"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
+ <meta http-equiv="Content-type" content="text/html;charset=UTF-8">
<title>LLVM Programmer's Manual</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<ul>
<li><a href="#isa">The <tt>isa<></tt>, <tt>cast<></tt>
and <tt>dyn_cast<></tt> templates</a> </li>
+ <li><a href="#string_apis">Passing strings (the <tt>StringRef</tt>
+and <tt>Twine</tt> classes)</a>
+ <ul>
+ <li><a href="#StringRef">The <tt>StringRef</tt> class</a> </li>
+ <li><a href="#Twine">The <tt>Twine</tt> class</a> </li>
+ </ul>
+ </li>
<li><a href="#DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt>
option</a>
<ul>
<li><a href="#dss_vector"><vector></a></li>
<li><a href="#dss_deque"><deque></a></li>
<li><a href="#dss_list"><list></a></li>
- <li><a href="#dss_ilist">llvm/ADT/ilist</a></li>
+ <li><a href="#dss_ilist">llvm/ADT/ilist.h</a></li>
<li><a href="#dss_other">Other Sequential Container Options</a></li>
</ul></li>
<li><a href="#ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
<li><a href="#dss_sortedvectorset">A sorted 'vector'</a></li>
<li><a href="#dss_smallset">"llvm/ADT/SmallSet.h"</a></li>
<li><a href="#dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a></li>
+ <li><a href="#dss_denseset">"llvm/ADT/DenseSet.h"</a></li>
<li><a href="#dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a></li>
<li><a href="#dss_set"><set></a></li>
<li><a href="#dss_setvector">"llvm/ADT/SetVector.h"</a></li>
<li><a href="#dss_stringmap">"llvm/ADT/StringMap.h"</a></li>
<li><a href="#dss_indexedmap">"llvm/ADT/IndexedMap.h"</a></li>
<li><a href="#dss_densemap">"llvm/ADT/DenseMap.h"</a></li>
+ <li><a href="#dss_valuemap">"llvm/ADT/ValueMap.h"</a></li>
<li><a href="#dss_map"><map></a></li>
<li><a href="#dss_othermap">Other Map-Like Container Options</a></li>
</ul></li>
+ <li><a href="#ds_string">String-like containers</a>
+ <!--<ul>
+ todo
+ </ul>--></li>
+ <li><a href="#ds_bit">BitVector-like containers</a>
+ <ul>
+ <li><a href="#dss_bitvector">A dense bitvector</a></li>
+ <li><a href="#dss_smallbitvector">A "small" dense bitvector</a></li>
+ <li><a href="#dss_sparsebitvector">A sparse bitvector</a></li>
+ </ul></li>
</ul>
</li>
<li><a href="#common">Helpful Hints for Common Operations</a>
the same way</a> </li>
<li><a href="#iterate_chains">Iterating over def-use &
use-def chains</a> </li>
+ <li><a href="#iterate_preds">Iterating over predecessors &
+successors of blocks</a></li>
</ul>
</li>
<li><a href="#simplechanges">Making simple changes</a>
<li><a href="#schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a> </li>
</ul>
</li>
+ <li><a href="#create_types">How to Create Types</a></li>
<!--
<li>Working with the Control Flow Graph
<ul>
</ul>
</li>
+ <li><a href="#threading">Threads and LLVM</a>
+ <ul>
+ <li><a href="#startmultithreaded">Entering and Exiting Multithreaded Mode
+ </a></li>
+ <li><a href="#shutdown">Ending execution with <tt>llvm_shutdown()</tt></a></li>
+ <li><a href="#managedstatic">Lazy initialization with <tt>ManagedStatic</tt></a></li>
+ <li><a href="#llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a></li>
+ <li><a href="#jitthreading">Threads and the JIT</a></li>
+ </ul>
+ </li>
+
<li><a href="#advanced">Advanced Topics</a>
<ul>
<li><a href="#TypeResolve">LLVM Type Resolution</a>
<li><a href="#AbstractTypeUser">The AbstractTypeUser Class</a></li>
</ul></li>
- <li><a href="#SymbolTable">The <tt>ValueSymbolTable</tt> and <tt>TypeSymbolTable</tt> classes </a></li>
+ <li><a href="#SymbolTable">The <tt>ValueSymbolTable</tt> and <tt>TypeSymbolTable</tt> classes</a></li>
+ <li><a href="#UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a></li>
</ul></li>
<li><a href="#coreclasses">The Core LLVM Class Hierarchy Reference</a>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
<a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>,
- <a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a>, and
- <a href="mailto:rspencer@x10sys.com">Reid Spencer</a></p>
+ <a href="mailto:ggreif@gmail.com">Gabor Greif</a>,
+ <a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a>,
+ <a href="mailto:rspencer@x10sys.com">Reid Spencer</a> and
+ <a href="mailto:owen@apple.com">Owen Anderson</a></p>
</div>
<!-- *********************************************************************** -->
standard C++ library.</li>
<li><a href="http://www.tempest-sw.com/cpp/">C++ In a Nutshell</a> - This is an
-O'Reilly book in the making. It has a decent
-Standard Library
-Reference that rivals Dinkumware's, and is unfortunately no longer free since the book has been
-published.</li>
+O'Reilly book in the making. It has a decent Standard Library
+Reference that rivals Dinkumware's, and is unfortunately no longer free since the
+book has been published.</li>
<li><a href="http://www.parashift.com/c++-faq-lite/">C++ Frequently Asked
Questions</a></li>
<dt><tt>cast<></tt>: </dt>
<dd><p>The <tt>cast<></tt> operator is a "checked cast" operation. It
- converts a pointer or reference from a base class to a derived cast, causing
+ converts a pointer or reference from a base class to a derived class, causing
an assertion failure if it is not really an instance of the right type. This
should be used in cases where you have some information that makes you believe
that something is of the right type. An example of the <tt>isa<></tt>
</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="string_apis">Passing strings (the <tt>StringRef</tt>
+and <tt>Twine</tt> classes)</a>
+</div>
+
+<div class="doc_text">
+
+<p>Although LLVM generally does not do much string manipulation, we do have
+several important APIs which take strings. Two important examples are the
+Value class -- which has names for instructions, functions, etc. -- and the
+StringMap class which is used extensively in LLVM and Clang.</p>
+
+<p>These are generic classes, and they need to be able to accept strings which
+may have embedded null characters. Therefore, they cannot simply take
+a <tt>const char *</tt>, and taking a <tt>const std::string&</tt> requires
+clients to perform a heap allocation which is usually unnecessary. Instead,
+many LLVM APIs use a <tt>StringRef</tt> or a <tt>const Twine&</tt> for
+passing strings efficiently.</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="StringRef">The <tt>StringRef</tt> class</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>StringRef</tt> data type represents a reference to a constant string
+(a character array and a length) and supports the common operations available
+on <tt>std:string</tt>, but does not require heap allocation.</p>
+
+<p>It can be implicitly constructed using a C style null-terminated string,
+an <tt>std::string</tt>, or explicitly with a character pointer and length.
