LLVM Programmer's Manual

In addition, all LLVM values can be named. The -"name" of the Value is symbolic string printed in the LLVM code:

+ -For example:

+ +

+ <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 +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.

+ +

-  Instruction *I = .. ;
-  BasicBlock *BB = I->getParent();
-  BB->getInstList().erase(I);
-

+ +

+ "llvm/ADT/SetVector.h" +

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. +

- -

Replacing an - `Instruction` with another `Value`

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.

Replacing individual instructions

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

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 AllocaInst that allocates memory for a single -integer with an null pointer to an integer.
+ +
+ "llvm/ADT/UniqueVector.h" +
-
```
-AllocaInst* instToReplace = ...;
-BasicBlock::iterator ii(instToReplace);
-ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii,
-                     Constant::getNullValue(PointerType::get(Type::IntTy)));
-
```
+
-
ReplaceInstWithInst +
+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.
-
This function replaces a particular instruction with another -instruction. The following example illustrates the replacement of one -AllocaInst with another.
+
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.
-
```
-AllocaInst* instToReplace = ...;
-BasicBlock::iterator ii(instToReplace);
-ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii,
-                    new AllocaInst(Type::IntTy, 0, "ptrToReplacedInt"));
-
```
+ -

Replacing multiple uses of Users and - Values

Value::replaceAllUsesWith

User::replaceUsesOfWith

Value Class

User Class

+ Other Set-Like Container Options +

-The Core LLVM Class Hierarchy Reference -

-The Core LLVM classes are the primary means of representing the program being -inspected or transformed. The core LLVM classes are defined in header files in -the include/llvm/ directory, and implemented in the lib/VMCore -directory.

+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).

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 Value class -

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.

#include "llvm/Value.h"

Value Class

+ + +

+ Map-Like Containers (std::map, DenseMap, etc) +

Value

Value

+Map-like containers are useful when you want to associate data to a key. As +usual, there are a lot of different ways to do this. :) +

Value

Argument

Value

User

Value

use_

+ +

+ A sorted 'vector' +

Value

Type

getType()

-   %foo = add int 1, 2
-

+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. +

NOTE

ONLY

std::map

Value

+ +

+ "llvm/ADT/StringMap.h" +

+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 lookup 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.

