// Copyright 2020 The gVisor 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.

//go:build go1.13 && !go1.18
// +build go1.13,!go1.18

// //go:linkname directives type-checked by checklinkname. Any other
// non-linkname assumptions outside the Go 1 compatibility guarantee should
// have an accompanied vet check or version guard build tag.

// Check type signatures and Noescape when updating Go version.
//
// TODO(b/165820485): add these checks to checklinkname.

// Package gohacks contains utilities for subverting the Go compiler.
package gohacks

import (
	"unsafe"
)

// SliceHeader is equivalent to reflect.SliceHeader, but represents the pointer
// to the underlying array as unsafe.Pointer rather than uintptr, allowing
// SliceHeaders to be directly converted to slice objects.
type SliceHeader struct {
	Data unsafe.Pointer
	Len  int
	Cap  int
}

// StringHeader is equivalent to reflect.StringHeader, but represents the
// pointer to the underlying array as unsafe.Pointer rather than uintptr,
// allowing StringHeaders to be directly converted to strings.
type StringHeader struct {
	Data unsafe.Pointer
	Len  int
}

// Noescape hides a pointer from escape analysis. Noescape is the identity
// function but escape analysis doesn't think the output depends on the input.
// Noescape is inlined and currently compiles down to zero instructions.
// USE CAREFULLY!
//
// (Noescape is copy/pasted from Go's runtime/stubs.go:noescape().)
//
//go:nosplit
func Noescape(p unsafe.Pointer) unsafe.Pointer {
	x := uintptr(p)
	return unsafe.Pointer(x ^ 0)
}

// ImmutableBytesFromString is equivalent to []byte(s), except that it uses the
// same memory backing s instead of making a heap-allocated copy. This is only
// valid if the returned slice is never mutated.
func ImmutableBytesFromString(s string) (bs []byte) {
	shdr := (*StringHeader)(unsafe.Pointer(&s))
	bshdr := (*SliceHeader)(unsafe.Pointer(&bs))
	bshdr.Data = shdr.Data
	bshdr.Len = shdr.Len
	bshdr.Cap = shdr.Len
	return
}

// StringFromImmutableBytes is equivalent to string(bs), except that it uses
// the same memory backing bs instead of making a heap-allocated copy. This is
// only valid if bs is never mutated after StringFromImmutableBytes returns.
func StringFromImmutableBytes(bs []byte) string {
	// This is cheaper than messing with StringHeader and SliceHeader, which as
	// of this writing produces many dead stores of zeroes. Compare
	// strings.Builder.String().
	return *(*string)(unsafe.Pointer(&bs))
}

// Note that go:linkname silently doesn't work if the local name is exported,
// necessitating an indirection for exported functions.

// Memmove is runtime.memmove, exported for SeqAtomicLoad/SeqAtomicTryLoad<T>.
//
//go:nosplit
func Memmove(to, from unsafe.Pointer, n uintptr) {
	memmove(to, from, n)
}

//go:linkname memmove runtime.memmove
//go:noescape
func memmove(to, from unsafe.Pointer, n uintptr)