+For example, the <tt>StringRef</tt> find function is declared as:</p>
+
+<pre class="doc_code">
+ iterator find(StringRef Key);
+</pre>
+
+<p>and clients can call it using any one of:</p>
+
+<pre class="doc_code">
+ Map.find("foo"); <i>// Lookup "foo"</i>
+ Map.find(std::string("bar")); <i>// Lookup "bar"</i>
+ Map.find(StringRef("\0baz", 4)); <i>// Lookup "\0baz"</i>
+</pre>
+
+<p>Similarly, APIs which need to return a string may return a <tt>StringRef</tt>
+instance, which can be used directly or converted to an <tt>std::string</tt>
+using the <tt>str</tt> member function. See
+"<tt><a href="/doxygen/classllvm_1_1StringRef_8h-source.html">llvm/ADT/StringRef.h</a></tt>"
+for more information.</p>
+
+<p>You should rarely use the <tt>StringRef</tt> class directly, because it contains
+pointers to external memory it is not generally safe to store an instance of the
+class (unless you know that the external storage will not be freed). StringRef is
+small and pervasive enough in LLVM that it should always be passed by value.</p>
+
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="Twine">The <tt>Twine</tt> class</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>Twine</tt> class is an efficient way for APIs to accept concatenated
+strings. For example, a common LLVM paradigm is to name one instruction based on
+the name of another instruction with a suffix, for example:</p>
+
+<div class="doc_code">
+<pre>
+ New = CmpInst::Create(<i>...</i>, SO->getName() + ".cmp");
+</pre>
+</div>
+
+<p>The <tt>Twine</tt> class is effectively a
+lightweight <a href="http://en.wikipedia.org/wiki/Rope_(computer_science)">rope</a>
+which points to temporary (stack allocated) objects. Twines can be implicitly
+constructed as the result of the plus operator applied to strings (i.e., a C
+strings, an <tt>std::string</tt>, or a <tt>StringRef</tt>). The twine delays the
+actual concatenation of strings until it is actually required, at which point
+it can be efficiently rendered directly into a character array. This avoids
+unnecessary heap allocation involved in constructing the temporary results of
+string concatenation. See
+"<tt><a href="/doxygen/classllvm_1_1Twine_8h-source.html">llvm/ADT/Twine.h</a></tt>"
+for more information.</p>
+
+<p>As with a <tt>StringRef</tt>, <tt>Twine</tt> objects point to external memory
+and should almost never be stored or mentioned directly. They are intended
+solely for use when defining a function which should be able to efficiently
+accept concatenated strings.</p>
+
+</div>
+
+
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt> option</a>
<div class="doc_code">
<pre>
-DOUT << "I am here!\n";
+DEBUG(errs() << "I am here!\n");
</pre>
</div>
<div class="doc_code">
<pre>
-DOUT << "No debug type\n";
#undef DEBUG_TYPE
+DEBUG(errs() << "No debug type\n");
#define DEBUG_TYPE "foo"
-DOUT << "'foo' debug type\n";
+DEBUG(errs() << "'foo' debug type\n");
#undef DEBUG_TYPE
#define DEBUG_TYPE "bar"
-DOUT << "'bar' debug type\n";
+DEBUG(errs() << "'bar' debug type\n"));
#undef DEBUG_TYPE
#define DEBUG_TYPE ""
-DOUT << "No debug type (2)\n";
+DEBUG(errs() << "No debug type (2)\n");
</pre>
</div>
for instruction scheduling to be enabled with <tt>-debug-type=InstrSched</tt>,
even if the source lives in multiple files.</p>
+<p>The <tt>DEBUG_WITH_TYPE</tt> macro is also available for situations where you
+would like to set <tt>DEBUG_TYPE</tt>, but only for one specific <tt>DEBUG</tt>
+statement. It takes an additional first parameter, which is the type to use. For
+example, the preceding example could be written as:</p>
+
+
+<div class="doc_code">
+<pre>
+DEBUG_WITH_TYPE("", errs() << "No debug type\n");
+DEBUG_WITH_TYPE("foo", errs() << "'foo' debug type\n");
+DEBUG_WITH_TYPE("bar", errs() << "'bar' debug type\n"));
+DEBUG_WITH_TYPE("", errs() << "No debug type (2)\n");
+</pre>
+</div>
+
</div>
<!-- ======================================================================= -->
<div class="doc_code">
<pre>
- 7646 bytecodewriter - Number of normal instructions
- 725 bytecodewriter - Number of oversized instructions
- 129996 bytecodewriter - Number of bytecode bytes written
+ 7646 bitcodewriter - Number of normal instructions
+ 725 bitcodewriter - Number of oversized instructions
+ 129996 bitcodewriter - Number of bitcode bytes written
2817 raise - Number of insts DCEd or constprop'd
3213 raise - Number of cast-of-self removed
5046 raise - Number of expression trees converted
iteration, but do not support efficient look-up based on a key.
</li>
+<li>a <a href="#ds_string">string</a> container is a specialized sequential
+ container or reference structure that is used for character or byte
+ arrays.</li>
+
+<li>a <a href="#ds_bit">bit</a> container provides an efficient way to store and
+ perform set operations on sets of numeric id's, while automatically
+ eliminating duplicates. Bit containers require a maximum of 1 bit for each
+ identifier you want to store.
