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.
+LLVM has a plethora of data structures in the llvm/ADT/ directory, + and we commonly use STL data structures. This section describes the trade-offs + you should consider when you pick one.
+ ++The first step is a choose your own adventure: do you want a sequential +container, a set-like container, or a map-like container? The most important +thing when choosing a container is the algorithmic properties of how you plan to +access the container. Based on that, you should use:
+ ++Once the proper category of container is determined, you can fine tune the +memory use, constant factors, and cache behaviors of access by intelligently +picking a member of the category. Note that constant factors and cache behavior +can be a big deal. If you have a vector that usually only contains a few +elements (but could contain many), for example, it's much better to use +SmallVector than vector +. Doing so avoids (relatively) expensive malloc/free calls, which dwarf the +cost of adding the elements to the container.
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.
- +There are a variety of sequential containers available for you, based on your +needs. Pick the first in this section that will do what you want.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"; -+
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.
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.
+ + +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).
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.
-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:
+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.
--// 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"; --
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.
-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";.
+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 :). +
-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:
+
One worthwhile note about std::vector: avoid code like this:
-#include "llvm/Support/InstIterator.h" - -// F is a ptr to a Function instance -for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) - llvm::cerr << *i << "\n"; +for ( ... ) { + std::vector<foo> V; + use V; +}
Easy, isn't it? You can also use InstIterators to fill a -worklist with its initial contents. For example, if you wanted to -initialize a worklist to contain all instructions in a Function -F, all you would need to do is something like:
+Instead, write this as:
-std::set<Instruction*> worklist;
-worklist.insert(inst_begin(F), inst_end(F));
+std::vector<foo> V;
+for ( ... ) {
+ use V;
+ V.clear();
+}
The STL set worklist would now contain all instructions in the -Function pointed to by F.
+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.
-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; -+
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.
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
+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.
--Instruction* pinst = i; -+
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.
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";
-}
-
+
+
+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.
Other STL containers are available, such as std::string.
-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 pseudocode, this -is what we want to do:
+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.
--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 +- private: - unsigned callCounter; -}; -Set-like containers are useful when you need to canonicalize multiple values +into a single representation. There are several different choices for how to do +this, providing various trade-offs.
- if (callInst->getCalledFunction() == targetFunc) - ++callCounter; - } - } - } - } +
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.
+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.
+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).
-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.
+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.
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:
+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.
--Function* F = ...; +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.
-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; - // ... -} - -++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. +
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.
+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. +
-Instantiating Instructions
+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.
-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:
++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.
--AllocaInst* ai = new AllocaInst(Type::IntTy); -
will create an AllocaInst instance that represents the allocation of -one integer in the current stack frame, at runtime. 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 -runtime. 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:
++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.
--AllocaInst* pa = new AllocaInst(Type::IntTy, 0, "indexLoc"); -
where indexLoc is now the logical name of the instruction's -execution value, which is a pointer to an integer on the runtime stack.
-Inserting instructions
+ + -There are essentially two ways to insert an Instruction -into an existing sequence of instructions that form a BasicBlock:
++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).
-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:
+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.
--BasicBlock *pb = ...; -Instruction *pi = ...; -Instruction *newInst = new Instruction(...); +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.
-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.
++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. +
+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); - +-+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.
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.
- ++- ++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).
-- +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 = .. ; -BasicBlock *BB = I->getParent(); +-+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. +
-BB->getInstList().erase(I); -+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.
--Replacing individual instructions
++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.
-Including "llvm/Transforms/Utils/BasicBlockUtils.h" -permits use of two very useful replace functions: ReplaceInstWithValue -and ReplaceInstWithInst.
+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).
-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); ++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).
-ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii, - Constant::getNullValue(PointerType::get(Type::IntTy))); -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.
-- ReplaceInstWithInst +
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.
-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::IntTy, 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.
- - - -- Advanced Topics -- +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.
--+-This section describes some of the advanced or obscure API's that most clients -do not need to be aware of. These API's tend manage the inner workings of the -LLVM system, and only need to be accessed in unusual circumstances. -
-- +-The 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 bytecode reader, -assembly parser, and linker also have to be aware of the inner workings of this -system. -
+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.
