- The isa<>, cast<> and dyn_cast<> templates +
- Passing strings (the StringRef +and Twine classes) + +
- The DEBUG() macro and -debug
option
-
@@ -48,25 +56,38 @@ option
- Sequential Containers (std::vector, std::list, etc) +
- String-like containers +
- Set-Like Containers (std::set, SmallSet, SetVector, etc)
- Map-Like Containers (std::map, DenseMap, etc)
-
@@ -74,9 +95,20 @@ option
- "llvm/ADT/StringMap.h"
- "llvm/ADT/IndexedMap.h"
- "llvm/ADT/DenseMap.h" +
- "llvm/ADT/ValueMap.h" +
- "llvm/ADT/IntervalMap.h"
- <map> +
- "llvm/ADT/MapVector.h" +
- "llvm/ADT/IntEqClasses.h" +
- "llvm/ADT/ImmutableMap.h"
- Other Map-Like Container Options
- BitVector-like containers +
Introduction -
This document is meant to highlight some of the important classes and interfaces available in the LLVM source-base. This manual is not @@ -201,24 +244,22 @@ href="/doxygen/InstVisitor_8h-source.html">InstVisitor template.
This section contains general information that is useful if you are working in the LLVM source-base, but that isn't specific to any particular API.
-LLVM makes heavy use of the C++ Standard Template Library (STL), perhaps much more than you are used to, or have seen before. Because of @@ -231,15 +272,14 @@ can get, so it will not be discussed in this document.
-
-
- Dinkumware C++ Library -reference - an excellent reference for the STL and other parts of the -standard C++ library. +
- Dinkumware +C++ Library reference - an excellent reference for the STL and other parts +of the standard C++ library.
- C++ In a Nutshell - 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. +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.
- C++ Frequently Asked Questions @@ -265,41 +305,39 @@ to write maintainable code more than where to put your curly braces.
Here we highlight some LLVM APIs that are generally useful and good to know about when writing transformations.
-The LLVM source-base makes extensive use of a custom form of RTTI. These templates have many similarities to the C++ dynamic_cast<> @@ -322,7 +360,7 @@ file (note that you very rarely have to include this file directly).
The cast<> 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 isa<> @@ -395,19 +433,119 @@ if (AllocationInst *AI = dyn_cast<How to set up LLVM-style +RTTI for your class hierarchy . +
+ Passing strings (the StringRef +and Twine classes) +
+ +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.
+ +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 const char *, and taking a const std::string& requires +clients to perform a heap allocation which is usually unnecessary. Instead, +many LLVM APIs use a StringRef or a const Twine& for +passing strings efficiently.
+ + ++ The StringRef class +
+ +The StringRef data type represents a reference to a constant string +(a character array and a length) and supports the common operations available +on std:string, but does not require heap allocation.
+ +It can be implicitly constructed using a C style null-terminated string, +an std::string, or explicitly with a character pointer and length. +For example, the StringRef find function is declared as:
+ ++ iterator find(StringRef Key); ++ +
and clients can call it using any one of:
+ +
+ Map.find("foo"); // Lookup "foo"
+ Map.find(std::string("bar")); // Lookup "bar"
+ Map.find(StringRef("\0baz", 4)); // Lookup "\0baz"
+
+
+Similarly, APIs which need to return a string may return a StringRef +instance, which can be used directly or converted to an std::string +using the str member function. See +"llvm/ADT/StringRef.h" +for more information.
+ +You should rarely use the StringRef 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.
+ ++ The Twine class +
+ +The Twine 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:
+ ++ New = CmpInst::Create(..., SO->getName() + ".cmp"); ++
The Twine class is effectively a lightweight +rope +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 std::string, or a StringRef). 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 +"llvm/ADT/Twine.h" +and here for more information.
+ +As with a StringRef, Twine 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.
++ The DEBUG() macro and -debug option +
+ +Often when working on your pass you will put a bunch of debugging printouts and other code into your pass. After you get it working, you want to remove @@ -426,7 +564,7 @@ tool) is run with the '-debug' command line argument:
-DOUT << "I am here!\n"; +DEBUG(errs() << "I am here!\n");
Sometimes you may find yourself in a situation where enabling -debug just turns on too much information (such as when working on the code @@ -471,16 +607,16 @@ option as follows:
-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");
The DEBUG_WITH_TYPE macro is also available for situations where you +would like to set DEBUG_TYPE, but only for one specific DEBUG +statement. It takes an additional first parameter, which is the type to use. For +example, the preceding example could be written as:
+ + +
+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");
+
+The "llvm/ADT/Statistic.h" file @@ -580,9 +733,9 @@ suite, it gives a report that looks like this:
- 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
@@ -615,11 +768,11 @@ maintainable and useful.
Several of the important data structures in LLVM are graphs: for example CFGs made out of LLVM BasicBlocks, CFGs made out of @@ -661,15 +814,21 @@ found at Graph Attributes.) If you want to restart and clear all the current graph attributes, then you can call DAG.clearGraphAttrs().
+Note that graph visualization features are compiled out of Release builds +to reduce file size. This means that you need a Debug+Asserts or +Release+Asserts build to use these features.
+ +LLVM has a plethora of data structures in the llvm/ADT/ directory, and we commonly use STL data structures. This section describes the trade-offs @@ -704,6 +863,15 @@ access the container. Based on that, you should use:
iteration, but do not support efficient look-up based on a key. +@@ -716,35 +884,47 @@ elements (but could contain many), for example, it's much better to use . Doing so avoids (relatively) expensive malloc/free calls, which dwarf the cost of adding the elements to the container.
-+ llvm/ADT/ArrayRef.h +
+ +The llvm::ArrayRef class is the preferred class to use in an interface that + accepts a sequential list of elements in memory and just reads from them. By + taking an ArrayRef, the API can be passed a fixed size array, an std::vector, + an llvm::SmallVector and anything else that is contiguous in memory. +
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.
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, @@ -756,11 +936,27 @@ construct those elements actually used).
+ "llvm/ADT/TinyPtrVector.h" +
+ + +TinyPtrVector<Type> is a highly specialized collection class +that is optimized to avoid allocation in the case when a vector has zero or one +elements. It has two major restrictions: 1) it can only hold values of pointer +type, and 2) it cannot hold a null pointer.
+ +Since this container is highly specialized, it is rarely used.
++ "llvm/ADT/SmallVector.h" +
-SmallVector<Type, N> is a simple class that looks and smells just like vector<Type>: it supports efficient iteration, lays out elements in memory order (so you can @@ -785,11 +981,11 @@ SmallVectors are most useful when on the stack.
<vector> -
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 @@ -803,8 +999,8 @@ vector is also useful when interfacing with code that expects vectors :).
for ( ... ) {
- std::vector V;
- use V;
+ std::vector<foo> V;
+ // make use of V.
}
-std::vector@@ -827,11 +1023,11 @@ the loop.V; +std::vector<foo> V; for ( ... ) { - use V; + // make use of V. V.clear(); }
<deque> -
std::deque is, in some senses, a generalized version of std::vector. Like std::vector, it provides constant time random access and other similar properties, but it also provides efficient access to the front of the list. It @@ -843,11 +1039,11 @@ something cheaper.
