vendor.github.com.cespare.xxhash.v2.xxhash_unsafe.go Maven / Gradle / Ivy
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//go:build !appengine
// +build !appengine
// This file encapsulates usage of unsafe.
// xxhash_safe.go contains the safe implementations.
package xxhash
import (
"unsafe"
)
// In the future it's possible that compiler optimizations will make these
// XxxString functions unnecessary by realizing that calls such as
// Sum64([]byte(s)) don't need to copy s. See https://go.dev/issue/2205.
// If that happens, even if we keep these functions they can be replaced with
// the trivial safe code.
// NOTE: The usual way of doing an unsafe string-to-[]byte conversion is:
//
// var b []byte
// bh := (*reflect.SliceHeader)(unsafe.Pointer(&b))
// bh.Data = (*reflect.StringHeader)(unsafe.Pointer(&s)).Data
// bh.Len = len(s)
// bh.Cap = len(s)
//
// Unfortunately, as of Go 1.15.3 the inliner's cost model assigns a high enough
// weight to this sequence of expressions that any function that uses it will
// not be inlined. Instead, the functions below use a different unsafe
// conversion designed to minimize the inliner weight and allow both to be
// inlined. There is also a test (TestInlining) which verifies that these are
// inlined.
//
// See https://github.com/golang/go/issues/42739 for discussion.
// Sum64String computes the 64-bit xxHash digest of s.
// It may be faster than Sum64([]byte(s)) by avoiding a copy.
func Sum64String(s string) uint64 {
b := *(*[]byte)(unsafe.Pointer(&sliceHeader{s, len(s)}))
return Sum64(b)
}
// WriteString adds more data to d. It always returns len(s), nil.
// It may be faster than Write([]byte(s)) by avoiding a copy.
func (d *Digest) WriteString(s string) (n int, err error) {
d.Write(*(*[]byte)(unsafe.Pointer(&sliceHeader{s, len(s)})))
// d.Write always returns len(s), nil.
// Ignoring the return output and returning these fixed values buys a
// savings of 6 in the inliner's cost model.
return len(s), nil
}
// sliceHeader is similar to reflect.SliceHeader, but it assumes that the layout
// of the first two words is the same as the layout of a string.
type sliceHeader struct {
s string
cap int
}