LLVM Programmer's Manual

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 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" 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.

Sometimes you may find yourself in a situation where enabling -debug just turns on too much information (such as when working on the code generator). If you want to enable debug information with more fine-grained control, you define the DEBUG_TYPE macro and the -debug only option as follows:

#undef  DEBUG_TYPE
DEBUG(errs() << "No debug type\n");
#define DEBUG_TYPE "foo"
DEBUG(errs() << "'foo' debug type\n");
#undef  DEBUG_TYPE
#define DEBUG_TYPE "bar"
DEBUG(errs() << "'bar' debug type\n"));
#undef  DEBUG_TYPE
#define DEBUG_TYPE ""
DEBUG(errs() << "No debug type (2)\n");

Then you can run your pass like this:

$ opt < a.bc > /dev/null -mypass
<no output>
$ opt < a.bc > /dev/null -mypass -debug
No debug type
'foo' debug type
'bar' debug type
No debug type (2)
$ opt < a.bc > /dev/null -mypass -debug-only=foo
'foo' debug type
$ opt < a.bc > /dev/null -mypass -debug-only=bar
'bar' debug type

Of course, in practice, you should only set DEBUG_TYPE at the top of a file, to specify the debug type for the entire module (if you do this before you #include "llvm/Support/Debug.h", you don't have to insert the ugly #undef's). Also, you should use names more meaningful than "foo" and "bar", because there is no system in place to ensure that names do not conflict. If two different modules use the same string, they will all be turned on when the name is specified. This allows, for example, all debug information for instruction scheduling to be enabled with -debug-type=InstrSched, even if the source lives in multiple files.

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::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, consider a SmallVector). The cost of a heap allocated array is the cost of the new/delete (aka malloc/free). Also note that if you are allocating an array of a type with a constructor, the constructor and destructors will be run for every element in the array (re-sizable vectors only construct those elements actually used).

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.

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 do pointer arithmetic between elements), supports efficient push_back/pop_back operations, supports efficient random access to its elements, etc.

The advantage of SmallVector is that it allocates space for some number of elements (N) in the object itself. Because of this, if the SmallVector is dynamically smaller than N, no malloc is performed. This can be a big win in cases where the malloc/free call is far more expensive than the code that fiddles around with the elements.

This is good for vectors that are "usually small" (e.g. the number of predecessors/successors of a block is usually less than 8). On the other hand, this makes the size of the SmallVector itself large, so you don't want to allocate lots of them (doing so will waste a lot of space). As such, SmallVectors are most useful when on the stack.

SmallVector also provides a nice portable and efficient replacement for alloca.

std::vector is well loved and respected. It is useful when SmallVector isn't: when the size of the vector is often large (thus the small optimization will rarely be a benefit) or if you will be allocating many instances of the vector itself (which would waste space for elements that aren't in the container). vector is also useful when interfacing with code that expects vectors :).

One worthwhile note about std::vector: avoid code like this:

for ( ... ) {
   std::vector<foo> V;
   use V;
}

Instead, write this as:

std::vector<foo> V;
for ( ... ) {
   use V;
   V.clear();
}

Doing so will save (at least) one heap allocation and free per iteration of the loop.

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 does not guarantee continuity of elements within memory.

In exchange for this extra flexibility, std::deque has significantly higher constant factor costs than std::vector. If possible, use std::vector or something cheaper.

std::list is an extremely inefficient class that is rarely useful. It performs a heap allocation for every element inserted into it, thus having an extremely high constant factor, particularly for small data types. std::list also only supports bidirectional iteration, not random access iteration.

In exchange for this high cost, std::list supports efficient access to both ends of the list (like std::deque, but unlike std::vector or SmallVector). In addition, the iterator invalidation characteristics of std::list are stronger than that of a vector class: inserting or removing an element into the list does not invalidate iterator or pointers to other elements in the list.

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.

These properties are exactly what we want for things like Instructions and basic blocks, which is why these are implemented with ilists.

• ilist_traits
• iplist
• llvm/ADT/ilist_node.h
• Sentinels

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.