<list> -
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 @@ -861,32 +1057,157 @@ not invalidate iterator or pointers to other elements in the list.
+ llvm/ADT/ilist.h +
-ilist<T> 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.
-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. +
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.
+ +These properties are exactly what we want for things like +Instructions and basic blocks, which is why these are implemented with +ilists.
+ +Related classes of interest are explained in the following subsections: +-
+
- ilist_traits +
- iplist +
- llvm/ADT/ilist_node.h +
- Sentinels +
+ llvm/ADT/PackedVector.h +
+ ++Useful for storing a vector of values using only a few number of bits for each +value. Apart from the standard operations of a vector-like container, it can +also perform an 'or' set operation.
-These properties are exactly what we want for things like Instructions and -basic blocks, which is why these are implemented with ilists.
+For example:
+ +
+enum State {
+ None = 0x0,
+ FirstCondition = 0x1,
+ SecondCondition = 0x2,
+ Both = 0x3
+};
+
+State get() {
+ PackedVector<State, 2> Vec1;
+ Vec1.push_back(FirstCondition);
+
+ PackedVector<State, 2> Vec2;
+ Vec2.push_back(SecondCondition);
+
+ Vec1 |= Vec2;
+ return Vec1[0]; // returns 'Both'.
+}
+
++ ilist_traits +
+ +ilist_traits<T> is ilist<T>'s customization +mechanism. iplist<T> (and consequently ilist<T>) +publicly derive from this traits class.
+ iplist +
+ +iplist<T> is ilist<T>'s base and as such +supports a slightly narrower interface. Notably, inserters from +T& are absent.
+ +ilist_traits<T> is a public base of this class and can be +used for a wide variety of customizations.
++ llvm/ADT/ilist_node.h +
+ +ilist_node<T> implements a the forward and backward links +that are expected by the ilist<T> (and analogous containers) +in the default manner.
+ +ilist_node<T>s are meant to be embedded in the node type +T, usually T publicly derives from +ilist_node<T>.
++ Sentinels +
+ +ilists 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 begin and end iterators, etc. Also, the +operator-- must work correctly on the end iterator in the +case of non-empty ilists.
+ +The only sensible solution to this problem is to allocate a so-called +sentinel along with the intrusive list, which serves as the end +iterator, providing the back-link to the last element. However conforming to the +C++ convention it is illegal to operator++ beyond the sentinel and it +also must not be dereferenced.
+ +These constraints allow for some implementation freedom to the ilist +how to allocate and store the sentinel. The corresponding policy is dictated +by ilist_traits<T>. By default a T gets heap-allocated +whenever the need for a sentinel arises.
+ +While the default policy is sufficient in most cases, it may break down when +T does not provide a default constructor. Also, in the case of many +instances of ilists, the memory overhead of the associated sentinels +is wasted. To alleviate the situation with numerous and voluminous +T-sentinels, sometimes a trick is employed, leading to ghostly +sentinels.
+ +Ghostly sentinels are obtained by specially-crafted ilist_traits<T> +which superpose the sentinel with the ilist instance in memory. Pointer +arithmetic is used to obtain the sentinel, which is relative to the +ilist's this pointer. The ilist 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.
++ Other Sequential Container options +
+ +Other STL containers are available, such as std::string.
There are also various STL adapter classes such as std::queue, @@ -894,28 +1215,204 @@ std::priority_queue, std::stack, etc. These provide simplified access to an underlying container but don't affect the cost of the container itself.
+ String-like containers +
+ ++There are a variety of ways to pass around and use strings in C and C++, and +LLVM adds a few new options to choose from. Pick the first option on this list +that will do what you need, they are ordered according to their relative cost. +
++Note that is is generally preferred to not pass strings around as +"const char*"'s. These have a number of problems, including the fact +that they cannot represent embedded nul ("\0") characters, and do not have a +length available efficiently. The general replacement for 'const +char*' is StringRef. +
+ +For more information on choosing string containers for APIs, please see +Passing strings.
+ + + ++ llvm/ADT/StringRef.h +
+ ++The StringRef class is a simple value class that contains a pointer to a +character and a length, and is quite related to the ArrayRef class (but specialized for arrays of +characters). Because StringRef carries a length with it, it safely handles +strings with embedded nul characters in it, getting the length does not require +a strlen call, and it even has very convenient APIs for slicing and dicing the +character range that it represents. +
+ ++StringRef is ideal for passing simple strings around that are known to be live, +either because they are C string literals, std::string, a C array, or a +SmallVector. Each of these cases has an efficient implicit conversion to +StringRef, which doesn't result in a dynamic strlen being executed. +
+ +StringRef has a few major limitations which make more powerful string +containers useful:
+ +-
+
- You cannot directly convert a StringRef to a 'const char*' because there is +no way to add a trailing nul (unlike the .c_str() method on various stronger +classes). + + +
- StringRef doesn't own or keep alive the underlying string bytes. +As such it can easily lead to dangling pointers, and is not suitable for +embedding in datastructures in most cases (instead, use an std::string or +something like that). + +
- For the same reason, StringRef cannot be used as the return value of a +method if the method "computes" the result string. Instead, use +std::string. + +
- StringRef's do not allow you to mutate the pointed-to string bytes and it +doesn't allow you to insert or remove bytes from the range. For editing +operations like this, it interoperates with the Twine class. +
Because of its strengths and limitations, it is very common for a function to +take a StringRef and for a method on an object to return a StringRef that +points into some string that it owns.
+ ++ llvm/ADT/Twine.h +
+ ++ The Twine class is used as an intermediary datatype for APIs that want to take + a string that can be constructed inline with a series of concatenations. + Twine works by forming recursive instances of the Twine datatype (a simple + value object) on the stack as temporary objects, linking them together into a + tree which is then linearized when the Twine is consumed. Twine is only safe + to use as the argument to a function, and should always be a const reference, + e.g.: +
+ ++ void foo(const Twine &T); + ... + StringRef X = ... + unsigned i = ... + foo(X + "." + Twine(i)); ++ +
This example forms a string like "blarg.42" by concatenating the values + together, and does not form intermediate strings containing "blarg" or + "blarg.". +
+ +Because Twine is constructed with temporary objects on the stack, and + because these instances are destroyed at the end of the current statement, + it is an inherently dangerous API. For example, this simple variant contains + undefined behavior and will probably crash:
+ ++ void foo(const Twine &T); + ... + StringRef X = ... + unsigned i = ... + const Twine &Tmp = X + "." + Twine(i); + foo(Tmp); ++ +
... because the temporaries are destroyed before the call. That said, + Twine's are much more efficient than intermediate std::string temporaries, and + they work really well with StringRef. Just be aware of their limitations.
+ ++ llvm/ADT/SmallString.h +
+ +SmallString is a subclass of SmallVector that +adds some convenience APIs like += that takes StringRef's. SmallString avoids +allocating memory in the case when the preallocated space is enough to hold its +data, and it calls back to general heap allocation when required. Since it owns +its data, it is very safe to use and supports full mutation of the string.
