b.ReportAllocs()
	str := strings.Repeat("x", 16<<10)
	for i := 0; i < b.N; i++ {
		br := NewReader(strings.NewReader(str))
		n, err := io.Copy(io.Discard, br)
		if err != nil {
			b.Fatal(err)
		}
		if n != int64(len(str)) {
			b.Fatal("wrong length")
		}
	}
}

func BenchmarkWriterEmpty(b *testing.B) {
	b.ReportAllocs()
	str := strings.Repeat("x", 1<<10)

	for _, call := range f.Calls {
		if call.Done {
			continue
		}
		start := f.offset(call.Call.Pos())
		end := f.offset(call.Call.End())
		str, nu := p.rewriteCall(f, call)
		if str != "" {
			f.Edit.Replace(start, end, str)
			if nu {
				needsUnsafe = true
			}
		}
	}
	return needsUnsafe
}

// rewriteCall rewrites one call to add pointer checks.

	FMOVD	8(R1), F2 // 220440fd
	FMOVQ	32(R5), F2 // a208c03d
	FMOVQ	65520(R10), F10 // 4afdff3d
	FMOVQ	64(RSP), F11    // eb13c03d

// medium offsets that either fit a single instruction or can use add+ldr/str
	MOVD -4095(R17), R3                        // 3bfe3fd1630340f9
	MOVD -391(R17), R3                         // 3b1e06d1630340f9
	MOVD -257(R17), R3                         // 3b0604d1630340f9