func BenchmarkReaderWriteToOptimal(b *testing.B) {
	const bufSize = 16 << 10
	buf := make([]byte, bufSize)
	r := bytes.NewReader(buf)
	srcReader := NewReaderSize(onlyReader{r}, 1<<10)
	if _, ok := io.Discard.(io.ReaderFrom); !ok {
		b.Fatal("io.Discard doesn't support ReaderFrom")
	}
	for i := 0; i < b.N; i++ {
		r.Seek(0, io.SeekStart)
		srcReader.Reset(onlyReader{r})
		n, err := srcReader.WriteTo(io.Discard)

	needsUnsafe := false
	f.walk(arg, ctxExpr, func(f *File, arg interface{}, context astContext) {
		px, ok := arg.(*ast.Expr)
		if !ok {
			return
		}
		sel, ok := (*px).(*ast.SelectorExpr)
		if ok {
			if l, ok := sel.X.(*ast.Ident); !ok || l.Name != "C" {
				return
			}

			for _, r := range f.Ref {
				if r.Expr == px {

	// EVEX: VCVTPD2DQ with Y suffix (VL=2).
	VCVTPD2DQY (BX), X20  // 62e1ff28e623
	VCVTPD2DQY (R11), X30 // 6241ff28e633
	// XED encoder uses EVEX.X=0 for these; most x86 tools use EVEX.X=1.
	// Either way is OK.
	VMOVQ SP, X20  // 62e1fd086ee4 or 62a1fd086ee4
	VMOVQ BP, X20  // 62e1fd086ee5 or 62a1fd086ee5
	VMOVQ R14, X20 // 62c1fd086ee6 or 6281fd086ee6
	// "VMOVQ r/m64, xmm1"/6E vs "VMOVQ xmm2/m64, xmm1"/7E with mem operand.

	// Hex constant 0xFFFFFFFE00010001
	ADD $-8589869055, R5            // 3fe0fffe63ff00017bff83e463ff00017cbf2a14 or 0602000138a50001

	//TODO: this compiles to add r5,r6,r0. It should be addi r5,r6,0.
	//      this is OK since r0 == $0, but the latter is preferred.
	ADD $0, R6, R5             	// 7ca60214

        //TODO: the assembler rewrites these into ADDIS $19, R5, Rx and ADD $-10617, Rx, Rx, but the test only sees the first ADDIS