Important Public Members of -the `Value` class

- -

Value::use_iterator - Typedef for iterator over the use-list
- Value::use_const_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.
+
+ "llvm/ADT/IndexedMap.h" +
-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.
+
+
+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).
+ +
+ + +
+ "llvm/ADT/DenseMap.h" +
+ +
+ +
+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.
+ +
+ + +
+ <map> +
+ +
+ +
+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).
+ +
+ + +
+ 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).
+ +
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 fixed 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). +
+
+ + +
+ 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). +
+
+ + +
+ Helpful Hints for Common Operations +
+ + +
+ +
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 +practical side of LLVM transformations.
Because this is a "how-to" section, +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.
+ +
+ + + +
+ Basic Inspection and Traversal Routines +
+ +
+ +
The LLVM compiler infrastructure have many different data structures that may +be traversed. Following the example of the C++ standard template library, the +techniques used to traverse these various data structures are all basically the +same. For a enumerable sequence of values, the XXXbegin() function (or +method) returns an iterator to the start of the sequence, the XXXend() +function returns an iterator pointing to one past the last valid element of the +sequence, and there is some XXXiterator data type that is common +between the two operations.
+ +
Because the pattern for iteration is common across many different aspects of +the program representation, the standard template library algorithms may be used +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 +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
+  llvm::cerr << "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.
+ +
+ + +
+ 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 +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&
+   llvm::cerr << *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";.
+ +
+ + +
+ 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, +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)
+  llvm::cerr << *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.
+ +
+ + +
+ Turning an iterator into a class pointer (and + vice-versa) +
+ +
+ +
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()) llvm::cerr << *it << "\n";
+}
+
```
+
+ +
+ + +
+ Finding call sites: a slightly more complex + example +
+ +
+ +
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;
+};
+
```
+
+ +
+ + +
+ Treating calls and invokes the same way +
+ +
+ +
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.
+ +
+ + +
+ Iterating over def-use & use-def chains +
+ +
+ +
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)) {
+    llvm::cerr << "F is used in instruction:\n";
+    llvm::cerr << *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):
+ +
+
```
+Instruction *pi = ...;
+
+for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
+  Value *v = *i;
+  // ...
+}
+
```
+
+ + + +
+ + +
+ 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:
+ +
+
```
+#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.
+ +
+ + + +
+ Making simple changes +
+ +
+ +
There are some primitive transformation operations present in the LLVM +infrastructure that are worth knowing about. When performing +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.
+ +
+ + +
+ Creating and inserting new + Instructions +
+ +
+ +
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 Instructions +
+ +
+ +
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:
+ +
+
```
+Instruction *I = .. ;
+I->eraseFromParent();
+
```
+
+ +
+ + +
+ Replacing an Instruction with another + Value +
+ +
+ +
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 (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 + 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::get(Type::Int32Ty)));
+
```
- ReplaceInstWithInst + +
  This function replaces a particular instruction with another + 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 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();
+
```
+
+ +
+ + +
+ 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. +
+
+ + +
+ LLVM Type Resolution +
+ +
+ +
+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. +
+ +
+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 bitcode reader, +assembly parser, and linker also have to be aware of the inner workings of this +system. +
+ +
+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 }"). +
+ +
+ + +
+ Basic Recursive Type Construction +
+ +
+ +
+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: +
+ +
+
```
+%mylist = type { %mylist*, i32 }
+
```
+
+ +
+To build this, use the following LLVM APIs: +
+ +
+
```
+// Create the initial outer struct
+PATypeHolder StructTy = OpaqueType::get();
+std::vector<const Type*> Elts;
+Elts.push_back(PointerType::get(StructTy));
+Elts.push_back(Type::Int32Ty);
+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);
+
```
+
+ +
+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. +
+ +
+ + +
+ The refineAbstractTypeTo method +
+ +
+
+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). +
+ +
+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. +
+ +
+ + +
+ The PATypeHolder Class +
+ +
+
+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. +
+ +