+ +Like SmallVector's, the big downside to SmallString is their sizeof. While +they are optimized for small strings, they themselves are not particularly +small. This means that they work great for temporary scratch buffers on the +stack, but should not generally be put into the heap: it is very rare to +see a SmallString as the member of a frequently-allocated heap data structure +or returned by-value. +
+ ++ std::string +
+The standard C++ std::string class is a very general class that (like + SmallString) owns its underlying data. sizeof(std::string) is very reasonable + so it can be embedded into heap data structures and returned by-value. + On the other hand, std::string is highly inefficient for inline editing (e.g. + concatenating a bunch of stuff together) and because it is provided by the + standard library, its performance characteristics depend a lot of the host + standard library (e.g. libc++ and MSVC provide a highly optimized string + class, GCC contains a really slow implementation). +
+ +The major disadvantage of std::string is that almost every operation that + makes them larger can allocate memory, which is slow. As such, it is better + to use SmallVector or Twine as a scratch buffer, but then use std::string to + persist the result.
+ + +Set-like containers are useful when you need to canonicalize multiple values into a single representation. There are several different choices for how to do this, providing various trade-offs.
-If you intend to insert a lot of elements, then do a lot of queries, a great approach is to use a vector (or other sequential container) with @@ -933,11 +1430,11 @@ efficiently queried with a standard binary or radix search.
If you have a set-like data structure that is usually small and whose elements are reasonably small, a SmallSet<Type, N> is a good choice. This set @@ -956,31 +1453,68 @@ and erasing, but does not support iteration.
SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is -transparently implemented with a SmallPtrSet), but also supports iterators. If +
SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is +transparently implemented with a SmallPtrSet), 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.
-Note that, unlike std::set, the iterators of SmallPtrSet are invalidated +
Note that, unlike std::set, the iterators of SmallPtrSet are invalidated whenever an insertion occurs. Also, the values visited by the iterators are not visited in sorted order.
+ "llvm/ADT/DenseSet.h" +
+ ++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 SmallPtrSet for pointers). Note that DenseSet has +the same requirements for the value type that DenseMap has. +
+ ++ "llvm/ADT/SparseSet.h" +
+ +SparseSet holds a small number of objects identified by unsigned keys of +moderate size. It uses a lot of memory, but provides operations that are +almost as fast as a vector. Typical keys are physical registers, virtual +registers, or numbered basic blocks.
+ +SparseSet is useful for algorithms that need very fast clear/find/insert/erase +and fast iteration over small sets. It is not intended for building composite +data structures.
++ "llvm/ADT/FoldingSet.h" +
+ +FoldingSet is an aggregate class that is really good at uniquing @@ -1013,11 +1547,11 @@ elements.
<set> -
std::set is a reasonable all-around set class, which is decent at many things but great at nothing. std::set allocates memory for each element @@ -1038,11 +1572,11 @@ std::set is almost never a good choice.
LLVM's SetVector<Type> is an adapter class that combines your choice of a set-like container along with a Sequential Container. The important property @@ -1068,21 +1602,22 @@ elements out of (linear time), unless you use it's "pop_back" method, which is faster.
-SetVector is an adapter class that defaults to using std::vector and std::set -for the underlying containers, so it is quite expensive. However, -"llvm/ADT/SetVector.h" also provides a SmallSetVector class, which -defaults to using a SmallVector and SmallSet of a specified size. If you use -this, and if your sets are dynamically smaller than N, you will save a lot of -heap traffic.
+SetVector is an adapter class that defaults to + using std::vector and a size 16 SmallSet for the underlying + containers, so it is quite expensive. However, + "llvm/ADT/SetVector.h" also provides a SmallSetVector + class, which defaults to using a SmallVector and SmallSet + of a specified size. If you use this, and if your sets are dynamically + smaller than N, you will save a lot of heap traffic.
UniqueVector is similar to SetVector, but it @@ -1096,49 +1631,68 @@ factors, and produces a lot of malloc traffic. It should be avoided.
+ "llvm/ADT/ImmutableSet.h" +
+ ++ImmutableSet is an immutable (functional) set implementation based on an AVL +tree. +Adding or removing elements is done through a Factory object and results in the +creation of a new ImmutableSet object. +If an ImmutableSet already exists with the given contents, then the existing one +is returned; equality is compared with a FoldingSetNodeID. +The time and space complexity of add or remove operations is logarithmic in the +size of the original set. + +
+There is no method for returning an element of the set, you can only check for +membership. + +
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).
+"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. +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.
-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.
+If your usage pattern follows a strict insert-then-query approach, you can @@ -1151,11 +1705,11 @@ vectors for sets.
Strings are commonly used as keys in maps, and they are difficult to support @@ -1174,7 +1728,7 @@ to the key string for a value.
The StringMap is very fast for several reasons: quadratic probing is very cache efficient for lookups, the hash value of strings in buckets is not -recomputed when lookup up an element, StringMap rarely has to touch the +recomputed when looking up an element, StringMap rarely has to touch the memory for unrelated objects when looking up a value (even when hash collisions happen), hash table growth does not recompute the hash values for strings already in the table, and each pair in the map is store in a single allocation @@ -1182,14 +1736,17 @@ already in the table, and each pair in the map is store in a single allocation
StringMap also provides query methods that take byte ranges, so it only ever copies a string if a value is inserted into the table.
+ +StringMap iteratation order, however, is not guaranteed to be deterministic, +so any uses which require that should instead use a std::map.
IndexedMap is a specialized container for mapping small dense integers (or values that can be mapped to small dense integers) to some other type. It is @@ -1205,11 +1762,11 @@ virtual register ID).
DenseMap is a simple quadratically probed hash table. It excels at supporting @@ -1220,22 +1777,64 @@ pointers, or map other small types to each other.
There are several aspects of DenseMap that you should be aware of, however. The -iterators in a densemap are invalidated whenever an insertion occurs, unlike +iterators in a DenseMap are invalidated whenever an insertion occurs, unlike 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.
++DenseMap's find_as() method supports lookup operations using an alternate key +type. This is useful in cases where the normal key type is expensive to +construct, but cheap to compare against. The DenseMapInfo is responsible for +defining the appropriate comparison and hashing methods for each alternate +key type used. +
++ "llvm/ADT/ValueMap.h" +
+ +
+ValueMap is a wrapper around a DenseMap 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 Config parameter to the ValueMap template.
+ "llvm/ADT/IntervalMap.h" +
+ +IntervalMap is a compact map for small keys and values. It maps key +intervals instead of single keys, and it will automatically coalesce adjacent +intervals. When then map only contains a few intervals, they are stored in the +map object itself to avoid allocations.
+ +The IntervalMap iterators are quite big, so they should not be passed around +as STL iterators. The heavyweight iterators allow a smaller data structure.
+ ++ <map> +
+ +std::map has similar characteristics to std::set: it uses @@ -1250,39 +1849,160 @@ another element takes place).
+ "llvm/ADT/MapVector.h" +
+MapVector<KeyT,ValueT> provides a subset of the DenseMap interface. + The main difference is that the iteration order is guaranteed to be + the insertion order, making it an easy (but somewhat expensive) solution + for non-deterministic iteration over maps of pointers.
+ +It is implemented by mapping from key to an index in a vector of key,value + pairs. This provides fast lookup and iteration, but has two main drawbacks: + The key is stored twice and it doesn't support removing elements.