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<T> is ilist<T>'s customization mechanism. iplist<T> (and consequently ilist<T>) publicly derive from this traits class.

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.

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>.

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 STL containers are available, such as std::string.

There are also various STL adapter classes such as std::queue, std::priority_queue, std::stack, etc. These provide simplified access to an underlying container but don't affect the cost of the container itself.

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:

1. 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).
2. 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).
3. 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.
4. 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.

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.

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.

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.

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 std::sort+std::unique to remove duplicates. This approach works really well if your usage pattern has these two distinct phases (insert then query), and can be coupled with a good choice of sequential container.

This combination provides the several nice properties: the result data is contiguous in memory (good for cache locality), has few allocations, is easy to address (iterators in the final vector are just indices or pointers), and can be 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 has space for N elements in place (thus, if the set is dynamically smaller than N, no malloc traffic is required) and accesses them with a simple linear search. When the set grows beyond 'N' elements, it allocates a more expensive representation that guarantees efficient access (for most types, it falls back to std::set, but for pointers it uses something far better, SmallPtrSet).

The magic of this class is that it handles small sets extremely efficiently, but gracefully handles extremely large sets without loss of efficiency. The drawback is that the interface is quite small: it supports insertion, queries 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 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 whenever an insertion occurs. Also, the values visited by the iterators are not visited in sorted order.

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.

FoldingSet is an aggregate class that is really good at uniquing expensive-to-create or polymorphic objects. It is a combination of a chained hash table with intrusive links (uniqued objects are required to inherit from FoldingSetNode) that uses SmallVector as part of its ID process.

Consider a case where you want to implement a "getOrCreateFoo" method for a complex object (for example, a node in the code generator). The client has a description of *what* it wants to generate (it knows the opcode and all the operands), but we don't want to 'new' a node, then try inserting it into a set only to find out it already exists, at which point we would have to delete it and return the node that already exists.

To support this style of client, FoldingSet perform a query with a FoldingSetNodeID (which wraps SmallVector) that can be used to describe the element that we want to query for. The query either returns the element matching the ID or it returns an opaque ID that indicates where insertion should take place. Construction of the ID usually does not require heap traffic.

Because FoldingSet uses intrusive links, it can support polymorphic objects in the set (for example, you can have SDNode instances mixed with LoadSDNodes). Because the elements are individually allocated, pointers to the elements are stable: inserting or removing elements does not invalidate any pointers to other elements.

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 inserted (thus it is very malloc intensive) and typically stores three pointers per element in the set (thus adding a large amount of per-element space overhead). It offers guaranteed log(n) performance, which is not particularly fast from a complexity standpoint (particularly if the elements of the set are expensive to compare, like strings), and has extremely high constant factors for lookup, insertion and removal.

The advantages of std::set are that its iterators are stable (deleting or inserting an element from the set does not affect iterators or pointers to other elements) and that iteration over the set is guaranteed to be in sorted order. If the elements in the set are large, then the relative overhead of the pointers and malloc traffic is not a big deal, but if the elements of the set are small, 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 that this provides is efficient insertion with uniquing (duplicate elements are ignored) with iteration support. It implements this by inserting elements into both a set-like container and the sequential container, using the set-like container for uniquing and the sequential container for iteration.

The difference between SetVector and other sets is that the order of iteration is guaranteed to match the order of insertion into the SetVector. This property is really important for things like sets of pointers. Because pointer values are non-deterministic (e.g. vary across runs of the program on different machines), iterating over the pointers in the set will not be in a well-defined order.

The drawback of SetVector is that it requires twice as much space as a normal set and has the sum of constant factors from the set-like container and the sequential container that it uses. Use it *only* if you need to iterate over the elements in a deterministic order. SetVector is also expensive to delete 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 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 retains a unique ID for each element inserted into the set. It internally contains a map and a vector, and it assigns a unique ID for each value inserted into the set.

UniqueVector is very expensive: its cost is the sum of the cost of maintaining both the map and vector, it has high complexity, high constant factors, and produces a lot of malloc traffic. It should be avoided.

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). 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.

