| // 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. |
| |
| // +build go1.13 |
| // +build !go1.18 |
| |
| // Check type signatures when updating Go version. |
| |
| // 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)) |
| } |