+ ++ "llvm/ADT/IntEqClasses.h" +
+ +IntEqClasses provides a compact representation of equivalence classes of +small integers. Initially, each integer in the range 0..n-1 has its own +equivalence class. Classes can be joined by passing two class representatives to +the join(a, b) method. Two integers are in the same class when findLeader() +returns the same representative.
+ +Once all equivalence classes are formed, the map can be compressed so each +integer 0..n-1 maps to an equivalence class number in the range 0..m-1, where m +is the total number of equivalence classes. The map must be uncompressed before +it can be edited again.
++ "llvm/ADT/ImmutableMap.h" +
+ ++ImmutableMap is an immutable (functional) map implementation based on an AVL +tree. +Adding or removing elements is done through a Factory object and results in the +creation of a new ImmutableMap object. +If an ImmutableMap already exists with the given key set, then the existing one +is returned; equality is compared with a FoldingSetNodeID. +The time and space complexity of add or remove operations is logarithmic in the +size of the original map. + +
+ Other Map-Like Container Options +
+ +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).
+"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.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.
-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.
++ Bit storage containers (BitVector, SparseBitVector) +
+ +Unlike the other containers, there are only two bit storage containers, and +choosing when to use each is relatively straightforward.
+ +One additional option is +std::vector<bool>: 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.
+ + ++ BitVector +
+ +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). +
++ SmallBitVector +
+ +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. +
++At this time, SmallBitVector does not support set operations (and, or, xor), +and its operator[] does not provide an assignable lvalue. +
+ SparseBitVector +
+ +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). +
+This section describes how to perform some very simple transformations of LLVM code. This is meant to give examples of common idioms used, showing the @@ -1291,15 +2011,13 @@ you should also read about the main classes that you will be working with. The Core LLVM Class Hierarchy Reference contains details and descriptions of the main classes that you should know about.
-The LLVM compiler infrastructure have many different data structures that may be traversed. Following the example of the C++ standard template library, the @@ -1316,16 +2034,14 @@ on them, and it is easier to remember how to iterate. First we show a few common examples of the data structures that need to be traversed. Other data structures are traversed in very similar ways.
-Iterating over the BasicBlocks in a Function -
It's quite common to have a Function instance that you'd like to transform in some way; in particular, you'd like to manipulate its @@ -1340,7 +2056,7 @@ an example that prints the name of a BasicBlock and the number of for (Function::iterator i = func->begin(), e = func->end(); i != e; ++i) // Print out the name of the basic block if it has one, and then the // number of instructions that it contains - llvm::cerr << "Basic block (name=" << i->getName() << ") has " + errs() << "Basic block (name=" << i->getName() << ") has " << i->size() << " instructions.\n";
Iterating over the Instructions in a BasicBlock -
Just like when dealing with BasicBlocks in Functions, it's easy to iterate over the individual instructions that make up @@ -1373,25 +2089,25 @@ a BasicBlock:
for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i) // The next statement works since operator<<(ostream&,...) // is overloaded for Instruction& - llvm::cerr << *i << "\n"; + errs() << *i << "\n";However, this isn't really the best way to print out the contents of a BasicBlock! 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: llvm::cerr << *blk << "\n";.
+basic block itself: errs() << *blk << "\n";.Iterating over the Instructions in a Function -
If you're finding that you commonly iterate over a Function's BasicBlocks and then that BasicBlock's Instructions, @@ -1405,8 +2121,8 @@ small example that shows how to dump all instructions in a function to the stand #include "llvm/Support/InstIterator.h" // F is a pointer to a Function instance -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";
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);
Sometimes, it'll be useful to grab a reference (or pointer) to a class instance when all you've got at hand is an iterator. Well, extracting @@ -1459,7 +2178,7 @@ the last line of the last example,
-Instruction* pinst = &*i; +Instruction *pinst = &*i;
-Instruction* pinst = i; +Instruction *pinst = i;
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 B is a BasicBlock, +from compiling:
+ ++ llvm::SmallVector<llvm::Instruction *, 16>(B->begin(), B->end()); ++
Because of this, these implicit conversions may be removed some day, +and operator* changed to return a pointer instead of a reference.
+Say that you're writing a FunctionPass and would like to count all the locations in the entire module (that is, across every Function) where a @@ -1530,13 +2264,12 @@ class OurFunctionPass : public FunctionPass { 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 (CallInst* callInst = dyn_cast<CallInst>(&*i)) { // We know we've encountered a call instruction, so we // need to determine if it's a call to the - // function pointed to by m_func or not - + // function pointed to by m_func or not. if (callInst->getCalledFunction() == targetFunc) ++callCounter; } @@ -1545,7 +2278,7 @@ class OurFunctionPass : public FunctionPass { } private: - unsigned callCounter; + unsigned callCounter; };
You may have noticed that the previous example was a bit oversimplified in that it did not deal with call sites generated by 'invoke' instructions. In @@ -1580,11 +2313,11 @@ If you look at its definition, it has only a single pointer member.
Frequently, we might have an instance of the Value Class and we want to @@ -1597,48 +2330,86 @@ of F:
-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";
}
Alternately, it's common to have an instance of the User Class and need to know what -Values are used by it. The list of all Values used by a -User is known as a use-def chain. Instances of class -Instruction are common Users, so we might want to iterate over -all of the values that a particular instruction uses (that is, the operands of -the particular Instruction):
+Note that dereferencing a Value::use_iterator is not a very cheap +operation. Instead of performing *i above several times, consider +doing it only once in the loop body and reusing its result.
+ +Alternatively, it's common to have an instance of the User Class and need to know what +Values are used by it. The list of all Values used by a +User is known as a use-def chain. Instances of class +Instruction are common Users, so we might want to iterate over +all of the values that a particular instruction uses (that is, the operands of +the particular Instruction):
+ +
+Instruction *pi = ...;
+
+for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
+ Value *v = *i;
+ // ...
+}
+
+Declaring objects as const is an important tool of enforcing +mutation free algorithms (such as analyses, etc.). For this purpose above +iterators come in constant flavors as Value::const_use_iterator +and Value::const_op_iterator. They automatically arise when +calling use/op_begin() on const Value*s or +const User*s respectively. Upon dereferencing, they return +const Use*s. Otherwise the above patterns remain unchanged.
+ ++ Iterating over predecessors & +successors of blocks +
+ +Iterating over the predecessors and successors of a block is quite easy +with the routines defined in "llvm/Support/CFG.h". Just use code like +this to iterate over all predecessors of BB:
-Instruction* pi = ...;
+#include "llvm/Support/CFG.h"
+BasicBlock *BB = ...;
-for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
- Value* v = *i;
+for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
+ BasicBlock *Pred = *PI;
// ...
}
Similarly, to iterate over successors use +succ_iterator/succ_begin/succ_end.
+ +There are some primitive transformation operations present in the LLVM infrastructure that are worth knowing about. When performing @@ -1646,15 +2417,13 @@ transformations, it's fairly common to manipulate the contents of basic blocks. This section describes some of the common methods for doing so and gives example code.
