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Add some notes about choice of container.

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Chris Lattner 2007-02-03 03:04:03 +00:00
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@ -44,6 +44,20 @@ option</a></li>
<li><a href="#ViewGraph">Viewing graphs while debugging code</a></li>
</ul>
</li>
<li><a href="#datastructure">Picking the Right Data Structure for a Task</a>
<ul>
<li><a href="#ds_sequential">Sequential Containers (std::vector, std::list, etc)</a><ul>
<li><a href="#dss_fixedarrays">Fixed Size Arrays</a></li>
<li><a href="#dss_heaparrays">Heap Allocated Arrays</a></li>
<li><a href="#dss_smallvector">"llvm/ADT/SmallVector.h"</a></li>
<li><a href="#dss_vector">&lt;vector&gt;</a></li>
<li><a href="#dss_ilist">llvm/ADT/ilist</a></li>
<li><a href="#dss_list">&lt;list&gt;</a></li>
</ul></li>
<li><a href="#ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a></li>
<li><a href="#ds_map">Map-Like Containers (std::map, DenseMap, etc)</a></li>
</ul>
</li>
<li><a href="#common">Helpful Hints for Common Operations</a>
<ul>
<li><a href="#inspection">Basic Inspection and Traversal Routines</a>
@ -632,6 +646,231 @@ attributes, then you can <tt>call DAG.clearGraphAttrs()</tt>. </p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="datastructure">Picking the Right Data Structure for a Task</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>LLVM has a plethora of datastructures in the <tt>llvm/ADT/</tt> directory,
and we commonly use STL datastructures. This section describes the tradeoffs
you should consider when you pick one.</p>
<p>
The first step is a choose your own adventure: do you want a sequential
container, a set-like container, or a map-like container? The most important
thing when choosing a container is the algorithmic properties of how you plan to
access the container. Based on that, you should use:</p>
<ul>
<li>a <a href="#ds_map">map-like</a> container if you need efficient lookup
of an value based on another value. Map-like containers also support
efficient queries for containment (whether a key is in the map). Map-like
containers generally do not support efficient reverse mapping (values to
keys). If you need that, use two maps. Some map-like containers also
support efficient iteration through the keys in sorted order. Map-like
containers are the most expensive sort, only use them if you need one of
these capabilities.</li>
<li>a <a href="#ds_set">set-like</a> container if you need to put a bunch of
stuff into a container that automatically eliminates duplicates. Some
set-like containers support efficient iteration through the elements in
sorted order. Set-like containers are more expensive than sequential
containers.
</li>
<li>a <a href="#ds_sequential">sequential</a> container provides
the most efficient way to add elements and keeps track of the order they are
added to the collection. They permit duplicates and support efficient
iteration, but do not support efficient lookup based on a key.
</li>
</ul>
<p>
Once the proper catagory of container is determined, you can fine tune the
memory use, constant factors, and cache behaviors of access by intelligently
picking a member of the catagory. Note that constant factors and cache behavior
can be a big deal. If you have a vector that usually only contains a few
elements (but could contain many), for example, it's much better to use
<a href="#dss_smallvector">SmallVector</a> than <a href="#dss_vector">vector</a>
. Doing so avoids (relatively) expensive malloc/free calls, which dwarf the
cost of adding the elements to the container. </p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
</div>
<div class="doc_text">
There are a variety of sequential containers available for you, based on your
needs. Pick the first in this section that will do what you want.
