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doc/go1.3.html: add note about small map iteration order
LGTM=r R=r CC=golang-codereviews https://golang.org/cl/98290048
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@ -137,6 +137,28 @@ to <code>unsafe.Pointer</code> is illegal and must be rewritten.
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Such code can be identified by <code>go vet</code>.
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</p>
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<h3 id="map">Map iteration</h3>
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<p>
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Iterations over small maps no longer happen in a consistent order.
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Go 1 defines that “<a href="http://golang.org/ref/spec#For_statements">The iteration order over maps
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is not specified and is not guaranteed to be the same from one iteration to the next.</a>”
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To keep code from depending on map iteration order,
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Go 1.0 started each map iteration at a random index in the map.
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A new map implementation introduced in Go 1.1 neglected to randomize
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iteration for maps with eight or fewer entries, although the iteration order
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can still vary from system to system.
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This has allowed people to write Go 1.1 and Go 1.2 programs that
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depend on small map iteration order and therefore only work reliably on certain systems.
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Go 1.3 reintroduces random iteration for small maps in order to flush out these bugs.
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</p>
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<p>
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<em>Updating</em>: If code assumes a fixed iteration order for small maps,
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it will break and must be rewritten not to make that assumption.
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Because only small maps are affected, the problem arises most often in tests.
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</p>
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<h3 id="liblink">The linker</h3>
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<p>
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