--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, int }"). -Third, a concrete type is a type that is not an abstract type (e.g. "{ int, -float }"). -
+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.
--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: -
+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:
--%mylist = type { %mylist*, int } +// 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";-To build this, use the following LLVM APIs: -
+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.
---// At this point, NewSTy = "{ opaque*, int }". 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()); +-// Create the initial outer struct -PATypeHolder StructTy = OpaqueType::get(); -std::vector<const Type*> Elts; -Elts.push_back(PointerType::get(StructTy)); -Elts.push_back(Type::IntTy); -StructType *NewSTy = StructType::get(Elts); +-// Add a name for the type to the module symbol table (optional) -MyModule->addTypeName("mylist", NewSTy); +- +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";-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. -
+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";.
-+-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*, int}" 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. -
+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" - -- The PATypeHolder Class +// 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"; +--- +-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. -
+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:
--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. -
+++ ++std::set<Instruction*> worklist; +worklist.insert(inst_begin(F), inst_end(F)); ++The STL set worklist would now contain all instructions in the +Function pointed to by F.
--Some data structures need more to perform more complex updates when types get -resolved. The SymbolTable class, for example, needs -move and potentially merge type planes in its representation when a pointer -changes.
+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:
--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 an opaque objects -somewhere) can never be refined. -
++++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,
- -- The SymbolTable class ++-+Instruction* pinst = &*i; +-+ + + + +This class provides a symbol table that the Function and -Module classes use for naming definitions. The symbol table can -provide a name for any Value or Type. SymbolTable is an abstract data -type. It hides the data it contains and provides access to it through a -controlled interface.
- -Note that the symbol table class is 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. -
- -To use the SymbolTable well, you need to understand the -structure of the information it holds. The class contains two -std::map objects. The first, pmap, is a map of -Type* to maps of name (std::string) to Value*. -The second, tmap, is a map of names to Type*. Thus, Values -are stored in two-dimensions and accessed by Type and name. Types, -however, are stored in a single dimension and accessed only by name.
+is semantically equivalent to
-The interface of this class provides three basic types of operations: -
-
+- Accessors. Accessors provide read-only access to information - such as finding a value for a name with the - lookup method.
-- Mutators. Mutators allow the user to add information to the - SymbolTable with methods like - insert.
-- Iterators. Iterators allow the user to traverse the content - of the symbol table in well defined ways, such as the method - type_begin.
-+-+Instruction* pinst = i; ++Accessors
--
+- Value* lookup(const Type* Ty, const std::string& name) const: -
-- The lookup method searches the type plane given by the - Ty parameter for a Value with the provided name. - If a suitable Value is not found, null is returned.
- -- Type* lookupType( const std::string& name) const:
-- The lookupType method searches through the types for a - Type with the provided name. If a suitable Type - is not found, null is returned.
- -- bool hasTypes() const:
-- This function returns true if an entry has been made into the type - map.
- -- bool isEmpty() const:
-- This function returns true if both the value and types maps are - empty
-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"; +} +++ ++ + + + +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)) { + 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; + // ... +} +++ ++ + + + +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.
+ ++ ++ + + + +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::IntTy); ++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::IntTy, 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. 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 = .. ; +BasicBlock *BB = I->getParent(); + +BB->getInstList().erase(I); +++ ++ + + + +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::IntTy))); +- 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::IntTy, 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(); +++ Advanced Topics ++ + +++ + + + ++This section describes some of the advanced or obscure API's that most clients +do not need to be aware of. These API's tend manage the inner workings of the +LLVM system, and only need to be accessed in unusual circumstances. +
++ ++ + + + ++The 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 }"). +
+ ++ ++ + + + ++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::IntTy); +StructType *NewSTy = StructType::get(Elts); + +// At this point, NewSTy = "{ opaque*, i32 }". Tell VMCore that +// the struct and the opaque type are actually the same. +cast<OpaqueType>(StructTy.get())->refineAbstractTypeTo(NewSTy); + +// NewSTy is potentially invalidated, but StructTy (a PATypeHolder) is +// kept up-to-date +NewSTy = cast<StructType>(StructTy.get()); + +// Add a name for the type to the module symbol table (optional) +MyModule->addTypeName("mylist", NewSTy); +++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 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. +
+ +++ + + + ++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. +
+ ++ ++ + + + + ++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 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.
