| LLVM Programmer's Manual | -
This document is meant to highlight some of the important classes and +interfaces available in the LLVM source-base. This manual is not +intended to explain what LLVM is, how it works, and what LLVM code looks +like. It assumes that you know the basics of LLVM and are interested +in writing transformations or otherwise analyzing or manipulating the +code.
+ +This document should get you oriented so that you can find your +way in the continuously growing source code that makes up the LLVM +infrastructure. Note that this manual is not intended to serve as a +replacement for reading the source code, so if you think there should be +a method in one of these classes to do something, but it's not listed, +check the source. Links to the doxygen sources +are provided to make this as easy as possible.
+ +The first section of this document describes general information that is +useful to know when working in the LLVM infrastructure, and the second describes +the Core LLVM classes. In the future this manual will be extended with +information describing how to use extension libraries, such as dominator +information, CFG traversal routines, and useful utilities like the InstVisitor template.
+ +This section contains general information that is useful if you are working +in the LLVM source-base, but that isn't specific to any particular API.
+ +LLVM makes heavy use of the C++ Standard Template Library (STL), +perhaps much more than you are used to, or have seen before. Because of +this, you might want to do a little background reading in the +techniques used and capabilities of the library. There are many good +pages that discuss the STL, and several books on the subject that you +can get, so it will not be discussed in this document.
+ +Here are some useful links:
+ +Written by Dinakar Dhurjati - Chris Lattner, and - Joel Stanley
You are also encouraged to take a look at the LLVM Coding Standards guide which focuses on how +to write maintainable code more than where to put your curly braces.
+ +Here we highlight some LLVM APIs that are generally useful and good to +know about when writing transformations.
+ +The LLVM source-base makes extensive use of a custom form of RTTI. +These templates have many similarities to the C++ dynamic_cast<> +operator, but they don't have some drawbacks (primarily stemming from +the fact that dynamic_cast<> only works on classes that +have a v-table). Because they are used so often, you must know what they +do and how they work. All of these templates are defined in the llvm/Support/Casting.h +file (note that you very rarely have to include this file directly).
+ +The isa<> operator works exactly like the Java + "instanceof" operator. It returns true or false depending on whether + a reference or pointer points to an instance of the specified class. This can + be very useful for constraint checking of various sorts (example below).
+The cast<> operator is a "checked cast" operation. It + converts a pointer or reference from a base class to a derived cast, causing + an assertion failure if it is not really an instance of the right type. This + should be used in cases where you have some information that makes you believe + that something is of the right type. An example of the isa<> + and cast<> template is:
+ ++static bool isLoopInvariant(const Value *V, const Loop *L) { + if (isa<Constant>(V) || isa<Argument>(V) || isa<GlobalValue>(V)) + return true; + + // Otherwise, it must be an instruction... + return !L->contains(cast<Instruction>(V)->getParent()); +} ++
Note that you should not use an isa<> test followed + by a cast<>, for that use the dyn_cast<> + operator.
+ +The dyn_cast<> operator is a "checking cast" operation. + It checks to see if the operand is of the specified type, and if so, returns a + pointer to it (this operator does not work with references). If the operand is + not of the correct type, a null pointer is returned. Thus, this works very + much like the dynamic_cast<> operator in C++, and should be + used in the same circumstances. Typically, the dyn_cast<> + operator is used in an if statement or some other flow control + statement like this:
+ ++if (AllocationInst *AI = dyn_cast<AllocationInst>(Val)) { + // ... +} ++
This form of the if statement effectively combines together a call + to isa<> and a call to cast<> into one + statement, which is very convenient.
+ +Note that the dyn_cast<> operator, like C++'s + dynamic_cast<> or Java's instanceof operator, can be + abused. In particular, you should not use big chained if/then/else + blocks to check for lots of different variants of classes. If you find + yourself wanting to do this, it is much cleaner and more efficient to use the + InstVisitor class to dispatch over the instruction type directly.
+ +The cast_or_null<> operator works just like the + cast<> operator, except that it allows for a null pointer as an + argument (which it then propagates). This can sometimes be useful, allowing + you to combine several null checks into one.
The dyn_cast_or_null<> operator works just like the + dyn_cast<> operator, except that it allows for a null pointer + as an argument (which it then propagates). This can sometimes be useful, + allowing you to combine several null checks into one.
These five templates can be used with any classes, whether they have a +v-table or not. To add support for these templates, you simply need to add +classof static methods to the class you are interested casting +to. Describing this is currently outside the scope of this document, but there +are lots of examples in the LLVM source base.
