before using the corresponding
+// vector operations.
+//
+// # Notes
+//
+//   - This package is not portable, as the available types and operations depend
+//     on the target architecture. It is not recommended to expose the SIMD types
+//     defined in this package in public APIs.
+//   - For performance reasons, it is recommended to use the vector types directly
+//     as values. It is not recommended to take the address of a vector type,
+//     allocate it in the heap, or put it in an aggregate type.
+package archsimd
+
+// BUG(cherry): Using a vector type as a type parameter may not work.
+
+// BUG(cherry): Using reflect Call to call a vector function/method may not work.