+ +++ + + + +#include "llvm/Type.h" +
+ +
doxygen info: Type ClassThe 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.
+ ++ ++ + + + +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. +
++ ++ + + ++
+- bool isInteger() const: Returns true for any integer type.
+ +- 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).
+ +- bool isSized(): Return true if the type has known size. Things + that don't have a size are abstract types, labels and void.
-Mutators
+--
+- void insert(Value *Val):
-- This method adds the provided value to the symbol table. The Value must - have both a name and a type which are extracted and used to place the value - in the correct type plane under the value's name.
- -- void insert(const std::string& Name, Value *Val):
-- Inserts a constant or type into the symbol table with the specified - name. There can be a many to one mapping between names and constants - or types.
- -- void insert(const std::string& Name, Type *Typ):
-- Inserts a type into the symbol table with the specified name. There - can be a many-to-one mapping between names and types. This method - allows a type with an existing entry in the symbol table to get - a new name.
- -- void remove(Value* Val):
-- This method removes a named value from the symbol table. The - type and name of the Value are extracted from \p N and used to - lookup the Value in the correct type plane. If the Value is - not in the symbol table, this method silently ignores the - request.
- -- void remove(Type* Typ):
-- This method removes a named type from the symbol table. The - name of the type is extracted from \P T and used to look up - the Type in the type map. If the Type is not in the symbol - table, this method silently ignores the request.
- -- Value* remove(const std::string& Name, Value *Val):
-- Remove a constant or type with the specified name from the - symbol table.
- -- Type* remove(const std::string& Name, Type* T):
-- Remove a type with the specified name from the symbol table. - Returns the removed Type.
- -- Value *value_remove(const value_iterator& It):
-- Removes a specific value from the symbol table. - Returns the removed value.
- -- bool strip():
-- This method will strip the symbol table of its names leaving - the type and values.
- -- void clear():
-- Empty the symbol table completely.
+- 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.
Iteration
-The following functions describe three types of iterators you can obtain -the beginning or end of the sequence for both const and non-const. It is -important to keep track of the different kinds of iterators. There are -three idioms worth pointing out:
- --
-- Units Iterator Idiom - -Planes Of name/Value maps PI -- -for (SymbolTable::plane_const_iterator PI = ST.plane_begin(), - PE = ST.plane_end(); PI != PE; ++PI ) { - PI->first // This is the Type* of the plane - PI->second // This is the SymbolTable::ValueMap of name/Value pairs -} -- -All name/Type Pairs TI -- -for (SymbolTable::type_const_iterator TI = ST.type_begin(), - TE = ST.type_end(); TI != TE; ++TI ) { - TI->first // This is the name of the type - TI->second // This is the Type* value associated with the name -} -- -name/Value pairs in a plane VI -- -for (SymbolTable::value_const_iterator VI = ST.value_begin(SomeType), - VE = ST.value_end(SomeType); VI != VE; ++VI ) { - VI->first // This is the name of the Value - VI->second // This is the Value* value associated with the name -} -Using the recommended iterator names and idioms will help you avoid -making mistakes. Of particular note, make sure that whenever you use -value_begin(SomeType) that you always compare the resulting iterator -with value_end(SomeType) not value_end(SomeOtherType) or else you -will loop infinitely.
-+ +
+ The Module class +-- plane_iterator plane_begin():
-- Get an iterator that starts at the beginning of the type planes. - The iterator will iterate over the Type/ValueMap pairs in the - type planes.
+--- plane_const_iterator plane_begin() const:
-- Get a const_iterator that starts at the beginning of the type - planes. The iterator will iterate over the Type/ValueMap pairs - in the type planes.
+#include "llvm/Module.h"
-
doxygen info: +Module Class- plane_iterator plane_end():
-- Get an iterator at the end of the type planes. This serves as - the marker for end of iteration over the type planes.
+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.
-- plane_const_iterator plane_end() const:
-- Get a const_iterator at the end of the type planes. This serves as - the marker for end of iteration over the type planes.
+- value_iterator value_begin(const Type *Typ):
-- Get an iterator that starts at the beginning of a type plane. - The iterator will iterate over the name/value pairs in the type plane. - Note: The type plane must already exist before using this.