-Instantiating Instructions
@@ -1665,7 +2434,7 @@ parameters. For example, an AllocaInst only requires a-AllocaInst* ai = new AllocaInst(Type::IntTy); +AllocaInst* ai = new AllocaInst(Type::Int32Ty);
-AllocaInst* pa = new AllocaInst(Type::IntTy, 0, "indexLoc"); +AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc");
Deleting an instruction from an existing sequence of instructions that form a -BasicBlock is very straight-forward. First, -you must have a pointer to the instruction that you wish to delete. Second, you -need to obtain the pointer to that instruction's basic block. You use the -pointer to the basic block to get its list of instructions and then use the -erase function to remove your instruction. For example:
+BasicBlock is very straight-forward: just +call the instruction's eraseFromParent() method. For example:Instruction *I = .. ; -BasicBlock *BB = I->getParent(); - -BB->getInstList().erase(I); +I->eraseFromParent();
This unlinks the instruction from its containing basic block and deletes +it. If you'd just like to unlink the instruction from its containing basic +block but not delete it, you can use the removeFromParent() method.
+Replacing individual instructions
+Replacing individual instructions
Including "llvm/Transforms/Utils/BasicBlockUtils.h" permits use of two very useful replace functions: ReplaceInstWithValue and ReplaceInstWithInst.
-Deleting Instructions
+Deleting Instructions
+- ReplaceInstWithValue
-
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 +
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 AllocaInst that allocates memory for a single integer with a null pointer to an integer.
@@ -1845,14 +2614,16 @@ AllocaInst* instToReplace = ...; BasicBlock::iterator ii(instToReplace); ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii, - Constant::getNullValue(PointerType::get(Type::IntTy))); + Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty)));
This function replaces a particular instruction with another - instruction. The following example illustrates the replacement of one - AllocaInst with another.
+ 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 AllocaInst with another.@@ -1860,11 +2631,13 @@ AllocaInst* instToReplace = ...; BasicBlock::iterator ii(instToReplace); ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii, - new AllocaInst(Type::IntTy, 0, "ptrToReplacedInt")); + new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt"));
Replacing multiple uses of Users and Values
+Replacing multiple uses of Users and Values
You can use Value::replaceAllUsesWith and User::replaceUsesOfWith to change more than one use at a time. See the @@ -1878,354 +2651,550 @@ ReplaceInstWithValue, ReplaceInstWithInst -->
+ Deleting GlobalVariables +
+ +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:
+ ++GlobalVariable *GV = .. ; + +GV->eraseFromParent(); ++
-This section describes some of the advanced or obscure API's that most clients -do not need to be aware of. These API's tend manage the inner workings of the -LLVM system, and only need to be accessed in unusual circumstances. -
+ How to Create Types +
+ +In generating IR, you may need some complex types. If you know these types +statically, you can use TypeBuilder<...>::get(), defined +in llvm/Support/TypeBuilder.h, to retrieve them. TypeBuilder +has two forms depending on whether you're building types for cross-compilation +or native library use. TypeBuilder<T, true> requires +that T be independent of the host environment, meaning that it's built +out of types from +the llvm::types +namespace and pointers, functions, arrays, etc. built of +those. TypeBuilder<T, false> additionally allows native C types +whose size may depend on the host compiler. For example,
+ ++FunctionType *ft = TypeBuilder<types::i<8>(types::i<32>*), true>::get(); +
is easier to read and write than the equivalent
++std::vector<const Type*> params; +params.push_back(PointerType::getUnqual(Type::Int32Ty)); +FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false); ++
See the class +comment for more details.
+ ++ Threads and LLVM +
+ + +-The LLVM type system has a very simple goal: allow clients to compare types for -structural equality with a simple pointer comparison (aka a shallow compare). -This goal makes clients much simpler and faster, and is used throughout the LLVM -system. +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.
-Unfortunately achieving this goal is not a simple matter. In particular, -recursive types and late resolution of opaque types makes the situation very -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, -assembly parser, and linker also have to be aware of the inner workings of this -system. +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 must adhere to the guidelines specified below to +ensure proper operation in multithreaded mode.
-For our purposes below, we need three concepts. First, an "Opaque Type" is -exactly as defined in the language -reference. Second an "Abstract Type" is any type which includes an -opaque type as part of its type graph (for example "{ opaque, i32 }"). -Third, a concrete type is a type that is not an abstract type (e.g. "{ i32, -float }"). +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.
-+ Entering and Exiting Multithreaded Mode +
--Because the most common question is "how do I build a recursive type with LLVM", -we answer it now and explain it as we go. Here we include enough to cause this -to be emitted to an output .ll file: +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 llvm_start_multithreaded() before +making any concurrent LLVM API calls. To subsequently tear down these +structures, use the llvm_stop_multithreaded() call. You can also use +the llvm_is_multithreaded() call to check the status of multithreaded +mode.
-
-%mylist = type { %mylist*, i32 }
-
--To build this, use the following LLVM APIs: +Note that both of these calls must be made in isolation. That is to +say that no other LLVM API calls may be executing at any time during the +execution of llvm_start_multithreaded() or llvm_stop_multithreaded +. It's is the client's responsibility to enforce this isolation.
--// Create the initial outer struct -PATypeHolder StructTy = OpaqueType::get(); -std::vector<const Type*> Elts; -Elts.push_back(PointerType::get(StructTy)); -Elts.push_back(Type::IntTy); -StructType *NewSTy = StructType::get(Elts); - -// At this point, NewSTy = "{ opaque*, i32 }". Tell VMCore that -// the struct and the opaque type are actually the same. -cast<OpaqueType>(StructTy.get())->refineAbstractTypeTo(NewSTy); - -// NewSTy is potentially invalidated, but StructTy (a PATypeHolder) is -// kept up-to-date -NewSTy = cast<StructType>(StructTy.get()); - -// Add a name for the type to the module symbol table (optional) -MyModule->addTypeName("mylist", NewSTy); -+
+The return value of llvm_start_multithreaded() 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 will be safe for +hosting threaded applications in the JIT, though care +must be taken to ensure that side exits and the like do not accidentally +result in concurrent LLVM API calls. +
+ Ending Execution with llvm_shutdown() +
+ +-This code shows the basic approach used to build recursive types: build a -non-recursive type using 'opaque', then use type unification to close the cycle. -The type unification step is performed by the refineAbstractTypeTo method, which is -described next. After that, we describe the PATypeHolder class. +When you are done using the LLVM APIs, you should call llvm_shutdown() +to deallocate memory used for internal structures. This will also invoke +llvm_stop_multithreaded() if LLVM is operating in multithreaded mode. +As such, llvm_shutdown() requires the same isolation guarantees as +llvm_stop_multithreaded().
++Note that, if you use scope-based shutdown, you can use the +llvm_shutdown_obj class, which calls llvm_shutdown() in its +destructor.
+ Lazy Initialization with ManagedStatic +
--The refineAbstractTypeTo method starts the type unification process. -While this method is actually a member of the DerivedType class, it is most -often used on OpaqueType instances. Type unification is actually a recursive -process. After unification, types can become structurally isomorphic to -existing types, and all duplicates are deleted (to preserve pointer equality). +ManagedStatic is a utility class in LLVM used to implement static +initialization of static resources, such as the global type tables. Before the +invocation of llvm_shutdown(), it implements a simple lazy +initialization scheme. Once llvm_start_multithreaded() returns, +however, it uses double-checked locking to implement thread-safe lazy +initialization.