+ +Often when working on your pass you will put a bunch of debugging printouts +and other code into your pass. After you get it working, you want to remove +it, but you may need it again in the future (to work out new bugs that you run +across).
+ +Naturally, because of this, you don't want to delete the debug printouts, +but you don't want them to always be noisy. A standard compromise is to comment +them out, allowing you to enable them if you need them in the future.
+ +The "llvm/Support/Debug.h" +file provides a macro named DEBUG() that is a much nicer solution to +this problem. Basically, you can put arbitrary code into the argument of the +DEBUG macro, and it is only executed if 'opt' (or any other +tool) is run with the '-debug' command line argument:
+ ++DOUT << "I am here!\n"; ++
Then you can run your pass like this:
+ ++$ opt < a.bc > /dev/null -mypass +<no output> +$ opt < a.bc > /dev/null -mypass -debug +I am here! ++
Using the DEBUG() macro instead of a home-brewed solution allows you +to not have to create "yet another" command line option for the debug output for +your pass. Note that DEBUG() macros are disabled for optimized builds, +so they do not cause a performance impact at all (for the same reason, they +should also not contain side-effects!).
+ +One additional nice thing about the DEBUG() macro is that you can +enable or disable it directly in gdb. Just use "set DebugFlag=0" or +"set DebugFlag=1" from the gdb if the program is running. If the +program hasn't been started yet, you can always just run it with +-debug.
+ +Sometimes you may find yourself in a situation where enabling -debug +just turns on too much information (such as when working on the code +generator). If you want to enable debug information with more fine-grained +control, you define the DEBUG_TYPE macro and the -debug only +option as follows:
+ ++DOUT << "No debug type\n"; +#undef DEBUG_TYPE +#define DEBUG_TYPE "foo" +DOUT << "'foo' debug type\n"; +#undef DEBUG_TYPE +#define DEBUG_TYPE "bar" +DOUT << "'bar' debug type\n"; +#undef DEBUG_TYPE +#define DEBUG_TYPE "" +DOUT << "No debug type (2)\n"; ++
Then you can run your pass like this:
+ ++$ opt < a.bc > /dev/null -mypass +<no output> +$ opt < a.bc > /dev/null -mypass -debug +No debug type +'foo' debug type +'bar' debug type +No debug type (2) +$ opt < a.bc > /dev/null -mypass -debug-only=foo +'foo' debug type +$ opt < a.bc > /dev/null -mypass -debug-only=bar +'bar' debug type ++
Of course, in practice, you should only set DEBUG_TYPE at the top of +a file, to specify the debug type for the entire module (if you do this before +you #include "llvm/Support/Debug.h", you don't have to insert the ugly +#undef's). Also, you should use names more meaningful than "foo" and +"bar", because there is no system in place to ensure that names do not +conflict. If two different modules use the same string, they will all be turned +on when the name is specified. This allows, for example, all debug information +for instruction scheduling to be enabled with -debug-type=InstrSched, +even if the source lives in multiple files.
+ +The "llvm/ADT/Statistic.h" file +provides a class named Statistic that is used as a unified way to +keep track of what the LLVM compiler is doing and how effective various +optimizations are. It is useful to see what optimizations are contributing to +making a particular program run faster.
+ +Often you may run your pass on some big program, and you're interested to see +how many times it makes a certain transformation. Although you can do this with +hand inspection, or some ad-hoc method, this is a real pain and not very useful +for big programs. Using the Statistic class makes it very easy to +keep track of this information, and the calculated information is presented in a +uniform manner with the rest of the passes being executed.
+ +There are many examples of Statistic uses, but the basics of using +it are as follows:
+ +Define your statistic like this:
+ ++#define DEBUG_TYPE "mypassname" // This goes before any #includes. +STATISTIC(NumXForms, "The # of times I did stuff"); ++
The STATISTIC macro defines a static variable, whose name is + specified by the first argument. The pass name is taken from the DEBUG_TYPE + macro, and the description is taken from the second argument. The variable + defined ("NumXForms" in this case) acts like an unsigned integer.