+</li>
</ul>
<p>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="dss_ilist">llvm/ADT/ilist</a>
+ <a name="dss_ilist">llvm/ADT/ilist.h</a>
</div>
<div class="doc_text">
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><tt>ilist</tt> has the same drawbacks as <tt>std::list</tt>, and additionally
+requires an <tt>ilist_traits</tt> 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 <tt>ilist</tt>s 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>
+<p>These properties are exactly what we want for things like
+<tt>Instruction</tt>s and basic blocks, which is why these are implemented with
+<tt>ilist</tt>s.</p>
+
+Related classes of interest are explained in the following subsections:
+ <ul>
+ <li><a href="#dss_ilist_traits">ilist_traits</a></li>
+ <li><a href="#dss_iplist">iplist</a></li>
+ <li><a href="#dss_ilist_node">llvm/ADT/ilist_node.h</a></li>
+ <li><a href="#dss_ilist_sentinel">Sentinels</a></li>
+ </ul>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="dss_ilist_traits">ilist_traits</a>
+</div>
+
+<div class="doc_text">
+<p><tt>ilist_traits<T></tt> is <tt>ilist<T></tt>'s customization
+mechanism. <tt>iplist<T></tt> (and consequently <tt>ilist<T></tt>)
+publicly derive from this traits class.</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="dss_iplist">iplist</a>
+</div>
+
+<div class="doc_text">
+<p><tt>iplist<T></tt> is <tt>ilist<T></tt>'s base and as such
+supports a slightly narrower interface. Notably, inserters from
+<tt>T&</tt> are absent.</p>
+
+<p><tt>ilist_traits<T></tt> is a public base of this class and can be
+used for a wide variety of customizations.</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="dss_ilist_node">llvm/ADT/ilist_node.h</a>
+</div>
+
+<div class="doc_text">
+<p><tt>ilist_node<T></tt> implements a the forward and backward links
+that are expected by the <tt>ilist<T></tt> (and analogous containers)
+in the default manner.</p>
+
+<p><tt>ilist_node<T></tt>s are meant to be embedded in the node type
+<tt>T</tt>, usually <tt>T</tt> publicly derives from
+<tt>ilist_node<T></tt>.</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="dss_ilist_sentinel">Sentinels</a>
+</div>
+
+<div class="doc_text">
+<p><tt>ilist</tt>s have another specialty that must be considered. To be a good
+citizen in the C++ ecosystem, it needs to support the standard container
+operations, such as <tt>begin</tt> and <tt>end</tt> iterators, etc. Also, the
+<tt>operator--</tt> must work correctly on the <tt>end</tt> iterator in the
+case of non-empty <tt>ilist</tt>s.</p>
+
+<p>The only sensible solution to this problem is to allocate a so-called
+<i>sentinel</i> along with the intrusive list, which serves as the <tt>end</tt>
+iterator, providing the back-link to the last element. However conforming to the
+C++ convention it is illegal to <tt>operator++</tt> beyond the sentinel and it
+also must not be dereferenced.</p>
+
+<p>These constraints allow for some implementation freedom to the <tt>ilist</tt>
+how to allocate and store the sentinel. The corresponding policy is dictated
+by <tt>ilist_traits<T></tt>. By default a <tt>T</tt> gets heap-allocated
+whenever the need for a sentinel arises.</p>
+
+<p>While the default policy is sufficient in most cases, it may break down when
+<tt>T</tt> does not provide a default constructor. Also, in the case of many
+instances of <tt>ilist</tt>s, the memory overhead of the associated sentinels
+is wasted. To alleviate the situation with numerous and voluminous
+<tt>T</tt>-sentinels, sometimes a trick is employed, leading to <i>ghostly
+sentinels</i>.</p>
+
+<p>Ghostly sentinels are obtained by specially-crafted <tt>ilist_traits<T></tt>
+which superpose the sentinel with the <tt>ilist</tt> instance in memory. Pointer
+arithmetic is used to obtain the sentinel, which is relative to the
+<tt>ilist</tt>'s <tt>this</tt> pointer. The <tt>ilist</tt> is augmented by an
+extra pointer, which serves as the back-link of the sentinel. This is the only
+field in the ghostly sentinel which can be legally accessed.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_text">
-<p>SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is
-transparently implemented with a SmallPtrSet), but also supports iterators. If
+<p>SmallPtrSet has all the advantages of <tt>SmallSet</tt> (and a <tt>SmallSet</tt> of pointers is
+transparently implemented with a <tt>SmallPtrSet</tt>), but also supports iterators. If
more than 'N' insertions are performed, a single quadratically
probed hash table is allocated and grows as needed, providing extremely
efficient access (constant time insertion/deleting/queries with low constant
factors) and is very stingy with malloc traffic.</p>
-<p>Note that, unlike std::set, the iterators of SmallPtrSet are invalidated
+<p>Note that, unlike <tt>std::set</tt>, the iterators of <tt>SmallPtrSet</tt> are invalidated
whenever an insertion occurs. Also, the values visited by the iterators are not
visited in sorted order.</p>
</div>
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="dss_denseset">"llvm/ADT/DenseSet.h"</a>
+</div>
+
+<div class="doc_text">
+
+<p>
+DenseSet is a simple quadratically probed hash table. It excels at supporting
+small values: it uses a single allocation to hold all of the pairs that
+are currently inserted in the set. DenseSet is a great way to unique small
+values that are not simple pointers (use <a
+href="#dss_smallptrset">SmallPtrSet</a> for pointers). Note that DenseSet has
+the same requirements for the value type that <a
+href="#dss_densemap">DenseMap</a> has.
+</p>
+
+</div>
+
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a>
<p>
The STL provides several other options, such as std::multiset and the various
-"hash_set" like containers (whether from C++ TR1 or from the SGI library).</p>
+"hash_set" like containers (whether from C++ TR1 or from the SGI library). We
+never use hash_set and unordered_set because they are generally very expensive
+(each insertion requires a malloc) and very non-portable.
+</p>
<p>std::multiset is useful if you're not interested in elimination of
duplicates, but has all the drawbacks of std::set. A sorted vector (where you
don't delete duplicate entries) or some other approach is almost always
better.</p>
-<p>The various hash_set implementations (exposed portably by
-"llvm/ADT/hash_set") is a simple chained hashtable. This algorithm is as malloc
-intensive as std::set (performing an allocation for each element inserted,
-thus having really high constant factors) but (usually) provides O(1)
-insertion/deletion of elements. This can be useful if your elements are large
-(thus making the constant-factor cost relatively low) or if comparisons are
-expensive. Element iteration does not visit elements in a useful order.</p>
-
</div>
<!-- ======================================================================= -->
map. Also, because DenseMap allocates space for a large number of key/value
pairs (it starts with 64 by default), it will waste a lot of space if your keys
or values are large. Finally, you must implement a partial specialization of
-DenseMapKeyInfo for the key that you want, if it isn't already supported. This
+DenseMapInfo for the key that you want, if it isn't already supported. This
is required to tell DenseMap about two special marker values (which can never be
inserted into the map) that it needs internally.</p>
</div>
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="dss_valuemap">"llvm/ADT/ValueMap.h"</a>
+</div>
+
+<div class="doc_text">
+
+<p>
+ValueMap is a wrapper around a <a href="#dss_densemap">DenseMap</a> mapping
+Value*s (or subclasses) to another type. When a Value is deleted or RAUW'ed,
+ValueMap will update itself so the new version of the key is mapped to the same
+value, just as if the key were a WeakVH. You can configure exactly how this
+happens, and what else happens on these two events, by passing
+a <code>Config</code> parameter to the ValueMap template.</p>
+
+</div>
+
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="dss_map"><map></a>
<p>
The STL provides several other options, such as std::multimap and the various
-"hash_map" like containers (whether from C++ TR1 or from the SGI library).</p>
+"hash_map" like containers (whether from C++ TR1 or from the SGI library). We
+never use hash_set and unordered_set because they are generally very expensive
+(each insertion requires a malloc) and very non-portable.</p>
<p>std::multimap is useful if you want to map a key to multiple values, but has
all the drawbacks of std::map. A sorted vector or some other approach is almost
always better.</p>
-<p>The various hash_map implementations (exposed portably by
-"llvm/ADT/hash_map") are simple chained hash tables. This algorithm is as
-malloc intensive as std::map (performing an allocation for each element
-inserted, thus having really high constant factors) but (usually) provides O(1)
-insertion/deletion of elements. This can be useful if your elements are large
-(thus making the constant-factor cost relatively low) or if comparisons are
-expensive. Element iteration does not visit elements in a useful order.</p>
+</div>
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="ds_string">String-like containers</a>
</div>
+<div class="doc_text">
+
+<p>
+TODO: const char* vs stringref vs smallstring vs std::string. Describe twine,
+xref to #string_apis.