-In the example above, the OpaqueType object is definitely deleted. -Additionally, if there is an "{ \2*, i32}" type already created in the system, -the pointer and struct type created are also deleted. Obviously whenever -a type is deleted, any "Type*" pointers in the program are invalidated. As -such, it is safest to avoid having any "Type*" pointers to abstract types -live across a call to refineAbstractTypeTo (note that non-abstract -types can never move or be deleted). To deal with this, the PATypeHolder class is used to maintain a stable -reference to a possibly refined type, and the AbstractTypeUser class is used to update more -complex datastructures. +Note that, because no other threads are allowed to issue LLVM API calls before +llvm_start_multithreaded() returns, it is possible to have +ManagedStatics of llvm::sys::Mutexs.
++The llvm_acquire_global_lock() and llvm_release_global_lock +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! +
+ Achieving Isolation with LLVMContext +
--PATypeHolder is a form of a "smart pointer" for Type objects. When VMCore -happily goes about nuking types that become isomorphic to existing types, it -automatically updates all PATypeHolder objects to point to the new type. In the -example above, this allows the code to maintain a pointer to the resultant -resolved recursive type, even though the Type*'s are potentially invalidated. +LLVMContext 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, +LLVMContext exists to enable just this kind of scenario!
-PATypeHolder is an extremely light-weight object that uses a lazy union-find -implementation to update pointers. For example the pointer from a Value to its -Type is maintained by PATypeHolder objects. +Conceptually, LLVMContext provides isolation. Every LLVM entity +(Modules, Values, Types, Constants, etc.) +in LLVM's in-memory IR belongs to an LLVMContext. Entities in +different contexts cannot interact with each other: Modules in +different contexts cannot be linked together, Functions cannot be added +to Modules 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.
-+In practice, very few places in the API require the explicit specification of a +LLVMContext, other than the Type creation/lookup APIs. +Because every Type carries a reference to its owning context, most +other entities can determine what context they belong to by looking at their +own Type. If you are adding new entities to LLVM IR, please try to +maintain this interface design. +
- -+For clients that do not require the benefits of isolation, LLVM +provides a convenience API getGlobalContext(). This returns a global, +lazily initialized LLVMContext that may be used in situations where +isolation is not a concern. +
+ Threads and the JIT +
+-Some data structures need more to perform more complex updates when types get -resolved. The SymbolTable class, for example, needs -move and potentially merge type planes in its representation when a pointer -changes.
+LLVM's "eager" JIT compiler is safe to use in threaded programs. Multiple +threads can call ExecutionEngine::getPointerToFunction() or +ExecutionEngine::runFunction() 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 LLVMContext 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 modifies the IR by adding +CallbackVHs). Another way is to only +call getPointerToFunction() from the LLVMContext's thread. + --To support this, a class can derive from the AbstractTypeUser class. This class -allows it to get callbacks when certain types are resolved. To register to get -callbacks for a particular type, the DerivedType::{add/remove}AbstractTypeUser -methods can be called on a type. Note that these methods only work for - abstract types. Concrete types (those that do not include any opaque -objects) can never be refined. +
When the JIT is configured to compile lazily (using +ExecutionEngine::DisableLazyCompilation(false)), there is currently a +race condition 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.
+ Advanced Topics +
+ + ++This section describes some of the advanced or obscure API's that most clients +do not need to be aware of. These API's tend manage the inner workings of the +LLVM system, and only need to be accessed in unusual circumstances. +
+ - ++ The ValueSymbolTable class +
-This class provides a symbol table that the
+ The
+ValueSymbolTable class provides a symbol table that the Function and
-Module classes use for naming definitions. The symbol table can
-provide a name for any Value.
-SymbolTable is an abstract data type. It hides the data it contains
-and provides access to it through a controlled interface.
Note that the SymbolTable class should not be directly accessed 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 -Values have names, and those without names (i.e. they have +Values have names, and those without names (i.e. they have an empty name) do not exist in the symbol table.
-To use the SymbolTable well, you need to understand the -structure of the information it holds. The class contains two -std::map objects. The first, pmap, is a map of -Type* to maps of name (std::string) to Value*. -Thus, Values are stored in two-dimensions and accessed by Type and -name.
+Symbol tables support iteration over the values in the symbol +table with begin/end/iterator and supports querying to see if a +specific name is in the symbol table (with lookup). The +ValueSymbolTable class exposes no public mutator methods, instead, +simply call setName on a value, which will autoinsert it into the +appropriate symbol table.
-The interface of this class provides three basic types of operations: -
-
-
- Accessors. Accessors provide read-only access to information - such as finding a value for a name with the - lookup method. -
- Mutators. Mutators allow the user to add information to the - SymbolTable with methods like - insert. -
- Iterators. Iterators allow the user to traverse the content - of the symbol table in well defined ways, such as the method - plane_begin. -
Accessors
--
-
- Value* lookup(const Type* Ty, const std::string& name) const: - -
- The lookup method searches the type plane given by the - Ty parameter for a Value with the provided name. - If a suitable Value is not found, null is returned. - -
- bool isEmpty() const: -
- This function returns true if both the value and types maps are - empty -
Mutators
--
-
- void insert(Value *Val): -
- This method adds the provided value to the symbol table. The Value must - have both a name and a type which are extracted and used to place the value - in the correct type plane under the value's name. - -
- void remove(Value* Val): -
- This method removes a named value from the symbol table. The - type and name of the Value are extracted from \p N and used to - lookup the Value in the correct type plane. If the Value is - not in the symbol table, this method silently ignores the - request. -
+ The User and owned Use classes' memory layout +
+ +The +User class provides a basis for expressing the ownership of User +towards other +Values. The +Use helper class is employed to do the bookkeeping and to facilitate O(1) +addition and removal.
-Iteration
-The following functions describe three types of iterators you can obtain -the beginning or end of the sequence for both const and non-const. It is -important to keep track of the different kinds of iterators. There are -three idioms worth pointing out:
- -| Units | Iterator | Idiom |
|---|---|---|
| Planes Of name/Value maps | PI | -
-for (SymbolTable::plane_const_iterator PI = ST.plane_begin(),
- PE = ST.plane_end(); PI != PE; ++PI ) {
- PI->first // This is the Type* of the plane
- PI->second // This is the SymbolTable::ValueMap of name/Value pairs
-}
- |
-
| name/Value pairs in a plane | VI | -
-for (SymbolTable::value_const_iterator VI = ST.value_begin(SomeType),
- VE = ST.value_end(SomeType); VI != VE; ++VI ) {
- VI->first // This is the name of the Value
- VI->second // This is the Value* value associated with the name
-}
- |
-
+ + Interaction and relationship between User and Use objects + +
-Using the recommended iterator names and idioms will help you avoid -making mistakes. Of particular note, make sure that whenever you use -value_begin(SomeType) that you always compare the resulting iterator -with value_end(SomeType) not value_end(SomeOtherType) or else you -will loop infinitely.
++A subclass of User can choose between incorporating its Use objects +or refer to them out-of-line by means of a pointer. A mixed variant +(some Uses inline others hung off) is impractical and breaks the invariant +that the Use objects belonging to the same User form a contiguous array. +
--
+
Layout a) +The Use object(s) are inside (resp. at fixed offset) of the User +object and there are a fixed number of them.