Whenever you make a transformation, bump the counter:
+ ++++NumXForms; // I did stuff! ++
That's all you have to do. To get 'opt' to print out the + statistics gathered, use the '-stats' option:
+ ++$ opt -stats -mypassname < program.bc > /dev/null +... statistics output ... ++
When running opt on a C file from the SPEC benchmark +suite, it gives a report that looks like this:
+ ++ 7646 bytecodewriter - Number of normal instructions + 725 bytecodewriter - Number of oversized instructions + 129996 bytecodewriter - Number of bytecode bytes written + 2817 raise - Number of insts DCEd or constprop'd + 3213 raise - Number of cast-of-self removed + 5046 raise - Number of expression trees converted + 75 raise - Number of other getelementptr's formed + 138 raise - Number of load/store peepholes + 42 deadtypeelim - Number of unused typenames removed from symtab + 392 funcresolve - Number of varargs functions resolved + 27 globaldce - Number of global variables removed + 2 adce - Number of basic blocks removed + 134 cee - Number of branches revectored + 49 cee - Number of setcc instruction eliminated + 532 gcse - Number of loads removed + 2919 gcse - Number of instructions removed + 86 indvars - Number of canonical indvars added + 87 indvars - Number of aux indvars removed + 25 instcombine - Number of dead inst eliminate + 434 instcombine - Number of insts combined + 248 licm - Number of load insts hoisted + 1298 licm - Number of insts hoisted to a loop pre-header + 3 licm - Number of insts hoisted to multiple loop preds (bad, no loop pre-header) + 75 mem2reg - Number of alloca's promoted + 1444 cfgsimplify - Number of blocks simplified ++
Obviously, with so many optimizations, having a unified framework for this +stuff is very nice. Making your pass fit well into the framework makes it more +maintainable and useful.
+ +Several of the important data structures in LLVM are graphs: for example +CFGs made out of LLVM BasicBlocks, CFGs made out of +LLVM MachineBasicBlocks, and +Instruction Selection +DAGs. In many cases, while debugging various parts of the compiler, it is +nice to instantly visualize these graphs.
+ +LLVM provides several callbacks that are available in a debug build to do +exactly that. If you call the Function::viewCFG() method, for example, +the current LLVM tool will pop up a window containing the CFG for the function +where each basic block is a node in the graph, and each node contains the +instructions in the block. Similarly, there also exists +Function::viewCFGOnly() (does not include the instructions), the +MachineFunction::viewCFG() and MachineFunction::viewCFGOnly(), +and the SelectionDAG::viewGraph() methods. Within GDB, for example, +you can usually use something like call DAG.viewGraph() to pop +up a window. Alternatively, you can sprinkle calls to these functions in your +code in places you want to debug.
+ +Getting this to work requires a small amount of configuration. On Unix +systems with X11, install the graphviz +toolkit, and make sure 'dot' and 'gv' are in your path. If you are running on +Mac OS/X, download and install the Mac OS/X Graphviz program, and add +/Applications/Graphviz.app/Contents/MacOS/ (or wherever you install +it) to your path. Once in your system and path are set up, rerun the LLVM +configure script and rebuild LLVM to enable this functionality.
+ +SelectionDAG has been extended to make it easier to locate +interesting nodes in large complex graphs. From gdb, if you +call DAG.setGraphColor(node, "color"), then the +next call DAG.viewGraph() would highlight the node in the +specified color (choices of colors can be found at colors.) More +complex node attributes can be provided with call +DAG.setGraphAttrs(node, "attributes") (choices can be +found at Graph +Attributes.) If you want to restart and clear all the current graph +attributes, then you can call DAG.clearGraphAttrs().
+ +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.
+ +Fixed size arrays are very simple and very fast. They are good if you know +exactly how many elements you have, or you have a (low) upper bound on how many +you have.
+Heap allocated arrays (new[] + delete[]) are also simple. They are good if +the number of elements is variable, if you know how many elements you will need +before the array is allocated, and if the array is usually large (if not, +consider a SmallVector). The cost of a heap +allocated array is the cost of the new/delete (aka malloc/free). Also note that +if you are allocating an array of a type with a constructor, the constructor and +destructors will be run for every element in the array (re-sizable vectors only +construct those elements actually used).
+SmallVector<Type, N> is a simple class that looks and smells +just like vector<Type>: +it supports efficient iteration, lays out elements in memory order (so you can +do pointer arithmetic between elements), supports efficient push_back/pop_back +operations, supports efficient random access to its elements, etc.
+ +The advantage of SmallVector is that it allocates space for +some number of elements (N) in the object itself. Because of this, if +the SmallVector is dynamically smaller than N, no malloc is performed. This can +be a big win in cases where the malloc/free call is far more expensive than the +code that fiddles around with the elements.
+ +This is good for vectors that are "usually small" (e.g. the number of +predecessors/successors of a block is usually less than 8). On the other hand, +this makes the size of the SmallVector itself large, so you don't want to +allocate lots of them (doing so will waste a lot of space). As such, +SmallVectors are most useful when on the stack.
