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doc/go1.3.html: add note about small map iteration order

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Russ Cox 2014-05-16 12:15:21 -04:00
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@ -137,6 +137,28 @@ to <code>unsafe.Pointer</code> is illegal and must be rewritten.
Such code can be identified by <code>go vet</code>.
</p>
<h3 id="map">Map iteration</h3>
<p>
Iterations over small maps no longer happen in a consistent order.
Go 1 defines that &ldquo;<a href="http://golang.org/ref/spec#For_statements">The iteration order over maps
is not specified and is not guaranteed to be the same from one iteration to the next.</a>&rdquo;
To keep code from depending on map iteration order,
Go 1.0 started each map iteration at a random index in the map.
A new map implementation introduced in Go 1.1 neglected to randomize
iteration for maps with eight or fewer entries, although the iteration order
can still vary from system to system.
This has allowed people to write Go 1.1 and Go 1.2 programs that
depend on small map iteration order and therefore only work reliably on certain systems.
Go 1.3 reintroduces random iteration for small maps in order to flush out these bugs.
</p>
<p>
<em>Updating</em>: If code assumes a fixed iteration order for small maps,
it will break and must be rewritten not to make that assumption.
Because only small maps are affected, the problem arises most often in tests.
</p>
<h3 id="liblink">The linker</h3>
<p>