+ + -- value_const_iterator value_begin(const Type *Typ) const:
-- Get a const_iterator that starts at the beginning of a type plane. - The iterator will iterate over the name/value pairs in the type plane. - Note: The type plane must already exist before using this.
+-+- value_iterator value_end(const Type *Typ):
-- Get an iterator to the end of a type plane. This serves as the marker - for end of iteration of the type plane. - Note: The type plane must already exist before using this.
++
-- Module::Module(std::string name = "")
+- value_const_iterator value_end(const Type *Typ) const:
-- Get a const_iterator to the end of a type plane. This serves as the - marker for end of iteration of the type plane. - Note: the type plane must already exist before using this.
+Constructing a Module is easy. You can optionally +provide a name for it (probably based on the name of the translation unit).
-- type_iterator type_begin():
-- Get an iterator to the start of the name/Type map.
++
- -- Module::iterator - Typedef for function list iterator
-
+ Module::const_iterator - Typedef for const_iterator.
-- type_const_iterator type_begin() cons:
-- Get a const_iterator to the start of the name/Type map.
+ begin(), end() + size(), empty() -- type_iterator type_end():
-- Get an iterator to the end of the name/Type map. This serves as the - marker for end of iteration of the types.
+These are forwarding methods that make it easy to access the contents of + a Module object's Function + list.
- type_const_iterator type_end() const:
-- Get a const-iterator to the end of the name/Type map. This serves - as the marker for end of iteration of the types.
+- Module::FunctionListType &getFunctionList() -
+- plane_const_iterator find(const Type* Typ ) const:
-- This method returns a plane_const_iterator for iteration over - the type planes starting at a specific plane, given by \p Ty.
+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.
-- plane_iterator find( const Type* Typ :
-- This method returns a plane_iterator for iteration over the - type planes starting at a specific plane, given by \p Ty.
+
++
- 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.
-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.
+- 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.
+- - - - -#include "llvm/Value.h"
+doxygen info: Value Class
-doxygen info: Value ClassThe 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 @@ -1697,11 +2408,11 @@ method. In addition, all LLVM values can be named. The "name" of the
--%foo = add int 1, 2 +%foo = add i32 1, 2The name of this instruction is "foo". NOTE +
The name of this instruction is "foo". NOTE that the name of any value may be missing (an empty string), so names should ONLY be used for debugging (making the source code easier to read, debugging printouts), they should not be used to keep track of values or map @@ -1904,104 +2615,79 @@ this Instruction is embedded into.
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
- -- - - - -#include "llvm/BasicBlock.h"
- -
-doxygen info: BasicBlock -Class
-Superclass: ValueThis 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.
- -Note that BasicBlocks themselves are Values, because they are referenced by instructions -like branches and can go in the switch tables. BasicBlocks have type -label.
- -- --- -
-- 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() +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
+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() +
+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. +
-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.
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). +
The GlobalValue class @@ -2038,11 +2724,11 @@ 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 -int], then the GlobalVariable is a pointer to that array. Although +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 int]. The first element's type -is int. Because of this, accessing a global value requires you to +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. @@ -2142,7 +2828,7 @@ is its address (after linking) which is guaranteed to be constant. create and what type of linkage the function should have. The FunctionType argument specifies the formal arguments and return value for the function. The same - FunctionType value can be used to + FunctionType value can be used to create multiple functions. The Parent argument specifies the Module in which the function is defined. If this argument is provided, the function will automatically be inserted into that module's list of @@ -2158,393 +2844,221 @@ is its address (after linking) which is guaranteed to be constant. 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.
- -- - - - -#include "llvm/GlobalVariable.h" -
- -
-doxygen info: GlobalVariable - Class
-Superclasses: GlobalValue, -Constant, -User, -ValueGlobal 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 constant address). See -GlobalValue for more on this. 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).
-- -- - --
- -- 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, WeakLinkage, or LinkOnceLinkage, then - the resultant global variable will have internal linkage. AppendingLinkage - concatenates together all instances (in different translation units) of the - variable into a single variable but is only applicable to arrays. See - 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 intial value for a GlobalVariable. It is not legal - to call this method if there is no initializer.