If your usage pattern follows a strict insert-then-query approach, you can trivially use the same approach as sorted vectors for set-like containers. The only difference is that your query function (which uses std::lower_bound to get efficient log(n) lookup) should only compare the key, not both the key and value. This yields the same advantages as sorted vectors for sets.

Strings are commonly used as keys in maps, and they are difficult to support efficiently: they are variable length, inefficient to hash and compare when long, expensive to copy, etc. StringMap is a specialized container designed to cope with these issues. It supports mapping an arbitrary range of bytes to an arbitrary other object.

The StringMap implementation uses a quadratically-probed hash table, where the buckets store a pointer to the heap allocated entries (and some other stuff). The entries in the map must be heap allocated because the strings are variable length. The string data (key) and the element object (value) are stored in the same allocation with the string data immediately after the element object. This container guarantees the "(char*)(&Value+1)" points 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 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 (the string data is stored in the same allocation as the Value of a pair).

StringMap also provides query methods that take byte ranges, so it only ever copies a string if a value is inserted into the table.

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 internally implemented as a vector with a mapping function that maps the keys to the dense integer range.

This is useful for cases like virtual registers in the LLVM code generator: they have a dense mapping that is offset by a compile-time constant (the first virtual register ID).

DenseMap is a simple quadratically probed hash table. It excels at supporting small keys and values: it uses a single allocation to hold all of the pairs that are currently inserted in the map. DenseMap is a great way to map pointers to 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 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 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.

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.

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.

std::map has similar characteristics to std::set: it uses a single allocation per pair inserted into the map, it offers log(n) lookup with an extremely large constant factor, imposes a space penalty of 3 pointers per pair in the map, etc.

std::map is most useful when your keys or values are very large, if you need to iterate over the collection in sorted order, or if you need stable iterators into the map (i.e. they don't get invalidated if an insertion or deletion of another element takes place).

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.

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). 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 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).

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.

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).

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 BasicBlocks. To facilitate this, you'll need to iterate over all of the BasicBlocks that constitute the Function. The following is an example that prints the name of a BasicBlock and the number of Instructions it contains:

// func is a pointer to a Function instance
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
  errs() << "Basic block (name=" << i->getName() << ") has "
             << i->size() << " instructions.\n";

Note that i can be used as if it were a pointer for the purposes of invoking member functions of the Instruction class. This is because the indirection operator is overloaded for the iterator classes. In the above code, the expression i->size() is exactly equivalent to (*i).size() just like you'd expect.

Just like when dealing with BasicBlocks in Functions, it's easy to iterate over the individual instructions that make up BasicBlocks. Here's a code snippet that prints out each instruction in a BasicBlock:

// blk is a pointer to a BasicBlock instance
for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i)
   // The next statement works since operator<<(ostream&,...)
   // is overloaded for Instruction&
   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: errs() << *blk << "\n";.

If you're finding that you commonly iterate over a Function's BasicBlocks and then that BasicBlock's Instructions, InstIterator should be used instead. You'll need to include llvm/Support/InstIterator.h, and then instantiate InstIterators explicitly in your code. Here's a small example that shows how to dump all instructions in a function to the standard error stream:

#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)
  errs() << *I << "\n";

Easy, isn't it? You can also use InstIterators to fill a work list with its initial contents. For example, if you wanted to initialize a work list to contain all instructions in a Function F, all you would need to do is something like:

std::set<Instruction*> worklist;
// or better yet, SmallPtrSet<Instruction*, 64> worklist;

for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
   worklist.insert(&*I);

The STL set worklist would now contain all instructions in the Function pointed to by F.