+</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="ds_bit">Bit storage containers (BitVector, SparseBitVector)</a>
+</div>
+
+<div class="doc_text">
+<p>Unlike the other containers, there are only two bit storage containers, and
+choosing when to use each is relatively straightforward.</p>
+
+<p>One additional option is
+<tt>std::vector<bool></tt>: we discourage its use for two reasons 1) the
+implementation in many common compilers (e.g. commonly available versions of
+GCC) is extremely inefficient and 2) the C++ standards committee is likely to
+deprecate this container and/or change it significantly somehow. In any case,
+please don't use it.</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="dss_bitvector">BitVector</a>
+</div>
+
+<div class="doc_text">
+<p> The BitVector container provides a dynamic size set of bits for manipulation.
+It supports individual bit setting/testing, as well as set operations. The set
+operations take time O(size of bitvector), but operations are performed one word
+at a time, instead of one bit at a time. This makes the BitVector very fast for
+set operations compared to other containers. Use the BitVector when you expect
+the number of set bits to be high (IE a dense set).
+</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="dss_smallbitvector">SmallBitVector</a>
+</div>
+
+<div class="doc_text">
+<p> The SmallBitVector container provides the same interface as BitVector, but
+it is optimized for the case where only a small number of bits, less than
+25 or so, are needed. It also transparently supports larger bit counts, but
+slightly less efficiently than a plain BitVector, so SmallBitVector should
+only be used when larger counts are rare.
+</p>
+
+<p>
+At this time, SmallBitVector does not support set operations (and, or, xor),
+and its operator[] does not provide an assignable lvalue.
+</p>
+</div>
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="dss_sparsebitvector">SparseBitVector</a>
+</div>
+
+<div class="doc_text">
+<p> The SparseBitVector container is much like BitVector, with one major
+difference: Only the bits that are set, are stored. This makes the
+SparseBitVector much more space efficient than BitVector when the set is sparse,
+as well as making set operations O(number of set bits) instead of O(size of
+universe). The downside to the SparseBitVector is that setting and testing of random bits is O(N), and on large SparseBitVectors, this can be slower than BitVector. In our implementation, setting or testing bits in sorted order
+(either forwards or reverse) is O(1) worst case. Testing and setting bits within 128 bits (depends on size) of the current bit is also O(1). As a general statement, testing/setting bits in a SparseBitVector is O(distance away from last set bit).
+</p>
+</div>
<!-- *********************************************************************** -->
<div class="doc_section">
for (Function::iterator i = func->begin(), e = func->end(); i != e; ++i)
// <i>Print out the name of the basic block if it has one, and then the</i>
// <i>number of instructions that it contains</i>
- llvm::cerr << "Basic block (name=" << i->getName() << ") has "
+ errs() << "Basic block (name=" << i->getName() << ") has "
<< i->size() << " instructions.\n";
</pre>
</div>
for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i)
// <i>The next statement works since operator<<(ostream&,...)</i>
// <i>is overloaded for Instruction&</i>
- llvm::cerr << *i << "\n";
+ errs() << *i << "\n";
</pre>
</div>
<p>However, this isn't really the best way to print out the contents of a
<tt>BasicBlock</tt>! Since the ostream operators are overloaded for virtually
anything you'll care about, you could have just invoked the print routine on the
-basic block itself: <tt>llvm::cerr << *blk << "\n";</tt>.</p>
+basic block itself: <tt>errs() << *blk << "\n";</tt>.</p>
</div>
#include "<a href="/doxygen/InstIterator_8h-source.html">llvm/Support/InstIterator.h</a>"
// <i>F is a pointer to a Function instance</i>
-for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
- llvm::cerr << *i << "\n";
+for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
+ errs() << *I << "\n";
</pre>
</div>
<div class="doc_code">
<pre>
std::set<Instruction*> worklist;
-worklist.insert(inst_begin(F), inst_end(F));
+// or better yet, SmallPtrSet<Instruction*, 64> worklist;
+
+for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
+ worklist.insert(&*I);
</pre>
</div>
<div class="doc_code">
<pre>
-Instruction* pinst = &*i;
+Instruction *pinst = &*i;
</pre>
</div>
<div class="doc_code">
<pre>
-Instruction* pinst = i;
+Instruction *pinst = i;
</pre>
</div>
void printNextInstruction(Instruction* inst) {
BasicBlock::iterator it(inst);
++it; // <i>After this line, it refers to the instruction after *inst</i>
- if (it != inst->getParent()->end()) llvm::cerr << *it << "\n";
+ if (it != inst->getParent()->end()) errs() << *it << "\n";
}
</pre>
</div>
+<p>Unfortunately, these implicit conversions come at a cost; they prevent
+these iterators from conforming to standard iterator conventions, and thus
+from being usable with standard algorithms and containers. For example, they
+prevent the following code, where <tt>B</tt> is a <tt>BasicBlock</tt>,
+from compiling:</p>
+
+<div class="doc_code">
+<pre>
+ llvm::SmallVector<llvm::Instruction *, 16>(B->begin(), B->end());
+</pre>
+</div>
+
+<p>Because of this, these implicit conversions may be removed some day,
+and <tt>operator*</tt> changed to return a pointer instead of a reference.</p>
+
</div>
<!--_______________________________________________________________________-->
virtual runOnFunction(Function& F) {
for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) {
- for (BasicBlock::iterator i = b->begin(); ie = b->end(); i != ie; ++i) {
+ for (BasicBlock::iterator i = b->begin(), ie = b->end(); i != ie; ++i) {
if (<a href="#CallInst">CallInst</a>* callInst = <a href="#isa">dyn_cast</a><<a
href="#CallInst">CallInst</a>>(&*i)) {
// <i>We know we've encountered a call instruction, so we</i>
// <i>need to determine if it's a call to the</i>
- // <i>function pointed to by m_func or not</i>
-
+ // <i>function pointed to by m_func or not.</i>
if (callInst->getCalledFunction() == targetFunc)
++callCounter;
}
}
private:
- unsigned callCounter;
+ unsigned callCounter;
};
</pre>
</div>
<div class="doc_code">
<pre>
-Function* F = ...;
+Function *F = ...;
for (Value::use_iterator i = F->use_begin(), e = F->use_end(); i != e; ++i)
if (Instruction *Inst = dyn_cast<Instruction>(*i)) {
- llvm::cerr << "F is used in instruction:\n";
- llvm::cerr << *Inst << "\n";
+ errs() << "F is used in instruction:\n";
+ errs() << *Inst << "\n";
}
</pre>
</div>
-<p>Alternately, it's common to have an instance of the <a
+<p>Note that dereferencing a <tt>Value::use_iterator</tt> is not a very cheap
+operation. Instead of performing <tt>*i</tt> above several times, consider
+doing it only once in the loop body and reusing its result.</p>
+
+<p>Alternatively, it's common to have an instance of the <a
href="/doxygen/classllvm_1_1User.html">User Class</a> and need to know what
<tt>Value</tt>s are used by it. The list of all <tt>Value</tt>s used by a