+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. +
+ +
+ + +
+ The AbstractTypeUser Class +
+ +
+ +
+Some data structures need more to perform more complex updates when types get +resolved. 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. +
+
+ + + +
+ The ValueSymbolTable and + TypeSymbolTable classes +
+ +
+
The +ValueSymbolTable class provides a symbol table that the Function and +Module classes use for naming value definitions. The symbol table +can provide a name for any Value. +The +TypeSymbolTable class is used by the Module class to store +names for types.
+ +
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 +an empty name) do not exist in the symbol table. +
+ +
These symbol tables support iteration over the values/types 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. For types, use the Module::addTypeName method to +insert entries into the symbol table.
+ +
+ + + + +
+ The User and owned Use classes' memory layout +
+ +
+
The +User class provides a base 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.
+ + +
+ Interaction and relationship between User and Use objects +
+ +
+
+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.
  +
+
+Initially each layout will possess 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 will also store 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) +will enforce the following memory layouts:
+ +
- Layout a) will be modelled by prepending the User object by the Use[] array.
  + +
```
+...---.---.---.---.-------...
+  | P | P | P | P | User
+'''---'---'---'---'-------'''
+
```
  + +
- Layout b) will be modelled by pointing at the Use[] array.
  +
```
+.-------...
+| User
+'-------'''
+    |
+    v
+    .---.---.---.---...
+    | P | P | P | P |
+    '---'---'---'---'''
+
```
  +
+(In the above figures 'P' stands for the Use** that + is stored in each Use object in the member Use::Prev) + + +
+ The waymarking algorithm +
+ +
+
+Since the Use objects will be deprived of the direct pointer to +their User objects, there must be a fast and exact method to +recover it. This is accomplished by the following scheme:
+
+ +A bit-encoding in the 2 LSBits (least significant bits) of the Use::Prev will allow to find the +start of the User object: +
- 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 +
-
Type *getType() const
-This method returns the Type of the Value. +
+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 will find a pointer (User* with LSBit set). +Following this pointer brings us to the User. A portable trick will ensure +that the first bytes of User (if interpreted as a pointer) will never have +the LSBit set.
-
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.
+ +
+ The Core LLVM Class Hierarchy Reference +
+ +
+
#include "llvm/Type.h" +
doxygen info: Type Class
-
void replaceAllUsesWith(Value *V)
+
The Core LLVM classes are the primary means of representing the program +being inspected or transformed. The core LLVM classes are defined in +header files in the include/llvm/ directory, and implemented in +the lib/VMCore directory.
-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 Type class and Derived Types +
+ +
+ +
Type is a superclass of all type classes. Every Value has + a Type. Type cannot be instantiated directly but only + through its subclasses. Certain primitive types (VoidType, + LabelType, FloatType and DoubleType) have hidden + subclasses. They are hidden because they offer no useful functionality beyond + what the Type class offers except to distinguish themselves from + other subclasses of Type.
+
All other types are subclasses of DerivedType. Types can be + named, but this is not a requirement. There exists exactly + one instance of a given shape at any one time. This allows type equality to + 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 +
+
- -

- -The User class -

bool isInteger() const: Returns true for any integer type.

#include "llvm/User.h"

User Class

bool isFloatingPoint(): Return true if this is one of the two + floating point types.
bool isAbstract(): Return true if the type is abstract (contains + an OpaqueType anywhere in its definition).

User

User

Value

bool isSized(): Return true if the type has known size. Things + that don't have a size are abstract types, labels and void.

User

+ Important Derived Types +

Important Public Members of -the `User` class

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 and PointerType +

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 wherease 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 its 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.

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.

User

Value *getOperand(unsigned i)
- unsigned getNumOperands()
-These two methods expose the operands of the User in a convenient form -for direct access.
+ +
+ The Module class +
-
User::op_iterator - Typedef for iterator over the operand list
- User::op_const_iterator - use_iterator op_begin() - - Get an iterator to the start of the operand list.
- use_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.
+
#include "llvm/Module.h"
doxygen info: +Module Class
+
The Module class represents the top level structure present in LLVM +programs. An LLVM module is effectively either a translation unit of the +original program or a combination of several translation units merged by the +linker. The Module class keeps track of a list of Functions, a list of GlobalVariables, and a SymbolTable. Additionally, it contains a few +helpful member functions that try to make common operations easy.
