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describe SmallSetVector
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33847 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -1016,8 +1016,9 @@ std::set is almost never a good choice.</p>
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</div>
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<div class="doc_text">
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<p>LLVM's SetVector<Type> is actually a combination of a set along with
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a <a href="#ds_sequential">Sequential Container</a>. The important property
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<p>LLVM's SetVector<Type> is an adapter class that combines your choice of
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a set-like container along with a <a href="#ds_sequential">Sequential
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Container</a>. The important property
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that this provides is efficient insertion with uniquing (duplicate elements are
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ignored) with iteration support. It implements this by inserting elements into
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both a set-like container and the sequential container, using the set-like
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@ -1028,7 +1029,7 @@ container for uniquing and the sequential container for iteration.
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iteration is guaranteed to match the order of insertion into the SetVector.
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This property is really important for things like sets of pointers. Because
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pointer values are non-deterministic (e.g. vary across runs of the program on
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different machines), iterating over the pointers in a std::set or other set will
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different machines), iterating over the pointers in the set will
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not be in a well-defined order.</p>
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<p>
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@ -1036,9 +1037,17 @@ The drawback of SetVector is that it requires twice as much space as a normal
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set and has the sum of constant factors from the set-like container and the
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sequential container that it uses. Use it *only* if you need to iterate over
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the elements in a deterministic order. SetVector is also expensive to delete
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elements out of (linear time).
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elements out of (linear time), unless you use it's "pop_back" method, which is
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faster.
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</p>
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<p>SetVector is an adapter class that defaults to using std::vector and std::set
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for the underlying containers, so it is quite expensive. However,
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<tt>"llvm/ADT/SetVector.h"</tt> also provides a SmallSetVector class, which
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defaults to using a SmallVector and SmallSet of a specified size. If you use
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this, and if your sets are dynamically smaller than N, you will save a lot of
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heap traffic.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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