+ +SmallVector also provides a nice portable and efficient replacement for +alloca.
+ ++std::vector is well loved and respected. It is useful when SmallVector isn't: +when the size of the vector is often large (thus the small optimization will +rarely be a benefit) or if you will be allocating many instances of the vector +itself (which would waste space for elements that aren't in the container). +vector is also useful when interfacing with code that expects vectors :). +
+ +One worthwhile note about std::vector: avoid code like this:
+ +
+for ( ... ) {
+ std::vector<foo> V;
+ use V;
+}
+
+Instead, write this as:
+ +
+std::vector<foo> V;
+for ( ... ) {
+ use V;
+ V.clear();
+}
+
+Doing so will save (at least) one heap allocation and free per iteration of +the loop.
+ +std::deque is, in some senses, a generalized version of std::vector. Like +std::vector, it provides constant time random access and other similar +properties, but it also provides efficient access to the front of the list. It +does not guarantee continuity of elements within memory.
+ +In exchange for this extra flexibility, std::deque has significantly higher +constant factor costs than std::vector. If possible, use std::vector or +something cheaper.
+std::list is an extremely inefficient class that is rarely useful. +It performs a heap allocation for every element inserted into it, thus having an +extremely high constant factor, particularly for small data types. std::list +also only supports bidirectional iteration, not random access iteration.
+ +In exchange for this high cost, std::list supports efficient access to both +ends of the list (like std::deque, but unlike std::vector or SmallVector). In +addition, the iterator invalidation characteristics of std::list are stronger +than that of a vector class: inserting or removing an element into the list does +not invalidate iterator or pointers to other elements in the list.
+ilist<T> implements an 'intrusive' doubly-linked list. It is +intrusive, because it requires the element to store and provide access to the +prev/next pointers for the list.
+ +ilist has the same drawbacks as std::list, and additionally requires an +ilist_traits implementation for the element type, but it provides some novel +characteristics. In particular, it can efficiently store polymorphic objects, +the traits class is informed when an element is inserted or removed from the +list, and ilists are guaranteed to support a constant-time splice operation. +
+ +These properties are exactly what we want for things like Instructions and +basic blocks, which is why these are implemented with ilists.
+Other STL containers are available, such as std::string.
+ +There are also various STL adapter classes such as std::queue, +std::priority_queue, std::stack, etc. These provide simplified access to an +underlying container but don't affect the cost of the container itself.
+ +Set-like containers are useful when you need to canonicalize multiple values +into a single representation. There are several different choices for how to do +this, providing various trade-offs.
+ +If you intend to insert a lot of elements, then do a lot of queries, a +great approach is to use a vector (or other sequential container) with +std::sort+std::unique to remove duplicates. This approach works really well if +your usage pattern has these two distinct phases (insert then query), and can be +coupled with a good choice of sequential container. +
+ ++This combination provides the several nice properties: the result data is +contiguous in memory (good for cache locality), has few allocations, is easy to +address (iterators in the final vector are just indices or pointers), and can be +efficiently queried with a standard binary or radix search.
+ +If you have a set-like data structure that is usually small and whose elements +are reasonably small, a SmallSet<Type, N> is a good choice. This set +has space for N elements in place (thus, if the set is dynamically smaller than +N, no malloc traffic is required) and accesses them with a simple linear search. +When the set grows beyond 'N' elements, it allocates a more expensive representation that +guarantees efficient access (for most types, it falls back to std::set, but for +pointers it uses something far better, SmallPtrSet).
+ +The magic of this class is that it handles small sets extremely efficiently, +but gracefully handles extremely large sets without loss of efficiency. The +drawback is that the interface is quite small: it supports insertion, queries +and erasing, but does not support iteration.
+ +SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is +transparently implemented with a SmallPtrSet), but also supports iterators. If +more than 'N' insertions are performed, a single quadratically +probed hash table is allocated and grows as needed, providing extremely +efficient access (constant time insertion/deleting/queries with low constant +factors) and is very stingy with malloc traffic.
+ +Note that, unlike std::set, the iterators of SmallPtrSet are invalidated +whenever an insertion occurs. Also, the values visited by the iterators are not +visited in sorted order.
+ ++FoldingSet is an aggregate class that is really good at uniquing +expensive-to-create or polymorphic objects. It is a combination of a chained +hash table with intrusive links (uniqued objects are required to inherit from +FoldingSetNode) that uses SmallVector as part of +its ID process.
