The latency impact of starting and stopping execution traces has been dramatically reduced.
      Execution traces may now begin or end during the garbage collection mark phase.
    </p>
    <p>
      To allow Go developers to take advantage of these improvements, an experimental
      trace reading package is available at <a href="/pkg/golang.org/x/exp/trace">golang.org/x/exp/trace</a>.

</p>

<pre>
const (
	bit0, mask0 = 1 &lt;&lt; iota, 1&lt;&lt;iota - 1  // bit0 == 1, mask0 == 0  (iota == 0)
	bit1, mask1                           // bit1 == 2, mask1 == 1  (iota == 1)
	_, _                                  //                        (iota == 2, unused)
	bit3, mask3                           // bit3 == 8, mask3 == 7  (iota == 3)
)
</pre>

<p>

</p>

<pre>
const (
	bit0, mask0 = 1 &lt;&lt; iota, 1&lt;&lt;iota - 1  // bit0 == 1, mask0 == 0  (iota == 0)
	bit1, mask1                           // bit1 == 2, mask1 == 1  (iota == 1)
	_, _                                  //                        (iota == 2, unused)
	bit3, mask3                           // bit3 == 8, mask3 == 7  (iota == 3)
)
</pre>

<p>

and declares <code>runtime·tlsoffset</code>, a 4-byte, implicitly zeroed variable that
contains no pointers.
</p>

<p>
There may be one or two arguments to the directives.
If there are two, the first is a bit mask of flags,
which can be written as numeric expressions, added or or-ed together,
or can be set symbolically for easier absorption by a human.