Allow Sum64String and (*Digest).WriteString to be inlined

xxhash_unsafe.go

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

xxhash_unsafe_test.go

@@ -3,6 +3,8 @@
 package xxhash
 
 import (
+	"os/exec"
+	"sort"
 	"strings"
 	"testing"
 )
@@ -22,3 +24,38 @@ func TestStringAllocs(t *testing.T) {
 		})
 	})
 }
+
+// This test is inspired by the Go runtime tests in https://golang.org/cl/57410.
+// It asserts that certain important functions may be inlined.
+func TestInlining(t *testing.T) {
+	funcs := map[string]struct{}{
+		"Sum64String":           {},
+		"(*Digest).WriteString": {},
+	}
+
+	// TODO: it would be better to use the go binary that is running
+	// 'go test' (if we are running under 'go test').
+	cmd := exec.Command("go", "test", "-gcflags=-m", "-run", "xxxx")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Log(string(out))
+		t.Fatal(err)
+	}
+
+	for _, line := range strings.Split(string(out), "\n") {
+		parts := strings.Split(line, ": can inline")
+		if len(parts) < 2 {
+			continue
+		}
+		delete(funcs, strings.TrimSpace(parts[1]))
+	}
+
+	var failed []string
+	for fn := range funcs {
+		failed = append(failed, fn)
+	}
+	sort.Strings(failed)
+	for _, fn := range failed {
+		t.Errorf("function %s not inlined", fn)
+	}
+}