- The Module class -- -- -- - - - -#include "llvm/Module.h"
- -
doxygen info: -Module ClassThe 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.
- -- --
- -- 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.
+ size(), empty() -
These are forwarding methods that make it easy to access the contents of + a Function object's BasicBlock + list.
-
+- Function::BasicBlockListType &getBasicBlockList() -
--
+ arg_begin(), arg_end() + arg_size(), arg_empty() -- SymbolTable *getSymbolTable() +
-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.
Return a reference to the SymbolTable - for this Module.
+- 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 Function object's Argument + list.
-
- Function *getFunction(const std::string - &Name, const FunctionType *Ty) +
- Function::ArgumentListType &getArgumentList() -
+Look up the specified function in the Module SymbolTable. If it does not exist, return - null.
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.
-- Function *getOrInsertFunction(const - std::string &Name, const FunctionType *T) +
- BasicBlock &getEntryBlock() -
+Look up the specified function in the Module SymbolTable. If it does not exist, add an - external declaration for the function and return it.
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.
-- std::string getTypeName(const Type *Ty) +
- Type *getReturnType()
+
+ FunctionType *getFunctionType() -If there is at least one entry in the SymbolTable for the specified Type, return it. Otherwise return the empty - string.
This traverses the Type of the + Function and returns the return type of the function, or the FunctionType of the actual + function.
-- bool addTypeName(const std::string &Name, const Type *Ty) +
- SymbolTable *getSymbolTable() -
+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.
Return a pointer to the SymbolTable + for this Function.
-- - -Constant represents a base class for different types of constants. It -is subclassed by ConstantBool, ConstantInt, ConstantArray etc for representing -the various types of Constants.
+#include "llvm/GlobalVariable.h" +
+
+doxygen info: GlobalVariable + Class
+Superclasses: GlobalValue, +Constant, +User, +ValueGlobal 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 constant address). See +GlobalValue for more on this. 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).
Important Subclasses of Constant++-
+- ConstantInt : This subclass of Constant represents an integer constant. -
--
-- int64_t getSExtValue() const: Returns the underlying value of - this constant as a sign extended signed integer value.
-- uint64_t getZExtValue() const: Returns the underlying value - of this constant as a zero extended unsigned integer value.
-- ConstantFP : This class represents a floating point constant. -
--
-- double getValue() const: Returns the underlying value of - this constant.
-- ConstantBool : This represents a boolean constant. -
--
-- bool 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). -
+- 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, WeakLinkage, or LinkOnceLinkage, then + the resultant global variable will have internal linkage. AppendingLinkage + concatenates together all instances (in different translation units) of the + variable into a single variable but is only applicable to arrays. See + 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 intial value for a GlobalVariable. It is not legal + to call this method if there is no initializer.
-Type as noted earlier is also a subclass of a Value class. Any primitive -type (like int, short etc) in LLVM is an instance of Type Class. All other -types are instances of subclasses of type like FunctionType, ArrayType -etc. DerivedType is the interface for all such dervied types including -FunctionType, ArrayType, PointerType, StructType. Types can have names. They can -be recursive (StructType). There exists exactly one instance of any type -structure at a time. This allows using pointer equality of Type *s for comparing -types.
+#include "llvm/BasicBlock.h"
+ +
+doxygen info: BasicBlock +Class
+Superclass: ValueThis 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.
+ +Note that BasicBlocks themselves are Values, because they are referenced by instructions +like branches and can go in the switch tables. BasicBlocks have type +label.
-+-
-- bool isInteger() const: True for any integer type.
-- bool isIntegral() const: Returns true if this is an integral - type, which is either Bool type or one of the Integer types.
+- BasicBlock(const std::string &Name = "", Function *Parent = 0) -
- bool isFloatingPoint(): Return true if this is one of the two - floating point types.
+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.
-- isLosslesslyConvertableTo (const Type *Ty) const: Return true if - this type can be converted to 'Ty' without any reinterpretation of bits. For - example, uint to int or one pointer type to another.
-- 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.
-+-
+- 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 interface for - array types. -
--
-- unsigned getNumElements() const: Returns the number of - elements in the array.
-- PointerType : Subclass of SequentialType for pointer types.
-- 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.
-