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 a reference or a pointer from an iterator is very straight-forward. Assuming that i is a BasicBlock::iterator and j is a BasicBlock::const_iterator:

Instruction& inst = *i;   // Grab reference to instruction reference
Instruction* pinst = &*i; // Grab pointer to instruction reference
const Instruction& inst = *j;

However, the iterators you'll be working with in the LLVM framework are special: they will automatically convert to a ptr-to-instance type whenever they need to. Instead of dereferencing the iterator and then taking the address of the result, you can simply assign the iterator to the proper pointer type and you get the dereference and address-of operation as a result of the assignment (behind the scenes, this is a result of overloading casting mechanisms). Thus the last line of the last example,

Instruction *pinst = &*i;

is semantically equivalent to

Instruction *pinst = i;

It's also possible to turn a class pointer into the corresponding iterator, and this is a constant time operation (very efficient). The following code snippet illustrates use of the conversion constructors provided by LLVM iterators. By using these, you can explicitly grab the iterator of something without actually obtaining it via iteration over some structure:

void printNextInstruction(Instruction* inst) {
  BasicBlock::iterator it(inst);
  ++it; // After this line, it refers to the instruction after *inst
  if (it != inst->getParent()->end()) errs() << *it << "\n";
}

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 certain function (i.e., some Function*) is already in scope. As you'll learn later, you may want to use an InstVisitor to accomplish this in a much more straight-forward manner, but this example will allow us to explore how you'd do it if you didn't have InstVisitor around. In pseudo-code, this is what we want to do:

initialize callCounter to zero
for each Function f in the Module
  for each BasicBlock b in f
    for each Instruction i in b
      if (i is a CallInst and calls the given function)
        increment callCounter

And the actual code is (remember, because we're writing a FunctionPass, our FunctionPass-derived class simply has to override the runOnFunction method):

Function* targetFunc = ...;

class OurFunctionPass : public FunctionPass {
  public:
    OurFunctionPass(): callCounter(0) { }

    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) {
          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.
            if (callInst->getCalledFunction() == targetFunc)
              ++callCounter;
          }
        }
      }
    }

  private:
    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 this, and in other situations, you may find that you want to treat CallInsts and InvokeInsts the same way, even though their most-specific common base class is Instruction, which includes lots of less closely-related things. For these cases, LLVM provides a handy wrapper class called CallSite. It is essentially a wrapper around an Instruction pointer, with some methods that provide functionality common to CallInsts and InvokeInsts.

This class has "value semantics": it should be passed by value, not by reference and it should not be dynamically allocated or deallocated using operator new or operator delete. It is efficiently copyable, assignable and constructable, with costs equivalents to that of a bare pointer. 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 determine which Users use the Value. The list of all Users of a particular Value is called a def-use chain. For example, let's say we have a Function* named F to a particular function foo. Finding all of the instructions that use foo is as simple as iterating over the def-use chain of 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)) {
    errs() << "F is used in instruction:\n";
    errs() << *Inst << "\n";
  }

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 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:

#include "llvm/Support/CFG.h"
BasicBlock *BB = ...;

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.

Instantiating Instructions

Creation of Instructions is straight-forward: simply call the constructor for the kind of instruction to instantiate and provide the necessary parameters. For example, an AllocaInst only requires a (const-ptr-to) Type. Thus:

AllocaInst* ai = new AllocaInst(Type::Int32Ty);

will create an AllocaInst instance that represents the allocation of one integer in the current stack frame, at run time. Each Instruction subclass is likely to have varying default parameters which change the semantics of the instruction, so refer to the doxygen documentation for the subclass of Instruction that you're interested in instantiating.

Naming values

It is very useful to name the values of instructions when you're able to, as this facilitates the debugging of your transformations. If you end up looking at generated LLVM machine code, you definitely want to have logical names associated with the results of instructions! By supplying a value for the Name (default) parameter of the Instruction constructor, you associate a logical name with the result of the instruction's execution at run time. For example, say that I'm writing a transformation that dynamically allocates space for an integer on the stack, and that integer is going to be used as some kind of index by some other code. To accomplish this, I place an AllocaInst at the first point in the first BasicBlock of some Function, and I'm intending to use it within the same Function. I might do:

AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc");

where indexLoc is now the logical name of the instruction's execution value, which is a pointer to an integer on the run time stack.