+ +Consider a case where you want to implement a "getOrCreateFoo" method for +a complex object (for example, a node in the code generator). The client has a +description of *what* it wants to generate (it knows the opcode and all the +operands), but we don't want to 'new' a node, then try inserting it into a set +only to find out it already exists, at which point we would have to delete it +and return the node that already exists. +
+ +To support this style of client, FoldingSet perform a query with a +FoldingSetNodeID (which wraps SmallVector) that can be used to describe the +element that we want to query for. The query either returns the element +matching the ID or it returns an opaque ID that indicates where insertion should +take place. Construction of the ID usually does not require heap traffic.
+ +Because FoldingSet uses intrusive links, it can support polymorphic objects +in the set (for example, you can have SDNode instances mixed with LoadSDNodes). +Because the elements are individually allocated, pointers to the elements are +stable: inserting or removing elements does not invalidate any pointers to other +elements. +
+ +std::set is a reasonable all-around set class, which is decent at +many things but great at nothing. std::set allocates memory for each element +inserted (thus it is very malloc intensive) and typically stores three pointers +per element in the set (thus adding a large amount of per-element space +overhead). It offers guaranteed log(n) performance, which is not particularly +fast from a complexity standpoint (particularly if the elements of the set are +expensive to compare, like strings), and has extremely high constant factors for +lookup, insertion and removal.
+ +The advantages of std::set are that its iterators are stable (deleting or +inserting an element from the set does not affect iterators or pointers to other +elements) and that iteration over the set is guaranteed to be in sorted order. +If the elements in the set are large, then the relative overhead of the pointers +and malloc traffic is not a big deal, but if the elements of the set are small, +std::set is almost never a good choice.
+ +LLVM's SetVector<Type> is an adapter class that combines your choice of +a set-like container along with a Sequential +Container. The important property +that this provides is efficient insertion with uniquing (duplicate elements are +ignored) with iteration support. It implements this by inserting elements into +both a set-like container and the sequential container, using the set-like +container for uniquing and the sequential container for iteration. +
+ +The difference between SetVector and other sets is that the order of +iteration is guaranteed to match the order of insertion into the SetVector. +This property is really important for things like sets of pointers. Because +pointer values are non-deterministic (e.g. vary across runs of the program on +different machines), iterating over the pointers in the set will +not be in a well-defined order.
+ ++The drawback of SetVector is that it requires twice as much space as a normal +set and has the sum of constant factors from the set-like container and the +sequential container that it uses. Use it *only* if you need to iterate over +the elements in a deterministic order. SetVector is also expensive to delete +elements out of (linear time), unless you use it's "pop_back" method, which is +faster. +
+ +SetVector is an adapter class that defaults to using std::vector and 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.
+ ++UniqueVector is similar to SetVector, but it +retains a unique ID for each element inserted into the set. It internally +contains a map and a vector, and it assigns a unique ID for each value inserted +into the set.
+ +UniqueVector is very expensive: its cost is the sum of the cost of +maintaining both the map and vector, it has high complexity, high constant +factors, and produces a lot of malloc traffic. It should be avoided.
+ ++The STL provides several other options, such as std::multiset and the various +"hash_set" like containers (whether from C++ TR1 or from the SGI library).
+ +std::multiset is useful if you're not interested in elimination of +duplicates, but has all the drawbacks of std::set. A sorted vector (where you +don't delete duplicate entries) or some other approach is almost always +better.
+ +The various hash_set implementations (exposed portably by +"llvm/ADT/hash_set") is a simple chained hashtable. This algorithm is as malloc +intensive as std::set (performing an allocation for each element inserted, +thus having really high constant factors) but (usually) provides O(1) +insertion/deletion of elements. This can be useful if your elements are large +(thus making the constant-factor cost relatively low) or if comparisons are +expensive. Element iteration does not visit elements in a useful order.
+ ++If your usage pattern follows a strict insert-then-query approach, you can +trivially use the same approach as sorted vectors +for set-like containers. The only difference is that your query function +(which uses std::lower_bound to get efficient log(n) lookup) should only compare +the key, not both the key and value. This yields the same advantages as sorted +vectors for sets. +
++Strings are commonly used as keys in maps, and they are difficult to support +efficiently: they are variable length, inefficient to hash and compare when +long, expensive to copy, etc. StringMap is a specialized container designed to +cope with these issues. It supports mapping an arbitrary range of bytes to an +arbitrary other object.
