crbytes: add AllocAligned, CopyAligned

Add facilities for allocating byte slices such that the address of the first
byte of the slice is aligned.

crbytes/crbytes.go

@@ -0,0 +1,46 @@
+// Copyright 2024 The Cockroach Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+// implied. See the License for the specific language governing
+// permissions and limitations under the License.
+
+package crbytes
+
+import (
+	"fmt"
+	"unsafe"
+)
+
+// AllocAligned allocates a new byte slice of length n, ensuring the address of
+// the beginning of the slice is word aligned. Go does not guarantee that a
+// simple make([]byte, n) is aligned.
+func AllocAligned(n int) []byte {
+	if n == 0 {
+		return nil
+	}
+	a := make([]uint64, (n+7)/8)
+	b := unsafe.Slice((*byte)(unsafe.Pointer(&a[0])), n)
+
+	// Verify alignment.
+	ptr := uintptr(unsafe.Pointer(&b[0]))
+	if ptr&7 != 0 {
+		panic(fmt.Sprintf("allocated []uint64 slice not 8-aligned: pointer %p", &b[0]))
+	}
+	return b
+}
+
+// CopyAligned copies the provided byte slice into an aligned byte slice of the
+// same length.
+func CopyAligned(s []byte) []byte {
+	dst := AllocAligned(len(s))
+	copy(dst, s)
+	return dst
+}