Inserting instructions

There are essentially two ways to insert an Instruction into an existing sequence of instructions that form a BasicBlock:

• Insertion into an explicit instruction list

  Given a BasicBlock* pb, an Instruction* pi within that BasicBlock, and a newly-created instruction we wish to insert before *pi, we do the following:

  BasicBlock *pb = ...;
  Instruction *pi = ...;
  Instruction *newInst = new Instruction(...);
  
  pb->getInstList().insert(pi, newInst); // Inserts newInst before pi in pb
  

  Appending to the end of a BasicBlock is so common that the Instruction class and Instruction-derived classes provide constructors which take a pointer to a BasicBlock to be appended to. For example code that looked like:

  BasicBlock *pb = ...;
  Instruction *newInst = new Instruction(...);
  
  pb->getInstList().push_back(newInst); // Appends newInst to pb
  

  becomes:

  BasicBlock *pb = ...;
  Instruction *newInst = new Instruction(..., pb);
  

  which is much cleaner, especially if you are creating long instruction streams.

• Insertion into an implicit instruction list

  Instruction instances that are already in BasicBlocks are implicitly associated with an existing instruction list: the instruction list of the enclosing basic block. Thus, we could have accomplished the same thing as the above code without being given a BasicBlock by doing:

  Instruction *pi = ...;
  Instruction *newInst = new Instruction(...);
  
  pi->getParent()->getInstList().insert(pi, newInst);
  

  In fact, this sequence of steps occurs so frequently that the Instruction class and Instruction-derived classes provide constructors which take (as a default parameter) a pointer to an Instruction which the newly-created Instruction should precede. That is, Instruction constructors are capable of inserting the newly-created instance into the BasicBlock of a provided instruction, immediately before that instruction. Using an Instruction constructor with a insertBefore (default) parameter, the above code becomes:

  Instruction* pi = ...;
  Instruction* newInst = new Instruction(..., pi);
  

  which is much cleaner, especially if you're creating a lot of instructions and adding them to BasicBlocks.

Deleting an instruction from an existing sequence of instructions that form a BasicBlock is very straight-forward: just call the instruction's eraseFromParent() method. For example:

Instruction *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

Including "llvm/Transforms/Utils/BasicBlockUtils.h" permits use of two very useful replace functions: ReplaceInstWithValue and ReplaceInstWithInst.

Deleting Instructions

• ReplaceInstWithValue

  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.

  AllocaInst* instToReplace = ...;
  BasicBlock::iterator ii(instToReplace);
  
  ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii,
                       Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty)));
  

• ReplaceInstWithInst

  This function replaces a particular instruction 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.

  AllocaInst* instToReplace = ...;
  BasicBlock::iterator ii(instToReplace);
  
  ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii,
                      new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt"));
  

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 doxygen documentation for the Value Class and User Class, respectively, for more information.

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();

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.

We have 2 different layouts in the User (sub)classes:

• 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.

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:

• Layout a) is modelled by prepending the User object by the Use[] array.

  ...---.---.---.---.-------...
    | P | P | P | P | User
  '''---'---'---'---'-------'''
  

• Layout b) is modelled by pointing at the Use[] array.

  .-------...
  | User
  '-------'''
      |
      v
      .---.---.---.---...
      | P | P | P | P |
      '---'---'---'---'''
  

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.

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.

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.)

• bool isIntegerTy() const: Returns true for any integer type.
• bool isFloatingPointTy(): Return true if this is one of the five floating point types.
• bool isSized(): Return true if the type has known size. Things that don't have a size are abstract types, labels and void.

IntegerType

Subclass of DerivedType that represents integer types of any bit width. Any bit width between IntegerType::MIN_INT_BITS (1) and IntegerType::MAX_INT_BITS (~8 million) can be represented.

• static const IntegerType* get(unsigned NumBits): get an integer type of a specific bit width.
• unsigned getBitWidth() const: Get the bit width of an integer type.

SequentialType

This is subclassed by ArrayType, PointerType and VectorType.

• const Type * getElementType() const: Returns the type of each of the elements in the sequential type.

ArrayType

This is a subclass of SequentialType and defines the interface for array types.

• unsigned getNumElements() const: Returns the number of elements in the array.

PointerType

Subclass of SequentialType for pointer types.

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

Subclass of DerivedTypes for function types.