<tt>User</tt> is known as a <i>use-def</i> chain. Instances of class
<div class="doc_code">
<pre>
-Instruction* pi = ...;
+Instruction *pi = ...;
for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
- Value* v = *i;
+ Value *v = *i;
// <i>...</i>
}
</pre>
</div>
-<!--
- def-use chains ("finding all users of"): Value::use_begin/use_end
- use-def chains ("finding all values used"): User::op_begin/op_end [op=operand]
--->
+<p>Declaring objects as <tt>const</tt> is an important tool of enforcing
+mutation free algorithms (such as analyses, etc.). For this purpose above
+iterators come in constant flavors as <tt>Value::const_use_iterator</tt>
+and <tt>Value::const_op_iterator</tt>. They automatically arise when
+calling <tt>use/op_begin()</tt> on <tt>const Value*</tt>s or
+<tt>const User*</tt>s respectively. Upon dereferencing, they return
+<tt>const Use*</tt>s. Otherwise the above patterns remain unchanged.</p>
</div>
+<!--_______________________________________________________________________-->
+<div class="doc_subsubsection">
+ <a name="iterate_preds">Iterating over predecessors &
+successors of blocks</a>
+</div>
+
+<div class="doc_text">
+
+<p>Iterating over the predecessors and successors of a block is quite easy
+with the routines defined in <tt>"llvm/Support/CFG.h"</tt>. Just use code like
+this to iterate over all predecessors of BB:</p>
+
+<div class="doc_code">
+<pre>
+#include "llvm/Support/CFG.h"
+BasicBlock *BB = ...;
+
+for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
+ BasicBlock *Pred = *PI;
+ // <i>...</i>
+}
+</pre>
+</div>
+
+<p>Similarly, to iterate over successors use
+succ_iterator/succ_begin/succ_end.</p>
+
+</div>
+
+
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="simplechanges">Making simple changes</a>
<div class="doc_code">
<pre>
-AllocaInst* ai = new AllocaInst(Type::IntTy);
+AllocaInst* ai = new AllocaInst(Type::Int32Ty);
</pre>
</div>
<div class="doc_code">
<pre>
-AllocaInst* pa = new AllocaInst(Type::IntTy, 0, "indexLoc");
+AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc");
</pre>
</div>
<div class="doc_code">
<pre>
<a href="#Instruction">Instruction</a> *I = .. ;
-<a href="#BasicBlock">BasicBlock</a> *BB = I->getParent();
-
-BB->getInstList().erase(I);
+I->eraseFromParent();
</pre>
</div>
<ul>
<li><tt>ReplaceInstWithValue</tt>
- <p>This function replaces all uses (within a basic block) of a given
- instruction with a value, and then removes the original instruction. The
- following example illustrates the replacement of the result of a particular
+ <p>This function replaces all uses of a given instruction with a value,
+ and then removes the original instruction. The following example
+ illustrates the replacement of the result of a particular
<tt>AllocaInst</tt> that allocates memory for a single integer with a null
pointer to an integer.</p>
BasicBlock::iterator ii(instToReplace);
ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii,
- Constant::getNullValue(PointerType::get(Type::IntTy)));
+ Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty)));
</pre></div></li>
<li><tt>ReplaceInstWithInst</tt>
<p>This function replaces a particular instruction with another
- instruction. The following example illustrates the replacement of one
- <tt>AllocaInst</tt> with another.</p>
+ instruction, inserting the new instruction into the basic block at the
+ location where the old instruction was, and replacing any uses of the old
+ instruction with the new instruction. The following example illustrates
+ the replacement of one <tt>AllocaInst</tt> with another.</p>
<div class="doc_code">
<pre>
BasicBlock::iterator ii(instToReplace);
ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii,
- new AllocaInst(Type::IntTy, 0, "ptrToReplacedInt"));
+ new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt"));
</pre></div></li>
</ul>
<div class="doc_text">
-<p>Deleting a global variable from a module is similar to deleting an
-instruction. First, you must have a pointer to the global variable that you wish
- to delete. Second, you must have a pointer to the module the global variable
- belongs to. You use the pointer to the module to get its list of global
- variables and then use the erase function to remove your global variable.
+<p>Deleting a global variable from a module is just as easy as deleting an
+Instruction. First, you must have a pointer to the global variable that you wish
+ to delete. You use this pointer to erase it from its parent, the module.
For example:</p>
<div class="doc_code">
<pre>
<a href="#GlobalVariable">GlobalVariable</a> *GV = .. ;
-<a href="#Module">Module</a> *M = GV->getParent();
-M->getGlobaleList().erase(GV);
+GV->eraseFromParent();
</pre>
</div>
</div>
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="create_types">How to Create Types</a>
+</div>
+
+<div class="doc_text">
+
+<p>In generating IR, you may need some complex types. If you know these types
+statically, you can use <tt>TypeBuilder<...>::get()</tt>, defined
+in <tt>llvm/Support/TypeBuilder.h</tt>, to retrieve them. <tt>TypeBuilder</tt>
+has two forms depending on whether you're building types for cross-compilation
+or native library use. <tt>TypeBuilder<T, true></tt> requires
+that <tt>T</tt> be independent of the host environment, meaning that it's built
+out of types from
+the <a href="/doxygen/namespacellvm_1_1types.html"><tt>llvm::types</tt></a>
+namespace and pointers, functions, arrays, etc. built of
+those. <tt>TypeBuilder<T, false></tt> additionally allows native C types
+whose size may depend on the host compiler. For example,</p>
+
+<div class="doc_code">
+<pre>
+FunctionType *ft = TypeBuilder<types::i<8>(types::i<32>*), true>::get();
+</pre>
+</div>
+
+<p>is easier to read and write than the equivalent</p>
+
+<div class="doc_code">
+<pre>
+std::vector<const Type*> params;
+params.push_back(PointerType::getUnqual(Type::Int32Ty));
+FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false);
+</pre>
+</div>
+
+<p>See the <a href="/doxygen/TypeBuilder_8h-source.html#l00001">class
+comment</a> for more details.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section">
+ <a name="threading">Threads and LLVM</a>
+</div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+<p>
+This section describes the interaction of the LLVM APIs with multithreading,
+both on the part of client applications, and in the JIT, in the hosted
+application.
+</p>
+
+<p>
+Note that LLVM's support for multithreading is still relatively young. Up
+through version 2.5, the execution of threaded hosted applications was
+supported, but not threaded client access to the APIs. While this use case is
+now supported, clients <em>must</em> adhere to the guidelines specified below to
+ensure proper operation in multithreaded mode.
+</p>
+
+<p>
+Note that, on Unix-like platforms, LLVM requires the presence of GCC's atomic
+intrinsics in order to support threaded operation. If you need a
+multhreading-capable LLVM on a platform without a suitably modern system
+compiler, consider compiling LLVM and LLVM-GCC in single-threaded mode, and
+using the resultant compiler to build a copy of LLVM with multithreading
+support.