+ +The StringMap implementation uses a quadratically-probed hash table, where +the buckets store a pointer to the heap allocated entries (and some other +stuff). The entries in the map must be heap allocated because the strings are +variable length. The string data (key) and the element object (value) are +stored in the same allocation with the string data immediately after the element +object. This container guarantees the "(char*)(&Value+1)" points +to the key string for a value.
+ +The StringMap is very fast for several reasons: quadratic probing is very +cache efficient for lookups, the hash value of strings in buckets is not +recomputed when 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.
++IndexedMap is a specialized container for mapping small dense integers (or +values that can be mapped to small dense integers) to some other type. It is +internally implemented as a vector with a mapping function that maps the keys to +the dense integer range. +
+ ++This is useful for cases like virtual registers in the LLVM code generator: they +have a dense mapping that is offset by a compile-time constant (the first +virtual register ID).
+ ++DenseMap is a simple quadratically probed hash table. It excels at supporting +small keys and values: it uses a single allocation to hold all of the pairs that +are currently inserted in the map. DenseMap is a great way to map pointers to +pointers, or map other small types to each other. +
+ ++There are several aspects of DenseMap that you should be aware of, however. The +iterators in a densemap are invalidated whenever an insertion occurs, unlike +map. Also, because DenseMap allocates space for a large number of key/value +pairs (it starts with 64 by default), it will waste a lot of space if your keys +or values are large. Finally, you must implement a partial specialization of +DenseMapKeyInfo 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.
+ ++std::map has similar characteristics to std::set: it uses +a single allocation per pair inserted into the map, it offers log(n) lookup with +an extremely large constant factor, imposes a space penalty of 3 pointers per +pair in the map, etc.
+ +std::map is most useful when your keys or values are very large, if you need +to iterate over the collection in sorted order, or if you need stable iterators +into the map (i.e. they don't get invalidated if an insertion or deletion of +another element takes place).
+ ++The STL provides several other options, such as std::multimap and the various +"hash_map" like containers (whether from C++ TR1 or from the SGI library).
+ +std::multimap is useful if you want to map a key to multiple values, but has +all the drawbacks of std::map. A sorted vector or some other approach is almost +always better.
+ +The various hash_map implementations (exposed portably by +"llvm/ADT/hash_map") are simple chained hash tables. This algorithm is as +malloc intensive as std::map (performing an allocation for each element +inserted, thus having really high constant factors) but (usually) provides O(1) +insertion/deletion of elements. This can be useful if your elements are large +(thus making the constant-factor cost relatively low) or if comparisons are +expensive. Element iteration does not visit elements in a useful order.
+ +| -Introduction - |
+
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.
+ +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.
+ +It's quite common to have a Function instance that you'd like to +transform in some way; in particular, you'd like to manipulate its +BasicBlocks. To facilitate this, you'll need to iterate over all of +the BasicBlocks that constitute the Function. The following is +an example that prints the name of a BasicBlock and the number of +Instructions it contains:
+ ++// func is a pointer to a Function instance +for (Function::iterator i = func->begin(), e = func->end(); i != e; ++i) + // Print out the name of the basic block if it has one, and then the + // number of instructions that it contains + llvm::cerr << "Basic block (name=" << i->getName() << ") has " + << i->size() << " instructions.\n"; ++
Note that i can be used as if it were a pointer for the purposes of +invoking member functions of the Instruction class. This is +because the indirection operator is overloaded for the iterator +classes. In the above code, the expression i->size() is +exactly equivalent to (*i).size() just like you'd expect.
+ +Just like when dealing with BasicBlocks in Functions, it's +easy to iterate over the individual instructions that make up +BasicBlocks. Here's a code snippet that prints out each instruction in +a BasicBlock:
+ ++// blk is a pointer to a BasicBlock instance +for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i) + // The next statement works since operator<<(ostream&,...) + // is overloaded for Instruction& + llvm::cerr << *i << "\n"; ++
However, this isn't really the best way to print out the contents of a +BasicBlock! Since the ostream operators are overloaded for virtually +anything you'll care about, you could have just invoked the print routine on the +basic block itself: llvm::cerr << *blk << "\n";.
+ +If you're finding that you commonly iterate over a Function's +BasicBlocks and then that BasicBlock's Instructions, +InstIterator should be used instead. You'll need to include llvm/Support/InstIterator.h, +and then instantiate InstIterators explicitly in your code. Here's a +small example that shows how to dump all instructions in a function to the standard error stream:
+ +
+#include "llvm/Support/InstIterator.h" + +// F is a pointer to a Function instance +for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) + llvm::cerr << *i << "\n"; ++
Easy, isn't it? You can also use InstIterators to fill a +work list with its initial contents. For example, if you wanted to +initialize a work list to contain all instructions in a Function +F, all you would need to do is something like:
+ ++std::set<Instruction*> worklist; +worklist.insert(inst_begin(F), inst_end(F)); ++
The STL set worklist would now contain all instructions in the +Function pointed to by F.