+
- -

- -The Instruction class -

#include "llvm/Instruction.h"

Instruction Class

User

+ +

+ Important Public Members of the Module class +

Instruction

Instruction

-Because the Instruction class subclasses the User class, its operands can be accessed in the same -way as for other Users (with the -getOperand()/getNumOperands() and -op_begin()/op_end() methods).

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

-An important file for the Instruction class is the -llvm/Instruction.def file. This file contains some meta-data about the -various different types of instructions in LLVM. It describes the enum values -that are used as opcodes (for example Instruction::Add and -Instruction::SetLE), as well as the concrete sub-classes of -Instruction that implement the instruction (for example BinaryOperator and SetCondInst). Unfortunately, the use of macros in -this file confused doxygen, so these enum values don't show up correctly in the -doxygen output.

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.
- -

Important Public Members of -the `Instruction` class

+ begin(), end() + size(), empty() -

BasicBlock *getParent()
+
These are forwarding methods that make it easy to access the contents of + a Module object's Function + list.

Instruction

Module::FunctionListType &getFunctionList() -
bool mayWriteToMemory()
+
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.
-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.

Module::global_iterator - Typedef for global variable list iterator
-
Instruction *clone() const
+ Module::const_global_iterator - Typedef for const_iterator.
-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.
+ 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() -

---> +

SymbolTable *getSymbolTable() - -

- -The BasicBlock class -

#include "llvm/BasicBlock.h"

BasicBlock Class

- - -This class represents a single entry multiple 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. Matching the language definition, the last element of -this list of instructions is always a terminator instruction (a subclass of the -TerminatorInst class).

- -In addition to tracking the list of instructions that make up the block, the -BasicBlock class also keeps track of the Function that it is embedded into.

Return a reference to the SymbolTable + for this Module.

BasicBlock

BasicBlock

label

- -

Important Public Members of -the `BasicBlock` class

Function *getFunction(const std::string + &Name, const FunctionType *Ty) -
BasicBlock(const std::string &Name = "", Function *Parent = 0)
+
Look up the specified function in the Module SymbolTable. If it does not exist, return + null.

BasicBlock

Parent

BasicBlock

Function *getOrInsertFunction(const + std::string &Name, const FunctionType *T) -
BasicBlock::iterator - Typedef for instruction list iterator
- BasicBlock::const_iterator - Typedef for const_iterator.
- begin(), end(), front(), back(), - size(), empty(), rbegin(), rend()
+
Look up the specified function in the Module SymbolTable. If it does not exist, add an + external declaration for the function and return it.

getInstList()

std::string getTypeName(const Type *Ty) -
BasicBlock::InstListType &getInstList()
+
If there is at least one entry in the SymbolTable for the specified Type, return it. Otherwise return the empty + string.

BasicBlock

bool addTypeName(const std::string &Name, const Type *Ty) -
Function *getParent()
+
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.

+ -

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.
+ +
+ The Value class +
+
- -

- -The GlobalValue class -

#include "llvm/GlobalValue.h"

GlobalValue Class

User

#include "llvm/Value.h" +
+doxygen info: Value Class

GlobalVariable

GlobalValue

The Value class is the most important class in the LLVM Source +base. It represents a typed value that may be used (among other things) as an +operand to an instruction. There are many different types of Values, +such as Constants,Arguments. Even Instructions and Functions are Values.

GlobalValue

static

GlobalValue

Module

A particular Value may be used many times in the LLVM representation +for a program. For example, an incoming argument to a function (represented +with an instance of the Argument class) is "used" by +every instruction in the function that references the argument. To keep track +of this relationship, the Value class keeps a list of all of the Users that is using it (the User class is a base class for all nodes in the LLVM +graph that can refer to Values). This use list is how LLVM represents +def-use information in the program, and is accessible through the use_* +methods, shown below.

GlobalValue

Type

Because LLVM is a typed representation, every LLVM Value is typed, +and this Type is available through the getType() +method. In addition, all LLVM values can be named. The "name" of the +Value is a symbolic string printed in the LLVM code:

+%foo = add i32 1, 2
+

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 +between them. For this purpose, use a std::map of pointers to the +Value itself instead.

Important Public Members of -the `GlobalValue` class

One important aspect of LLVM is that there is no distinction between an SSA +variable and the operation that produces it. Because of this, any reference to +the value produced by an instruction (or the value available as an incoming +argument, for example) is represented as a direct pointer to the instance of +the class that +represents this value. Although this may take some getting used to, it +simplifies the representation and makes it easier to manipulate.