+ +Layout b) +The Use object(s) are referenced by a pointer to an +array from the User object and there may be a variable +number of them.
+Layout a) is modelled by prepending the User object by the Use[] array.
-- plane_iterator plane_begin():
-- Get an iterator that starts at the beginning of the type planes. - The iterator will iterate over the Type/ValueMap pairs in the - type planes.
++...---.---.---.---.-------... + | P | P | P | P | User +'''---'---'---'---'-------''' +
-- plane_const_iterator plane_begin() const:
-- Get a const_iterator that starts at the beginning of the type - planes. The iterator will iterate over the Type/ValueMap pairs - in the type planes.
+Layout b) is modelled by pointing at the Use[] array.
++.-------... +| User +'-------''' + | + v + .---.---.---.---... + | P | P | P | P | + '---'---'---'---''' +
+- plane_iterator plane_end(): -
- Get an iterator at the end of the type planes. This serves as - the marker for end of iteration over the type planes. +
+We have 2 different layouts in the User (sub)classes: +
-
+
+As of v2.4 each layout still possesses a direct pointer to the +start of the array of Uses. Though not mandatory for layout a), +we stick to this redundancy for the sake of simplicity. +The User object also stores the number of Use objects it +has. (Theoretically this information can also be calculated +given the scheme presented below.)
++Special forms of allocation operators (operator new) +enforce the following memory layouts:
+ +-
+
+ The waymarking algorithm +
-+Since the Use objects are deprived of the direct (back)pointer to +their User objects, there must be a fast and exact method to +recover it. This is accomplished by the following scheme:
--
+
- 00 —> binary digit 0 +
- 01 —> binary digit 1 +
- 10 —> stop and calculate (s) +
- 11 —> full stop (S) +
+Given a Use*, all we have to do is to walk till we get +a stop and we either have a User immediately behind or +we have to walk to the next stop picking up digits +and calculating the offset:
++.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---------------- +| 1 | s | 1 | 0 | 1 | 0 | s | 1 | 1 | 0 | s | 1 | 1 | s | 1 | S | User (or User*) +'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---------------- + |+15 |+10 |+6 |+3 |+1 + | | | | |__> + | | | |__________> + | | |______________________> + | |______________________________________> + |__________________________________________________________> ++
+Only the significant number of bits need to be stored between the +stops, so that the worst case is 20 memory accesses when there are +1000 Use objects associated with a User.
-+ Reference implementation +
-+The following literate Haskell fragment demonstrates the concept:
-+> 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 [] +> ++
+Printing <test> gives: "1s100000s11010s10100s1111s1010s110s11s1S"
++The reverse algorithm computes the length of the string just by examining +a certain prefix:
+ +
+> 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
+>
+
++Now, as expected, printing <pref test> gives 40.
++We can quickCheck this with following property:
+ ++> testcase = dist 2000 [] +> testcaseLength = length testcase +> +> identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr +> where arr = takeLast n testcase +> ++
+As expected <quickCheck identityProp> gives:
+ ++*Main> quickCheck identityProp +OK, passed 100 tests. ++
+Let's be a bit more exhaustive:
+ +
+>
+> deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
+>
+
++And here is the result of <deepCheck identityProp>:
+ ++*Main> deepCheck identityProp +OK, passed 500 tests. ++ +
+ Tagging considerations +
+ ++To maintain the invariant that the 2 LSBits of each Use** in Use +never change after being set up, setters of Use::Prev must re-tag the +new Use** on every modification. Accordingly getters must strip the +tag bits.
++For layout b) instead of the User we find a pointer (User* with LSBit set). +Following this pointer brings us to the User. A portable trick ensures +that the first bytes of User (if interpreted as a pointer) never has +the LSBit set. (Portability is relying on the fact that all known compilers place the +vptr in the first word of the instances.)
-#include "llvm/Type.h"
doxygen info: Type Class
Type is a superclass of all type classes. Every Value has a Type. Type cannot be instantiated directly but only @@ -2256,24 +3223,20 @@ the lib/VMCore directory.
be performed with address equality of the Type Instance. That is, given two Type* values, the types are identical if the pointers are identical. -+ Important Public Methods +
--
-
- bool isInteger() const: Returns true for any integer type. +
- bool isIntegerTy() const: Returns true for any integer type. -
- bool isFloatingPoint(): Return true if this is one of the two +
- bool isFloatingPointTy(): Return true if this is one of the five floating point types. -
- bool isAbstract(): Return true if the type is abstract (contains - an OpaqueType anywhere in its definition). -
- bool isSized(): Return true if the type has known size. Things that don't have a size are abstract types, labels and void. @@ -2281,10 +3244,10 @@ the lib/VMCore directory.
+ Important Derived Types +
+- IntegerType
- Subclass of DerivedType that represents integer types of any bit width. @@ -2298,7 +3261,7 @@ the lib/VMCore directory.
- SequentialType -
- This is subclassed by ArrayType and PointerType
+
- This is subclassed by ArrayType, PointerType and VectorType.
- const Type * getElementType() const: Returns the type of each of the elements in the sequential type. @@ -2314,18 +3277,18 @@ the lib/VMCore directory.
- This is subclassed by ArrayType, PointerType and VectorType.
- PointerType
- Subclass of SequentialType for pointer types. -
- PackedType -
- Subclass of SequentialType for packed (vector) types. A - packed type is similar to an ArrayType but is distinguished because it is - a first class type wherease ArrayType is not. Packed types are used for +
- VectorType +
- Subclass of SequentialType for vector types. A + vector type is similar to an ArrayType but is distinguished because it is + 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.
- StructType
- Subclass of DerivedTypes for struct types. -
- FunctionType +
- FunctionType
- Subclass of DerivedTypes for function types.
-
-
- bool isVarArg() const: Returns true if its a vararg +
- bool isVarArg() const: Returns true if it's a vararg function
- const Type * getReturnType() const: Returns the return type of the function. @@ -2335,25 +3298,17 @@ the lib/VMCore directory. number of formal parameters.
- - OpaqueType -
- Sublcass of DerivedType for abstract types. This class - defines no content and is used as a placeholder for some other type. Note - that OpaqueType is used (temporarily) during type resolution for forward - references of types. Once the referenced type is resolved, the OpaqueType - is replaced with the actual type. OpaqueType can also be used for data - abstraction. At link time opaque types can be resolved to actual types - of the same name.
The Module class -
#include "llvm/Module.h"
doxygen info:
@@ -2368,23 +3323,20 @@ href="#GlobalVariable">GlobalVariables, and a SymbolTable. Additionally, it contains a few
helpful member functions that try to make common operations easy.
-
-
- Module::Module(std::string name = "") -
Constructing a Module is easy. You can optionally +
Constructing a Module is easy. You can optionally provide a name for it (probably based on the name of the translation unit).
+- Module::iterator - Typedef for function list iterator
Module::const_iterator - Typedef for const_iterator.
@@ -2442,8 +3394,9 @@ provide a name for it (probably based on the name of the translation unit).
-
-
- Function *getFunction(const std::string - &Name, const FunctionType *Ty) + +
- Function *getFunction(StringRef Name) const
+
Look up the specified function in the Module SymbolTable. If it does not exist, return @@ -2474,13 +3427,14 @@ provide a name for it (probably based on the name of the translation unit).
