78 lines
2.6 KiB
Go
78 lines
2.6 KiB
Go
// Copyright 2020 The gVisor Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// +build go1.13
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// +build !go1.18
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// Check type signatures when updating Go version.
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// Package gohacks contains utilities for subverting the Go compiler.
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package gohacks
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import (
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"unsafe"
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)
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// SliceHeader is equivalent to reflect.SliceHeader, but represents the pointer
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// to the underlying array as unsafe.Pointer rather than uintptr, allowing
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// SliceHeaders to be directly converted to slice objects.
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type SliceHeader struct {
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Data unsafe.Pointer
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Len int
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Cap int
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}
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// StringHeader is equivalent to reflect.StringHeader, but represents the
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// pointer to the underlying array as unsafe.Pointer rather than uintptr,
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// allowing StringHeaders to be directly converted to strings.
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type StringHeader struct {
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Data unsafe.Pointer
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Len int
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}
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// Noescape hides a pointer from escape analysis. Noescape is the identity
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// function but escape analysis doesn't think the output depends on the input.
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// Noescape is inlined and currently compiles down to zero instructions.
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// USE CAREFULLY!
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//
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// (Noescape is copy/pasted from Go's runtime/stubs.go:noescape().)
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//
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//go:nosplit
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func Noescape(p unsafe.Pointer) unsafe.Pointer {
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x := uintptr(p)
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return unsafe.Pointer(x ^ 0)
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}
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// ImmutableBytesFromString is equivalent to []byte(s), except that it uses the
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// same memory backing s instead of making a heap-allocated copy. This is only
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// valid if the returned slice is never mutated.
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func ImmutableBytesFromString(s string) (bs []byte) {
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shdr := (*StringHeader)(unsafe.Pointer(&s))
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bshdr := (*SliceHeader)(unsafe.Pointer(&bs))
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bshdr.Data = shdr.Data
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bshdr.Len = shdr.Len
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bshdr.Cap = shdr.Len
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return
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}
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// StringFromImmutableBytes is equivalent to string(bs), except that it uses
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// the same memory backing bs instead of making a heap-allocated copy. This is
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// only valid if bs is never mutated after StringFromImmutableBytes returns.
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func StringFromImmutableBytes(bs []byte) string {
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// This is cheaper than messing with StringHeader and SliceHeader, which as
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// of this writing produces many dead stores of zeroes. Compare
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// strings.Builder.String().
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return *(*string)(unsafe.Pointer(&bs))
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}
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