+</p>
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="startmultithreaded">Entering and Exiting Multithreaded Mode</a>
+</div>
+
+<div class="doc_text">
+
+<p>
+In order to properly protect its internal data structures while avoiding
+excessive locking overhead in the single-threaded case, the LLVM must intialize
+certain data structures necessary to provide guards around its internals. To do
+so, the client program must invoke <tt>llvm_start_multithreaded()</tt> before
+making any concurrent LLVM API calls. To subsequently tear down these
+structures, use the <tt>llvm_stop_multithreaded()</tt> call. You can also use
+the <tt>llvm_is_multithreaded()</tt> call to check the status of multithreaded
+mode.
+</p>
+
+<p>
+Note that both of these calls must be made <em>in isolation</em>. That is to
+say that no other LLVM API calls may be executing at any time during the
+execution of <tt>llvm_start_multithreaded()</tt> or <tt>llvm_stop_multithreaded
+</tt>. It's is the client's responsibility to enforce this isolation.
+</p>
+
+<p>
+The return value of <tt>llvm_start_multithreaded()</tt> indicates the success or
+failure of the initialization. Failure typically indicates that your copy of
+LLVM was built without multithreading support, typically because GCC atomic
+intrinsics were not found in your system compiler. In this case, the LLVM API
+will not be safe for concurrent calls. However, it <em>will</em> be safe for
+hosting threaded applications in the JIT, though <a href="#jitthreading">care
+must be taken</a> to ensure that side exits and the like do not accidentally
+result in concurrent LLVM API calls.
+</p>
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="shutdown">Ending Execution with <tt>llvm_shutdown()</tt></a>
+</div>
+
+<div class="doc_text">
+<p>
+When you are done using the LLVM APIs, you should call <tt>llvm_shutdown()</tt>
+to deallocate memory used for internal structures. This will also invoke
+<tt>llvm_stop_multithreaded()</tt> if LLVM is operating in multithreaded mode.
+As such, <tt>llvm_shutdown()</tt> requires the same isolation guarantees as
+<tt>llvm_stop_multithreaded()</tt>.
+</p>
+
+<p>
+Note that, if you use scope-based shutdown, you can use the
+<tt>llvm_shutdown_obj</tt> class, which calls <tt>llvm_shutdown()</tt> in its
+destructor.
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="managedstatic">Lazy Initialization with <tt>ManagedStatic</tt></a>
+</div>
+
+<div class="doc_text">
+<p>
+<tt>ManagedStatic</tt> is a utility class in LLVM used to implement static
+initialization of static resources, such as the global type tables. Before the
+invocation of <tt>llvm_shutdown()</tt>, it implements a simple lazy
+initialization scheme. Once <tt>llvm_start_multithreaded()</tt> returns,
+however, it uses double-checked locking to implement thread-safe lazy
+initialization.
+</p>
+
+<p>
+Note that, because no other threads are allowed to issue LLVM API calls before
+<tt>llvm_start_multithreaded()</tt> returns, it is possible to have
+<tt>ManagedStatic</tt>s of <tt>llvm::sys::Mutex</tt>s.
+</p>
+
+<p>
+The <tt>llvm_acquire_global_lock()</tt> and <tt>llvm_release_global_lock</tt>
+APIs provide access to the global lock used to implement the double-checked
+locking for lazy initialization. These should only be used internally to LLVM,
+and only if you know what you're doing!
+</p>
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a>
+</div>
+
+<div class="doc_text">
+<p>
+<tt>LLVMContext</tt> is an opaque class in the LLVM API which clients can use
+to operate multiple, isolated instances of LLVM concurrently within the same
+address space. For instance, in a hypothetical compile-server, the compilation
+of an individual translation unit is conceptually independent from all the
+others, and it would be desirable to be able to compile incoming translation
+units concurrently on independent server threads. Fortunately,
+<tt>LLVMContext</tt> exists to enable just this kind of scenario!
+</p>
+
+<p>
+Conceptually, <tt>LLVMContext</tt> provides isolation. Every LLVM entity
+(<tt>Module</tt>s, <tt>Value</tt>s, <tt>Type</tt>s, <tt>Constant</tt>s, etc.)
+in LLVM's in-memory IR belongs to an <tt>LLVMContext</tt>. Entities in
+different contexts <em>cannot</em> interact with each other: <tt>Module</tt>s in
+different contexts cannot be linked together, <tt>Function</tt>s cannot be added
+to <tt>Module</tt>s in different contexts, etc. What this means is that is is
+safe to compile on multiple threads simultaneously, as long as no two threads
+operate on entities within the same context.
+</p>
+
+<p>
+In practice, very few places in the API require the explicit specification of a
+<tt>LLVMContext</tt>, other than the <tt>Type</tt> creation/lookup APIs.
+Because every <tt>Type</tt> carries a reference to its owning context, most
+other entities can determine what context they belong to by looking at their
+own <tt>Type</tt>. If you are adding new entities to LLVM IR, please try to
+maintain this interface design.
+</p>
+
+<p>
+For clients that do <em>not</em> require the benefits of isolation, LLVM
+provides a convenience API <tt>getGlobalContext()</tt>. This returns a global,
+lazily initialized <tt>LLVMContext</tt> that may be used in situations where
+isolation is not a concern.
+</p>
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="jitthreading">Threads and the JIT</a>
+</div>
+
+<div class="doc_text">
+<p>
+LLVM's "eager" JIT compiler is safe to use in threaded programs. Multiple
+threads can call <tt>ExecutionEngine::getPointerToFunction()</tt> or
+<tt>ExecutionEngine::runFunction()</tt> concurrently, and multiple threads can
+run code output by the JIT concurrently. The user must still ensure that only
+one thread accesses IR in a given <tt>LLVMContext</tt> while another thread
+might be modifying it. One way to do that is to always hold the JIT lock while
+accessing IR outside the JIT (the JIT <em>modifies</em> the IR by adding
+<tt>CallbackVH</tt>s). Another way is to only
+call <tt>getPointerToFunction()</tt> from the <tt>LLVMContext</tt>'s thread.
+</p>
+
+<p>When the JIT is configured to compile lazily (using
+<tt>ExecutionEngine::DisableLazyCompilation(false)</tt>), there is currently a
+<a href="http://llvm.org/bugs/show_bug.cgi?id=5184">race condition</a> in
+updating call sites after a function is lazily-jitted. It's still possible to
+use the lazy JIT in a threaded program if you ensure that only one thread at a
+time can call any particular lazy stub and that the JIT lock guards any IR
+access, but we suggest using only the eager JIT in threaded programs.
+</p>
+</div>
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="advanced">Advanced Topics</a>
difficult to handle. Fortunately, for the most part, our implementation makes
most clients able to be completely unaware of the nasty internal details. The
primary case where clients are exposed to the inner workings of it are when
-building a recursive type. In addition to this case, the LLVM bytecode reader,
+building a recursive type. In addition to this case, the LLVM bitcode reader,
assembly parser, and linker also have to be aware of the inner workings of this
system.