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_fixedarrays">Fixed Size Arrays</a>
</div>
<div class="doc_text">
<p>Fixed size arrays are very simple and very fast. They are good if you know
exactly how many elements you have, or you have a (low) upper bound on how many
you have.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_heaparrays">Heap Allocated Arrays</a>
</div>
<div class="doc_text">
<p>Heap allocated arrays (new[] + delete[]) are also simple. They are good if
the number of elements is variable, if you know how many elements you will need
before the array is allocated, and if the array is usually large (if not,
consider a <a href="#dss_smallvector">SmallVector</a>). The cost of a heap
allocated array is the cost of the new/delete (aka malloc/free). Also note that
if you are allocating an array of a type with a constructor, the constructor and
destructors will be run for every element in the array (resizable vectors only
construct those elements actually used).</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_smallvector">"llvm/ADT/SmallVector.h"</a>
</div>
<div class="doc_text">
<p><tt>SmallVector&lt;Type, N&gt;</tt> is a simple class that looks and smells
just like <tt>vector&lt;Type&gt;</tt>:
it supports efficient iteration, lays out elements in memory order (so you can
do pointer arithmetic between elements), supports efficient push_back/pop_back
operations, supports efficient random access to its elements, etc.</p>
<p>The advantage of SmallVector is that it allocates space for
some number of elements (N) <b>in the object itself</b>. Because of this, if
the SmallVector is dynamically smaller than N, no malloc is performed. This can
be a big win in cases where the malloc/free call is far more expensive than the
code that fiddles around with the elements.</p>
<p>This is good for vectors that are "usually small" (e.g. the number of
predecessors/successors of a block is usually less than 8). On the other hand,
this makes the size of the SmallVector itself large, so you don't want to
allocate lots of them (doing so will waste a lot of space). As such,
SmallVectors are most useful when on the stack.</p>
<p>SmallVector also provides a nice portable and efficient replacement for
<tt>alloca</tt>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_vector">&lt;vector&gt;</a>
</div>
<div class="doc_text">
<p>
std::vector is well loved and respected. It is useful when SmallVector isn't:
when the size of the vector is often large (thus the small optimization will
rarely be a benefit) or if you will be allocating many instances of the vector
itself (which would waste space for elements that aren't in the container).
vector is also useful when interfacing with code that expects vectors :).
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_list">&lt;list&gt;</a>
</div>
<div class="doc_text">
<p>std::list is an extremely inefficient class that is rarely useful.
It performs a heap allocation for every element inserted into it, thus having an
extremely high constant factor, particularly for small data types. std::list
also only supports bidirectional iteration, not random access iteration.</p>
<p>In exchange for this high cost, std::list supports efficient access to both
ends of the list (like std::deque, but unlike std::vector or SmallVector). In
addition, the iterator invalidation characteristics of std::list are stronger
than that of a vector class: inserting or removing an element into the list does
not invalidate iterator or pointers to other elements in the list.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_ilist">llvm/ADT/ilist</a>
</div>
<div class="doc_text">
<p><tt>ilist&lt;T&gt;</tt> implements an 'intrusive' doubly-linked list. It is
intrusive, because it requires the element to store and provide access to the
prev/next pointers for the list.</p>
<p>ilist has the same drawbacks as std::list, and additionally requires an
ilist_traits implementation for the element type, but it provides some novel
characteristics. In particular, it can efficiently store polymorphic objects,
the traits class is informed when an element is inserted or removed from the
list, and ilists are guaranteed to support a constant-time splice operation.
</p>
<p>These properties are exactly what we want for things like Instructions and
basic blocks, which is why these are implemented with ilists.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_other">Other options</a>
</div>
<div class="doc_text">
<p>Other STL containers are available, such as std::deque (which has similar
characteristics to std::vector, but has higher constant factors and provides
efficient push_front/pop_front methods) and std::string.</p>
<p>There are also various STL adapter classes such as std::queue,
std::priority_queue, std::stack, etc. These provide simplified access to an
underlying container but don't affect the cost of the container itself.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
</div>
<div class="doc_text">
<p>
SmallPtrSet
SmallSet
sorted vector
FoldingSet
hash_set
UniqueVector
SetVector
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
</div>
<div class="doc_text">
sorted vector
std::map
DenseMap
IndexedMap
hash_map
CStringMap
</div>
<!-- *********************************************************************** -->
<div class="doc_section">