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