</p>
// <i>Create the initial outer struct</i>
<a href="#PATypeHolder">PATypeHolder</a> StructTy = OpaqueType::get();
std::vector<const Type*> Elts;
-Elts.push_back(PointerType::get(StructTy));
-Elts.push_back(Type::IntTy);
+Elts.push_back(PointerType::getUnqual(StructTy));
+Elts.push_back(Type::Int32Ty);
StructType *NewSTy = StructType::get(Elts);
// <i>At this point, NewSTy = "{ opaque*, i32 }". Tell VMCore that</i>
by most clients. It should only be used when iteration over the symbol table
names themselves are required, which is very special purpose. Note that not
all LLVM
-<a href="#Value">Value</a>s have names, and those without names (i.e. they have
+<tt><a href="#Value">Value</a></tt>s have names, and those without names (i.e. they have
an empty name) do not exist in the symbol table.
</p>
-<!-- *********************************************************************** -->
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a>
+</div>
+
+<div class="doc_text">
+<p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1User.html">
+User</a></tt> class provides a basis for expressing the ownership of <tt>User</tt>
+towards other <tt><a href="http://llvm.org/doxygen/classllvm_1_1Value.html">
+Value</a></tt>s. The <tt><a href="http://llvm.org/doxygen/classllvm_1_1Use.html">
+Use</a></tt> helper class is employed to do the bookkeeping and to facilitate <i>O(1)</i>
+addition and removal.</p>
+
+<!-- ______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="Use2User">Interaction and relationship between <tt>User</tt> and <tt>Use</tt> objects</a>
+</div>
+
+<div class="doc_text">
+<p>
+A subclass of <tt>User</tt> can choose between incorporating its <tt>Use</tt> objects
+or refer to them out-of-line by means of a pointer. A mixed variant
+(some <tt>Use</tt>s inline others hung off) is impractical and breaks the invariant
+that the <tt>Use</tt> objects belonging to the same <tt>User</tt> form a contiguous array.
+</p>
+</div>
+
+<p>
+We have 2 different layouts in the <tt>User</tt> (sub)classes:
+<ul>
+<li><p>Layout a)
+The <tt>Use</tt> object(s) are inside (resp. at fixed offset) of the <tt>User</tt>
+object and there are a fixed number of them.</p>
+
+<li><p>Layout b)
+The <tt>Use</tt> object(s) are referenced by a pointer to an
+array from the <tt>User</tt> object and there may be a variable
+number of them.</p>
+</ul>
+<p>
+As of v2.4 each layout still possesses a direct pointer to the
+start of the array of <tt>Use</tt>s. Though not mandatory for layout a),
+we stick to this redundancy for the sake of simplicity.
+The <tt>User</tt> object also stores the number of <tt>Use</tt> objects it
+has. (Theoretically this information can also be calculated
+given the scheme presented below.)</p>
+<p>
+Special forms of allocation operators (<tt>operator new</tt>)
+enforce the following memory layouts:</p>
+
+<ul>
+<li><p>Layout a) is modelled by prepending the <tt>User</tt> object by the <tt>Use[]</tt> array.</p>
+
+<pre>
+...---.---.---.---.-------...
+ | P | P | P | P | User
+'''---'---'---'---'-------'''
+</pre>
+
+<li><p>Layout b) is modelled by pointing at the <tt>Use[]</tt> array.</p>
+<pre>
+.-------...
+| User
+'-------'''
+ |
+ v
+ .---.---.---.---...
+ | P | P | P | P |
+ '---'---'---'---'''
+</pre>
+</ul>
+<i>(In the above figures '<tt>P</tt>' stands for the <tt>Use**</tt> that
+ is stored in each <tt>Use</tt> object in the member <tt>Use::Prev</tt>)</i>
+
+<!-- ______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="Waymarking">The waymarking algorithm</a>
+</div>
+
+<div class="doc_text">
+<p>
+Since the <tt>Use</tt> objects are deprived of the direct (back)pointer to
+their <tt>User</tt> objects, there must be a fast and exact method to
+recover it. This is accomplished by the following scheme:</p>
+</div>
+
+A bit-encoding in the 2 LSBits (least significant bits) of the <tt>Use::Prev</tt> allows to find the
+start of the <tt>User</tt> object:
+<ul>
+<li><tt>00</tt> —> binary digit 0</li>
+<li><tt>01</tt> —> binary digit 1</li>
+<li><tt>10</tt> —> stop and calculate (<tt>s</tt>)</li>
+<li><tt>11</tt> —> full stop (<tt>S</tt>)</li>
+</ul>
+<p>
+Given a <tt>Use*</tt>, all we have to do is to walk till we get
+a stop and we either have a <tt>User</tt> immediately behind or
+we have to walk to the next stop picking up digits
+and calculating the offset:</p>
+<pre>
+.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.----------------
+| 1 | s | 1 | 0 | 1 | 0 | s | 1 | 1 | 0 | s | 1 | 1 | s | 1 | S | User (or User*)
+'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'----------------
+ |+15 |+10 |+6 |+3 |+1
+ | | | | |__>
+ | | | |__________>
+ | | |______________________>
+ | |______________________________________>
+ |__________________________________________________________>
+</pre>
+<p>
+Only the significant number of bits need to be stored between the
+stops, so that the <i>worst case is 20 memory accesses</i> when there are
+1000 <tt>Use</tt> objects associated with a <tt>User</tt>.</p>
+
+<!-- ______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="ReferenceImpl">Reference implementation</a>
+</div>
+
+<div class="doc_text">
+<p>
+The following literate Haskell fragment demonstrates the concept:</p>
+</div>
+
+<div class="doc_code">
+<pre>
+> import Test.QuickCheck
+>
+> digits :: Int -> [Char] -> [Char]
+> digits 0 acc = '0' : acc
+> digits 1 acc = '1' : acc
+> digits n acc = digits (n `div` 2) $ digits (n `mod` 2) acc
+>
+> dist :: Int -> [Char] -> [Char]
+> dist 0 [] = ['S']
+> dist 0 acc = acc
+> dist 1 acc = let r = dist 0 acc in 's' : digits (length r) r
+> dist n acc = dist (n - 1) $ dist 1 acc
+>
+> takeLast n ss = reverse $ take n $ reverse ss
+>
+> test = takeLast 40 $ dist 20 []
+>
+</pre>
+</div>
+<p>
+Printing <test> gives: <tt>"1s100000s11010s10100s1111s1010s110s11s1S"</tt></p>
+<p>
+The reverse algorithm computes the length of the string just by examining
+a certain prefix:</p>
+
+<div class="doc_code">
+<pre>
+> pref :: [Char] -> Int
+> pref "S" = 1
+> pref ('s':'1':rest) = decode 2 1 rest
+> pref (_:rest) = 1 + pref rest
+>
+> decode walk acc ('0':rest) = decode (walk + 1) (acc * 2) rest
+> decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest
+> decode walk acc _ = walk + acc
+>
+</pre>
+</div>
+<p>
+Now, as expected, printing <pref test> gives <tt>40</tt>.</p>
+<p>
+We can <i>quickCheck</i> this with following property:</p>
+
+<div class="doc_code">
+<pre>
+> testcase = dist 2000 []
+> testcaseLength = length testcase
+>
+> identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr
+> where arr = takeLast n testcase
+>
+</pre>
+</div>
+<p>
+As expected <quickCheck identityProp> gives:</p>
+
+<pre>
+*Main> quickCheck identityProp
+OK, passed 100 tests.