+ +Sometimes, it'll be useful to grab a reference (or pointer) to a class +instance when all you've got at hand is an iterator. Well, extracting +a reference or a pointer from an iterator is very straight-forward. +Assuming that i is a BasicBlock::iterator and j +is a BasicBlock::const_iterator:
+ ++Instruction& inst = *i; // Grab reference to instruction reference +Instruction* pinst = &*i; // Grab pointer to instruction reference +const Instruction& inst = *j; ++
However, the iterators you'll be working with in the LLVM framework are +special: they will automatically convert to a ptr-to-instance type whenever they +need to. Instead of dereferencing the iterator and then taking the address of +the result, you can simply assign the iterator to the proper pointer type and +you get the dereference and address-of operation as a result of the assignment +(behind the scenes, this is a result of overloading casting mechanisms). Thus +the last line of the last example,
+ ++Instruction* pinst = &*i; ++
is semantically equivalent to
+ ++Instruction* pinst = i; ++
It's also possible to turn a class pointer into the corresponding iterator, +and this is a constant time operation (very efficient). The following code +snippet illustrates use of the conversion constructors provided by LLVM +iterators. By using these, you can explicitly grab the iterator of something +without actually obtaining it via iteration over some structure:
+ +
+void printNextInstruction(Instruction* inst) {
+ BasicBlock::iterator it(inst);
+ ++it; // After this line, it refers to the instruction after *inst
+ if (it != inst->getParent()->end()) 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:
+ +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.
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.
+ ++
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.
-The first section of this document describes general information that is useful -to know when working in the LLVM infrastructure, and the second describes the -Core LLVM classes. In the future this manual will be extended with information -describing how to use extension libraries, such as dominator information, CFG -traversal routines, and useful utilities like the InstVisitor template.+
+AllocaInst* instToReplace = ...; +BasicBlock::iterator ii(instToReplace); +ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii, + Constant::getNullValue(PointerType::get(Type::IntTy))); +
| -General Information - |
+
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); - -
| - -The C++ Standard Template Library - |
- -Here are some useful links:
-
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.
-+
+ +
+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 isa<>, cast<> and dyn_cast<> templates - |
+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. +
+ ++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.
+ +| -Helpful Hints for Common Operations - |
+
#include "llvm/Type.h"
+
doxygen info: Type Class
- - +
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.
+| - -Basic Inspection and Traversal Routines - |
+
+ +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. +
++ +
+
- // func is a pointer to a Function instance
- for(Function::iterator i = func->begin(), e = func->end(); i != e; ++i) {
+ - // 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& - cerr << *i << "\n"; --However, this isn't really the best way to print out the contents of a -BasicBlock! Since the ostream operators are overloaded for -virtually anything you'll care about, you could have just invoked the -print routine on the basic block itself: cerr << *blk << -"\n";.
+ +
+
#include "llvm/Module.h"
doxygen info:
+Module Class
The Module class represents the top level structure present in LLVM +programs. An LLVM module is effectively either a translation unit of the +original program or a combination of several translation units merged by the +linker. The Module class keeps track of a list of Functions, a list of GlobalVariables, and a SymbolTable. Additionally, it contains a few +helpful member functions that try to make common operations easy.
+ +-#include "llvm/Support/InstIterator.h" -... -// Suppose F is a ptr to a function -for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) - cerr << **i << "\n"; -+
-std::set<Instruction*> worklist; -worklist.insert(inst_begin(F), inst_end(F)); -+
Constructing a Module is easy. You can optionally +provide a name for it (probably based on the name of the translation unit).
-The STL set worklist would now contain all instructions in -the Function pointed to by F. +These are forwarding methods that make it easy to access the contents of + a Module object's Function + list.
-- Instruction& inst = *i; // grab reference to instruction reference - Instruction* pinst = &*i; // grab pointer to instruction reference - const Instruction& inst = *j; --However, the iterators you'll be working with in the LLVM framework -are special: they will automatically convert to a ptr-to-instance type -whenever they need to. Instead of dereferencing the iterator and then -taking the address of the result, you can simply assign the iterator -to the proper pointer type and you get the dereference and address-of -operation as a result of the assignment (behind the scenes, this is a -result of overloading casting mechanisms). Thus the last line of the -last example, +
Instruction* pinst = &*i;+
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.