The Value class -
#include "llvm/Value.h"
@@ -2516,7 +3470,7 @@ method. In addition, all LLVM values can be named. The "name" of the
The name of this instruction is "foo". NOTE +
The name of this instruction is "foo". NOTE that the name of any value may be missing (an empty string), so names should ONLY be used for debugging (making the source code easier to read, debugging printouts), they should not be used to keep track of values or map @@ -2531,19 +3485,17 @@ the class that represents this value. Although this may take some getting used to, it simplifies the representation and makes it easier to manipulate.
-- Value::use_iterator - Typedef for iterator over the
use-list
- Value::use_const_iterator - Typedef for const_iterator over + Value::const_use_iterator - Typedef for const_iterator over the use-list
unsigned use_size() - Returns the number of users of the value.
@@ -2585,12 +3537,14 @@ Inst->replaceAllUsesWith(ConstVal);
The User class -
#include "llvm/User.h"
@@ -2609,14 +3563,12 @@ Single Assignment (SSA) form, there can only be one definition referred to,
allowing this direct connection. This connection provides the use-def
information in LLVM.
The User class exposes the operand list in two ways: through an index access interface and through an iterator based interface.
@@ -2639,12 +3591,14 @@ the operands of a User.#include "llvm/Instruction.h"
@@ -2675,14 +3629,13 @@ href="#CmpInst">CmpInst). Unfortunately, the use of macros in
this file confuses doxygen, so these enum values don't show up correctly in the
doxygen output.
+ + Important Subclasses of the Instruction class + +
+- BinaryOperator
This subclasses represents all two operand instructions whose operands @@ -2701,12 +3654,13 @@ this file confuses doxygen, so these enum values don't show up correctly in the
+ + Important Public Members of the Instruction class + +
-- BasicBlock *getParent() @@ -2726,12 +3680,14 @@ and it has no name
Constant represents a base class for different types of constants. It is subclassed by ConstantInt, ConstantArray, etc. for representing @@ -2739,19 +3695,27 @@ the various types of Constants. GlobalValue is also a subclass, which represents the address of a global variable or function.
-Important Subclasses of Constant
+- ConstantInt : This subclass of Constant represents an integer constant of
any width.
-
-
- int64_t getSExtValue() const: Returns the underlying value of - this constant as a sign extended signed integer value. -
- uint64_t getZExtValue() const: Returns the underlying value - of this constant as a zero extended unsigned integer value. +
- const APInt& getValue() const: Returns the underlying + value of this constant, an APInt value. +
- int64_t getSExtValue() const: Converts the underlying APInt + value to an int64_t via sign extension. If the value (not the bit width) + of the APInt is too large to fit in an int64_t, an assertion will result. + For this reason, use of this method is discouraged. +
- uint64_t getZExtValue() const: Converts the underlying APInt + value to a uint64_t via zero extension. IF the value (not the bit width) + of the APInt is too large to fit in a uint64_t, an assertion will result. + For this reason, use of this method is discouraged. +
- static ConstantInt* get(const APInt& Val): Returns the + ConstantInt object that represents the value provided by Val. + The type is implied as the IntegerType that corresponds to the bit width + of Val.
- static ConstantInt* get(const Type *Ty, uint64_t Val): Returns the ConstantInt object that represents the value provided by Val for integer type Ty. @@ -2781,13 +3745,14 @@ a subclass, which represents the address of a global variable or function.
#include "llvm/GlobalValue.h"
@@ -2827,15 +3792,14 @@ dereference the pointer with GetElementPtrInst first, then its elements
can be accessed. This is explained in the LLVM
Language Reference Manual.
+ + Important Public Members of the GlobalValue class + +
-- bool hasInternalLinkage() const
@@ -2851,12 +3815,14 @@ GlobalValue is currently embedded into.
#include "llvm/Function.h"
doxygen
@@ -2867,7 +3833,7 @@ Superclasses: GlobalValue,
Value
The Function 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 Function class keeps track of a list of BasicBlocks, a list of formal Arguments, and a @@ -2876,7 +3842,7 @@ of a list of BasicBlocks, a list of formal
The list of BasicBlocks is the most commonly used part of Function objects. The list imposes an implicit ordering of the blocks in the function, which indicate how the code will be -layed out by the backend. Additionally, the first BasicBlock is the implicit entry node for the Function. 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 @@ -2903,32 +3869,32 @@ href="#Argument">Arguments in the function body.
Note that Function is a GlobalValue and therefore also a Constant. The value of the function is its address (after linking) which is guaranteed to be constant.
-+ + Important Public Members of the Function class + +
-- Function(const FunctionType
*Ty, LinkageTypes Linkage, const std::string &N = "", Module* Parent = 0)
Constructor used when you need to create new Functions to add - the the program. The constructor must specify the type of the function to + the program. The constructor must specify the type of the function to create and what type of linkage the function should have. The FunctionType argument specifies the formal arguments and return value for the function. The same - FunctionType value can be used to + FunctionType value can be used to create multiple functions. The Parent argument specifies the Module in which the function is defined. If this argument is provided, the function will automatically be inserted into that module's list of functions.
- - bool isExternal() +
- bool isDeclaration()
Return whether or not the Function has a body defined. If the function is "external", it does not have a body, and thus must be resolved @@ -2989,12 +3955,14 @@ iterator
#include "llvm/GlobalVariable.h" @@ -3006,7 +3974,7 @@ Superclasses: GlobalValue, User, Value
-Global variables are represented with the (suprise suprise) +
Global variables are represented with the (surprise surprise) GlobalVariable class. Like functions, GlobalVariables are also subclasses of GlobalValue, and as such are always referenced by their address (global values must live in memory, so their @@ -3016,15 +3984,15 @@ variables may have an initial value (which must be a Constant), and if they have an initializer, they may be marked as "constant" themselves (indicating that their contents never change at runtime).
-+ + Important Public Members of the GlobalVariable class + +
-- GlobalVariable(const Type *Ty, bool
@@ -3034,11 +4002,12 @@ never change at runtime).
Create a new global variable of the specified type. If isConstant 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 LLVM Language Reference 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 @@ -3055,27 +4024,28 @@ never change at runtime).
- Constant *getInitializer()
-
Returns the intial value for a GlobalVariable. It is not legal +
Returns the initial value for a GlobalVariable. It is not legal to call this method if there is no initializer.
#include "llvm/BasicBlock.h"
-doxygen info: BasicBlock
+doxygen info: BasicBlock
Class
Superclass: Value
This class represents a single entry multiple exit section of the code, +
This class represents a single entry single exit section of the code, commonly known as a basic block by the compiler community. The BasicBlock class maintains a list of Instructions, which form the body of the block. @@ -3092,15 +4062,14 @@ href="#Value">Values, because they are referenced by instructions like branches and can go in the switch tables. BasicBlocks have type label.
-+ + Important Public Members of the BasicBlock class + +
- - + -This subclass of Value defines the interface for incoming formal arguments to a function. A Function maintains a list of its formal @@ -3166,17 +4136,19 @@ arguments. An argument has a pointer to the parent Function.
+ src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS">
- The LLVM Compiler Infrastructure
+ The LLVM Compiler Infrastructure
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