• bool isVarArg() const: Returns true if it's a vararg function
• const Type * getReturnType() const: Returns the return type of the function.
• const Type * getParamType (unsigned i): Returns the type of the ith parameter.
• const unsigned getNumParams() const: Returns the number of formal parameters.

• Module::Module(std::string name = "")

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.
  begin(), end() size(), empty()

  These are forwarding methods that make it easy to access the contents of a Module object's Function list.

• Module::FunctionListType &getFunctionList()

  Returns the list of Functions. This is necessary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

• Module::global_iterator - Typedef for global variable list iterator
  Module::const_global_iterator - Typedef for const_iterator.
  global_begin(), global_end() global_size(), global_empty()

  These are forwarding methods that make it easy to access the contents of a Module object's GlobalVariable list.

• Module::GlobalListType &getGlobalList()

  Returns the list of GlobalVariables. This is necessary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

• SymbolTable *getSymbolTable()

  Return a reference to the SymbolTable for this Module.

• Function *getFunction(const std::string &Name, const FunctionType *Ty)

  Look up the specified function in the Module SymbolTable. If it does not exist, return null.

• Function *getOrInsertFunction(const std::string &Name, const FunctionType *T)

  Look up the specified function in the Module SymbolTable. If it does not exist, add an external declaration for the function and return it.

• std::string getTypeName(const Type *Ty)

  If there is at least one entry in the SymbolTable for the specified Type, return it. Otherwise return the empty string.

• bool addTypeName(const std::string &Name, const Type *Ty)

  Insert an entry in the SymbolTable mapping Name to Ty. If there is already an entry for this name, true is returned and the SymbolTable is not modified.

• Value::use_iterator - Typedef for iterator over the use-list
  Value::const_use_iterator - Typedef for const_iterator over the use-list
  unsigned use_size() - Returns the number of users of the value.
  bool use_empty() - Returns true if there are no users.
  use_iterator use_begin() - Get an iterator to the start of the use-list.
  use_iterator use_end() - Get an iterator to the end of the use-list.
  User *use_back() - Returns the last element in the list.

  These methods are the interface to access the def-use information in LLVM. As with all other iterators in LLVM, the naming conventions follow the conventions defined by the STL.

• Type *getType() const

  This method returns the Type of the Value.

• bool hasName() const
  std::string getName() const
  void setName(const std::string &Name)

  This family of methods is used to access and assign a name to a Value, be aware of the precaution above.

• void replaceAllUsesWith(Value *V)

  This method traverses the use list of a Value changing all Users of the current value to refer to "V" instead. For example, if you detect that an instruction always produces a constant value (for example through constant folding), you can replace all uses of the instruction with the constant like this:

  Inst->replaceAllUsesWith(ConstVal);
  

The User class exposes the operand list in two ways: through an index access interface and through an iterator based interface.

• Value *getOperand(unsigned i)
  unsigned getNumOperands()

  These two methods expose the operands of the User in a convenient form for direct access.

• User::op_iterator - Typedef for iterator over the operand list
  op_iterator op_begin() - Get an iterator to the start of the operand list.
  op_iterator op_end() - Get an iterator to the end of the operand list.

  Together, these methods make up the iterator based interface to the operands of a User.

• BinaryOperator

  This subclasses represents all two operand instructions whose operands must be the same type, except for the comparison instructions.

• CastInst

  This subclass is the parent of the 12 casting instructions. It provides common operations on cast instructions.

• CmpInst

  This subclass respresents the two comparison instructions, ICmpInst (integer opreands), and FCmpInst (floating point operands).

• TerminatorInst

  This subclass is the parent of all terminator instructions (those which can terminate a block).

• BasicBlock *getParent()

  Returns the BasicBlock that this Instruction is embedded into.

• bool mayWriteToMemory()

  Returns true if the instruction writes to memory, i.e. it is a call,free,invoke, or store.

• unsigned getOpcode()

  Returns the opcode for the Instruction.