bool hasInternalLinkage() const
- bool hasExternalLinkage() const
- void setInternalLinkage(bool HasInternalLinkage)
+ -These methods manipulate the linkage characteristics of the -GlobalValue.
+ +
+ Important Public Members of the Value class +
-
Module *getParent()
+
-This returns the Module that the GlobalValue is -currently embedded into.
+
- Value::use_iterator - Typedef for iterator over the +use-list
  + Value::use_const_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 Function class -

#include "llvm/Function.h"

Function Class

GlobalValue

User

- -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 -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 SymbolTable.

- -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 explicitly branch to this initial -block. There are no implicit exit nodes, and in fact there may be multiple exit -nodes from a single Function. If the BasicBlock list is empty, this indicates that -the Function is actually a function declaration: the actual body of the -function hasn't been linked in yet.

+ The User class +

Function

Argument

+ +

+#include "llvm/User.h"
+doxygen info: User Class
+Superclass: Value

-The SymbolTable is a very rarely used LLVM -feature that is only used when you have to look up a value by name. Aside from -that, the SymbolTable is used internally to -make sure that there are not conflicts between the names of Instructions, BasicBlocks, or Arguments in the function body.

The User class is the common base class of all LLVM nodes that may +refer to Values. It exposes a list of "Operands" +that are all of the Values that the User is +referring to. The User class itself is a subclass of +Value.

+ +

The operands of a User point directly to the LLVM Value that it refers to. Because LLVM uses Static +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.

Important Public Members of -the `Function` class

- -

Function(const FunctionType *Ty, bool isInternal, const std::string &N = "")
+
+ Important Public Members of the User class +
-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 -whether or not it should start out with internal or external linkage.
+
-
bool isExternal()
+
The User class exposes the operand list in two ways: through +an index access interface and through an iterator based interface.
-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.
+
- 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.
+ -
Function::iterator - Typedef for basic block list iterator
- Function::const_iterator - Typedef for const_iterator.
- begin(), end(), front(), back(), - size(), empty(), rbegin(), rend()
+ +
+ The Instruction class +
-These are forwarding methods that make it easy to access the contents of a -Function object's BasicBlock -list.
+
-
Function::BasicBlockListType &getBasicBlockList()
+
#include "llvm/Instruction.h"
+doxygen info: Instruction Class
+Superclasses: User, Value
-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.
+
The Instruction class is the common base class for all LLVM +instructions. It provides only a few methods, but is a very commonly used +class. The primary data tracked by the Instruction class itself is the +opcode (instruction type) and the parent BasicBlock the Instruction is embedded +into. To represent a specific type of instruction, one of many subclasses of +Instruction are used.
+
Because the Instruction class subclasses the User class, its operands can be accessed in the same +way as for other Users (with the +getOperand()/getNumOperands() and +op_begin()/op_end() methods).

An important file for +the Instruction class is the llvm/Instruction.def file. This +file contains some meta-data about the various different types of instructions +in LLVM. It describes the enum values that are used as opcodes (for example +Instruction::Add and Instruction::ICmp), as well as the +concrete sub-classes of Instruction that implement the instruction (for +example BinaryOperator and CmpInst). Unfortunately, the use of macros in +this file confuses doxygen, so these enum values don't show up correctly in the +doxygen output.
+ + -
Function::aiterator - Typedef for the argument list iterator
- Function::const_aiterator - Typedef for const_iterator.
- abegin(), aend(), afront(), aback(), - asize(), aempty(), arbegin(), arend()
+ +
+ Important Subclasses of the Instruction + class +
+
+
- 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).
  +
+
-These are forwarding methods that make it easy to access the contents of a -Function object's Argument list.
+ +
+ Important Public Members of the Instruction + class +
-
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 *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
+
+ +
+ The Constant class and subclasses +
-
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.
+
Constant represents a base class for different types of constants. It +is subclassed by ConstantInt, ConstantArray, etc. for representing +the various types of Constants. GlobalValue is also +a subclass, which represents the address of a global variable or 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.
+ +
Important Subclasses of Constant
+
+
- 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). +
+
-
SymbolTable *getSymbolTable()
-Return a pointer to the SymbolTable for this -Function.
+ +
+ The GlobalValue class +
+
+
#include "llvm/GlobalValue.h"
+doxygen info: GlobalValue +Class
+Superclasses: Constant, +User, Value
- -

