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@@ -6,41 +6,52 @@
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package xxhash
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import (
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- "reflect"
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"unsafe"
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)
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-// Notes:
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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://golang.org/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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+
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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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-// See https://groups.google.com/d/msg/golang-nuts/dcjzJy-bSpw/tcZYBzQqAQAJ
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-// for some discussion about these unsafe conversions.
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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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-// In the future it's possible that compiler optimizations will make these
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-// unsafe operations unnecessary: https://golang.org/issue/2205.
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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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-// Both of these wrapper functions still incur function call overhead since they
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-// will not be inlined. We could write Go/asm copies of Sum64 and Digest.Write
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-// for strings to squeeze out a bit more speed. Mid-stack inlining should
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-// eventually fix this.
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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.
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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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- 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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+ 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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- 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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- return d.Write(b)
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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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+
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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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