• Instruction *clone() const

  Returns another instance of the specified instruction, identical in all ways to the original except that the instruction has no parent (ie it's not embedded into a BasicBlock), and it has no name

• ConstantInt : This subclass of Constant represents an integer constant of any width.
  • 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.
• ConstantFP : This class represents a floating point constant.
  • double getValue() const: Returns the underlying value of this constant.
• ConstantArray : This represents a constant array.
  • const std::vector<Use> &getValues() const: Returns a vector of component constants that makeup this array.
• ConstantStruct : This represents a constant struct.
  • const std::vector<Use> &getValues() const: Returns a vector of component constants that makeup this array.
• GlobalValue : This represents either a global variable or a function. In either case, the value is a constant fixed address (after linking).

• bool hasInternalLinkage() const
  bool hasExternalLinkage() const
  void setInternalLinkage(bool HasInternalLinkage)

  These methods manipulate the linkage characteristics of the GlobalValue.

• Module *getParent()

  This returns the Module that the GlobalValue is currently embedded into.

• 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 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 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 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 by linking with a function defined in a different translation unit.

• Function::iterator - Typedef for basic block list iterator
  Function::const_iterator - Typedef for const_iterator.
  begin(), end() size(), empty()

  These are forwarding methods that make it easy to access the contents of a Function object's BasicBlock list.

• Function::BasicBlockListType &getBasicBlockList()

  Returns the list of BasicBlocks. This is necessary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

• Function::arg_iterator - Typedef for the argument list iterator
  Function::const_arg_iterator - Typedef for const_iterator.
  arg_begin(), arg_end() arg_size(), arg_empty()

  These are forwarding methods that make it easy to access the contents of a Function object's Argument list.

• Function::ArgumentListType &getArgumentList()

  Returns the list of Arguments. This is necessary to use when you need to update the list or perform a complex action that doesn't have a forwarding method.

• BasicBlock &getEntryBlock()

  Returns the entry BasicBlock for the function. Because the entry block for the function is always the first block, this returns the first block of the Function.

• Type *getReturnType()
  FunctionType *getFunctionType()

  This traverses the Type of the Function and returns the return type of the function, or the FunctionType of the actual function.

• SymbolTable *getSymbolTable()

  Return a pointer to the SymbolTable for this Function.

• GlobalVariable(const Type *Ty, bool isConstant, LinkageTypes& Linkage, Constant *Initializer = 0, const std::string &Name = "", Module* Parent = 0)

  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, 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 well.

• bool isConstant() const

  Returns true if this is a global variable that is known not to be modified at runtime.

• bool hasInitializer()

  Returns true if this GlobalVariable has an intializer.

• Constant *getInitializer()

  Returns the initial value for a GlobalVariable. It is not legal to call this method if there is no initializer.

• BasicBlock(const std::string &Name = "", Function *Parent = 0)

  The BasicBlock constructor is used to create new basic blocks for insertion into a function. The constructor optionally takes a name for the new block, and a Function to insert it into. If the Parent parameter is specified, the new BasicBlock is automatically inserted at the end of the specified Function, if not specified, the BasicBlock must be manually inserted into the Function.

• BasicBlock::iterator - Typedef for instruction list iterator
  BasicBlock::const_iterator - Typedef for const_iterator.
  begin(), end(), front(), back(), size(), empty() STL-style functions for accessing the instruction list.

  These methods and typedefs are forwarding functions that have the same semantics as the standard library methods of the same names. These methods expose the underlying instruction list of a basic block in a way that is easy to manipulate. To get the full complement of container operations (including operations to update the list), you must use the getInstList() method.

• BasicBlock::InstListType &getInstList()

  This method is used to get access to the underlying container that actually holds the Instructions. This method must be used when there isn't a forwarding function in the BasicBlock class for the operation that you would like to perform. Because there are no forwarding functions for "updating" operations, you need to use this if you want to update the contents of a BasicBlock.

• Function *getParent()

  Returns a pointer to Function the block is embedded into, or a null pointer if it is homeless.

• TerminatorInst *getTerminator()

  Returns a pointer to the terminator instruction that appears at the end of the BasicBlock. If there is no terminator instruction, or if the last instruction in the block is not a terminator, then a null pointer is returned.