- -The GlobalVariable class -

#include "llvm/GlobalVariable.h"

GlobalVariable Class

GlobalValue

User

+ Important Public Members of the GlobalValue + class +

- -Global variables are represented with the (suprise suprise) -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 "name" refers to their address). Global 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).

Global values (GlobalVariables or Functions) are the only LLVM values that are +visible in the bodies of all Functions. +Because they are visible at global scope, they are also subject to linking with +other globals defined in different translation units. To control the linking +process, GlobalValues know their linkage rules. Specifically, +GlobalValues know whether they have internal or external linkage, as +defined by the LinkageTypes enumeration.

If a GlobalValue has internal linkage (equivalent to being +static in C), it is not visible to code outside the current translation +unit, and does not participate in linking. If it has external linkage, it is +visible to external code, and does participate in linking. In addition to +linkage information, GlobalValues keep track of which Module they are currently part of.

Because GlobalValues are memory objects, they are always referred to +by their address. As such, the Type of a +global is always a pointer to its contents. It is important to remember this +when using the GetElementPtrInst instruction because this pointer must +be dereferenced first. For example, if you have a GlobalVariable (a +subclass of GlobalValue) that is an array of 24 ints, type [24 x +i32], then the GlobalVariable is a pointer to that array. Although +the address of the first element of this array and the value of the +GlobalVariable are the same, they have different types. The +GlobalVariable's type is [24 x i32]. The first element's type +is i32. Because of this, accessing a global value requires you to +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 -`GlobalVariable` class

GlobalVariable(const Type *Ty, bool isConstant, bool -isInternal, Constant *Initializer = 0, const std::string -&Name = "")
+
-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, and -if isInternal is true the resultant global variable will have internal -linkage. Optionally an initializer and name may be specified for the global variable as well.
+
- 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.
+
-
bool isConstant() const
+ +
+ The Function class +
-Returns true if this is a global variable is known not to be modified at -runtime.
+
+
#include "llvm/Function.h"
doxygen +info: Function Class
+Superclasses: GlobalValue, +Constant, +User, +Value
-
bool hasInitializer()
+
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 +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 +SymbolTable.
+ +
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 +nodes from a single Function. If the BasicBlock list is empty, this indicates that +the Function is actually a function declaration: the actual body of the +function hasn't been linked in yet.
-Returns true if this GlobalVariable has an intializer.
+
In addition to a list of BasicBlocks, the +Function class also keeps track of the list of formal Arguments that the function receives. This +container manages the lifetime of the Argument +nodes, just like the BasicBlock list does for +the BasicBlocks.
+
The SymbolTable is a very rarely used +LLVM feature that is only used when you have to look up a value by name. Aside +from that, the SymbolTable is used +internally to make sure that there are not conflicts between the names of Instructions, BasicBlocks, or Arguments in the function body.
-
Constant *getInitializer()
+
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.
+ -Returns the intial value for a GlobalVariable. It is not legal to call -this method if there is no initializer.
+ +
+ Important Public Members of the Function + class +
+
- -

- -The Module 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 + 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 isExternal() + +
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.
-#include "llvm/Module.h"
-doxygen info: Module Class
+ begin(), end() + size(), empty() -The Module class represents the top level structure present in LLVM -programs. An LLVM module is effectively either a translation unit of the -original program or a combination of several translation units merged by the -linker. The Module class keeps track of a list of Functions, a list of GlobalVariables, and a SymbolTable. Additionally, it contains a few -helpful member functions that try to make common operations easy.
+
These are forwarding methods that make it easy to access the contents of + a Function object's BasicBlock + list.
Function::BasicBlockListType &getBasicBlockList() - -

Important Public Members of the -`Module` class

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.

Module::iterator - Typedef for function list iterator
- Module::const_iterator - Typedef for const_iterator.
- begin(), end(), front(), back(), - size(), empty(), rbegin(), rend()
+
Function::arg_iterator - Typedef for the argument list +iterator
+ Function::const_arg_iterator - Typedef for const_iterator.
-These are forwarding methods that make it easy to access the contents of a -Module object's Function -list.
+ arg_begin(), arg_end() + arg_size(), arg_empty() -
Module::FunctionListType &getFunctionList()
+
These are forwarding methods that make it easy to access the contents of + a Function object's Argument + list.