1 //go:build !appengine 2 // +build !appengine 3 4 // This file encapsulates usage of unsafe. 5 // xxhash_safe.go contains the safe implementations. 6 7 package xxhash 8 9 import ( 10 "unsafe" 11 ) 12 13 // In the future it's possible that compiler optimizations will make these 14 // XxxString functions unnecessary by realizing that calls such as 15 // Sum64([]byte(s)) don't need to copy s. See https://go.dev/issue/2205. 16 // If that happens, even if we keep these functions they can be replaced with 17 // the trivial safe code. 18 19 // NOTE: The usual way of doing an unsafe string-to-[]byte conversion is: 20 // 21 // var b []byte 22 // bh := (*reflect.SliceHeader)(unsafe.Pointer(&b)) 23 // bh.Data = (*reflect.StringHeader)(unsafe.Pointer(&s)).Data 24 // bh.Len = len(s) 25 // bh.Cap = len(s) 26 // 27 // Unfortunately, as of Go 1.15.3 the inliner's cost model assigns a high enough 28 // weight to this sequence of expressions that any function that uses it will 29 // not be inlined. Instead, the functions below use a different unsafe 30 // conversion designed to minimize the inliner weight and allow both to be 31 // inlined. There is also a test (TestInlining) which verifies that these are 32 // inlined. 33 // 34 // See https://github.com/golang/go/issues/42739 for discussion. 35 36 // Sum64String computes the 64-bit xxHash digest of s with a zero seed. 37 // It may be faster than Sum64([]byte(s)) by avoiding a copy. 38 func Sum64String(s string) uint64 { 39 b := *(*[]byte)(unsafe.Pointer(&sliceHeader{s, len(s)})) 40 return Sum64(b) 41 } 42 43 // WriteString adds more data to d. It always returns len(s), nil. 44 // It may be faster than Write([]byte(s)) by avoiding a copy. 45 func (d *Digest) WriteString(s string) (n int, err error) { 46 d.Write(*(*[]byte)(unsafe.Pointer(&sliceHeader{s, len(s)}))) 47 // d.Write always returns len(s), nil. 48 // Ignoring the return output and returning these fixed values buys a 49 // savings of 6 in the inliner's cost model. 50 return len(s), nil 51 } 52 53 // sliceHeader is similar to reflect.SliceHeader, but it assumes that the layout 54 // of the first two words is the same as the layout of a string. 55 type sliceHeader struct { 56 s string 57 cap int 58 } 59