+</pre>
+<p>
+Let's be a bit more exhaustive:</p>
+
+<div class="doc_code">
+<pre>
+>
+> deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
+>
+</pre>
+</div>
+<p>
+And here is the result of <deepCheck identityProp>:</p>
+
+<pre>
+*Main> deepCheck identityProp
+OK, passed 500 tests.
+</pre>
+
+<!-- ______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="Tagging">Tagging considerations</a>
+</div>
+
+<p>
+To maintain the invariant that the 2 LSBits of each <tt>Use**</tt> in <tt>Use</tt>
+never change after being set up, setters of <tt>Use::Prev</tt> must re-tag the
+new <tt>Use**</tt> on every modification. Accordingly getters must strip the
+tag bits.</p>
+<p>
+For layout b) instead of the <tt>User</tt> we find a pointer (<tt>User*</tt> with LSBit set).
+Following this pointer brings us to the <tt>User</tt>. A portable trick ensures
+that the first bytes of <tt>User</tt> (if interpreted as a pointer) never has
+the LSBit set. (Portability is relying on the fact that all known compilers place the
+<tt>vptr</tt> in the first word of the instances.)</p>
+
+</div>
+
+ <!-- *********************************************************************** -->
<div class="doc_section">
<a name="coreclasses">The Core LLVM Class Hierarchy Reference </a>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="m_Value">Important Public Methods</a>
+ <a name="m_Type">Important Public Methods</a>
</div>
<div class="doc_text">
<ul>
- <li><tt>bool isInteger() const</tt>: Returns true for any integer type.</li>
+ <li><tt>bool isIntegerTy() const</tt>: Returns true for any integer type.</li>
- <li><tt>bool isFloatingPoint()</tt>: Return true if this is one of the two
+ <li><tt>bool isFloatingPointTy()</tt>: Return true if this is one of the five
floating point types.</li>
<li><tt>bool isAbstract()</tt>: Return true if the type is abstract (contains
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="m_Value">Important Derived Types</a>
+ <a name="derivedtypes">Important Derived Types</a>
</div>
<div class="doc_text">
<dl>
<dt><tt>VectorType</tt></dt>
<dd>Subclass of SequentialType for vector types. A
vector type is similar to an ArrayType but is distinguished because it is
- a first class type wherease ArrayType is not. Vector types are used for
+ a first class type whereas ArrayType is not. Vector types are used for
vector operations and are usually small vectors of of an integer or floating
point type.</dd>
<dt><tt>StructType</tt></dt>
<dt><tt><a name="FunctionType">FunctionType</a></tt></dt>
<dd>Subclass of DerivedTypes for function types.
<ul>
- <li><tt>bool isVarArg() const</tt>: Returns true if its a vararg
+ <li><tt>bool isVarArg() const</tt>: Returns true if it's a vararg
function</li>
<li><tt> const Type * getReturnType() const</tt>: Returns the
return type of the function.</li>
<ul>
<li><tt>Value::use_iterator</tt> - Typedef for iterator over the
use-list<br>
- <tt>Value::use_const_iterator</tt> - Typedef for const_iterator over
+ <tt>Value::const_use_iterator</tt> - Typedef for const_iterator over
the use-list<br>
<tt>unsigned use_size()</tt> - Returns the number of users of the
value.<br>
<a href="#Value"><tt>Value</tt></a></p>
<p>The <tt>Function</tt> class represents a single procedure in LLVM. It is
-actually one of the more complex classes in the LLVM heirarchy because it must
+actually one of the more complex classes in the LLVM hierarchy because it must
keep track of a large amount of data. The <tt>Function</tt> class keeps track
of a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, a list of formal
<a href="#Argument"><tt>Argument</tt></a>s, and a
<p>The list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s is the most
commonly used part of <tt>Function</tt> objects. The list imposes an implicit
ordering of the blocks in the function, which indicate how the code will be
-la
yed out by the backend. Additionally, the first <a
+la
id out by the backend. Additionally, the first <a
href="#BasicBlock"><tt>BasicBlock</tt></a> is the implicit entry node for the
<tt>Function</tt>. It is not legal in LLVM to explicitly branch to this initial
block. There are no implicit exit nodes, and in fact there may be multiple exit
will automatically be inserted into that module's list of
functions.</p></li>
- <li><tt>bool isExternal()</tt>
+ <li><tt>bool isDeclaration()</tt>
<p>Return whether or not the <tt>Function</tt> has a body defined. If the
function is "external", it does not have a body, and thus must be resolved
<a href="#User"><tt>User</tt></a>,
<a href="#Value"><tt>Value</tt></a></p>
-<p>Global variables are represented with the (suprise suprise)
+<p>Global variables are represented with the (surprise surprise)
<tt>GlobalVariable</tt> class. Like functions, <tt>GlobalVariable</tt>s are also
subclasses of <a href="#GlobalValue"><tt>GlobalValue</tt></a>, and as such are
always referenced by their address (global values must live in memory, so their
<p>Create a new global variable of the specified type. If
<tt>isConstant</tt> is true then the global variable will be marked as
unchanging for the program. The Linkage parameter specifies the type of
- linkage (internal, external, weak, linkonce, appending) for the variable. If
- the linkage is InternalLinkage, WeakLinkage, or LinkOnceLinkage, then
- the resultant global variable will have internal linkage. AppendingLinkage
- concatenates together all instances (in different translation units) of the
- variable into a single variable but is only applicable to arrays. See
+ linkage (internal, external, weak, linkonce, appending) for the variable.
+ If the linkage is InternalLinkage, WeakAnyLinkage, WeakODRLinkage,
+ LinkOnceAnyLinkage or LinkOnceODRLinkage, then the resultant
+ global variable will have internal linkage. AppendingLinkage concatenates
+ together all instances (in different translation units) of the variable
+ into a single variable but is only applicable to arrays. See
the <a href="LangRef.html#modulestructure">LLVM Language Reference</a> for
further details on linkage types. Optionally an initializer, a name, and the
module to put the variable into may be specified for the global variable as
<li><tt><a href="#Constant">Constant</a> *getInitializer()</tt>
- <p>Returns the intial value for a <tt>GlobalVariable</tt>. It is not legal
+ <p>Returns the initial value for a <tt>GlobalVariable</tt>. It is not legal
to call this method if there is no initializer.</p></li>
</ul>
<p><tt>#include "<a
href="/doxygen/BasicBlock_8h-source.html">llvm/BasicBlock.h</a>"</tt><br>
-doxygen info: <a href="/doxygen/structllvm_1_1BasicBlock.html">BasicBlock
+doxygen info: <a href="/doxygen/classllvm_1_1BasicBlock.html">BasicBlock
Class</a><br>
Superclass: <a href="#Value"><tt>Value</tt></a></p>
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
- src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+ src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
<a href="http://validator.w3.org/check/referer"><img
- src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!" /></a>
+ src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01 Strict"></a>
<a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a> and
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
projects / oota-llvm.git / blobdiff