-is semantically equivalent to +Instruction* pinst = i;+
-BasicBlock::iterator bbi = ...; -BranchInst* b = dyn_cast<BranchInst>(&*bbi); -+ Module::const_global_iterator - Typedef for const_iterator.
-void printNextInstruction(Instruction* inst) {
- BasicBlock::iterator it(inst);
- ++it; // after this line, it refers to the instruction after *inst.
- if(it != inst->getParent()->end()) cerr << *it << "\n";
-}
-
-Of course, this example is strictly pedagogical, because it'd be much
-better to explicitly grab the next instruction directly from inst.
+ These are forwarding methods that make it easy to access the contents of + a Module object's GlobalVariable list.
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.
--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, since we're writing a -FunctionPass, our FunctionPass-derived class simply -has to override the runOnFunction method...): +
+
Return a reference to the SymbolTable + for this Module.
-class OurFunctionPass : public FunctionPass { - public: - OurFunctionPass(): callCounter(0) { } +Look up the specified function in the Module SymbolTable. If it does not exist, return + null.
| - -Making simple changes - |
Look up the specified function in the Module SymbolTable. If it does not exist, add an + external declaration for the function and return it.
+| -The Core LLVM Class Hierarchy Reference - |
If there is at least one entry in the SymbolTable for the specified Type, return it. Otherwise return the empty + string.
+ +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.
+
| - -The Value class - |
+
#include "llvm/Value.h"
+
+doxygen info: Value Class
+href="#Function">Functions are Values.
-A particular Value may be used many times in the LLVM representation +A particular Value may be used many times in the LLVM representation for a program. For example, an incoming argument to a function (represented with an instance of the Argument class) is "used" by every instruction in the function that references the argument. To keep track @@ -512,790 +2376,692 @@ href="#User">Users that is using it (the User class is a base class for all nodes in the LLVM graph that can refer to Values). This use list is how LLVM represents def-use information in the program, and is accessible through the use_* -methods, shown below.
+methods, shown below.
-Because LLVM is a typed representation, every LLVM Value is typed, and -this Type is available through the getType() -method. In addition, all LLVM values can be named. The -"name" of the Value is symbolic string printed in the LLVM code:+
Because LLVM is a typed representation, every LLVM Value is typed, +and this Type is available through the getType() +method. In addition, all LLVM values can be named. The "name" of the +Value is a symbolic string printed in the LLVM code:
+- %foo = add int 1, 2 +%foo = add i32 1, 2+
+
The name of this instruction is "foo". NOTE +that the name of any value may be missing (an empty string), so names should +ONLY be used for debugging (making the source code easier to read, +debugging printouts), they should not be used to keep track of values or map +between them. For this purpose, use a std::map of pointers to the +Value itself instead.
-One important aspect of LLVM is that there is no distinction between an SSA +One important aspect of LLVM is that there is no distinction between an SSA variable and the operation that produces it. Because of this, any reference to the value produced by an instruction (or the value available as an incoming -argument, for example) is represented as a direct pointer to the class that +argument, for example) is represented as a direct pointer to the instance of +the class that represents this value. Although this may take some getting used to, it -simplifies the representation and makes it easier to manipulate.
+simplifies the representation and makes it easier to manipulate.
+- -These methods are the interface to access the def-use information in LLVM. As with all other iterators in LLVM, the naming conventions follow the conventions defined by the STL.
- -
-This method returns the Type of the Value. - -
These methods are the interface to access the def-use +information in LLVM. As with all other iterators in LLVM, the naming +conventions follow the conventions defined by the STL.
+This method returns the Type of the Value.
+- -This family of methods is used to access and assign a name to a Value, -be aware of the precaution above.
- - -
- -This method traverses the use list of a Value changing all User's of the current value to refer to "V" -instead. For example, if you detect that an instruction always produces a -constant value (for example through constant folding), you can replace all uses -of the instruction with the constant like this:
- + void setName(const std::string &Name) +
This family of methods is used to access and assign a name to a Value, +be aware of the precaution above.
+This method traverses the use list of a Value changing all Users of the current value to refer to + "V" instead. For example, if you detect that an instruction always + produces a constant value (for example through constant folding), you can + replace all uses of the instruction with the constant like this:
+ +- Inst->replaceAllUsesWith(ConstVal); -
+Inst->replaceAllUsesWith(ConstVal); + +
| - -The User class - |
+
+#include "llvm/User.h"
+doxygen info: User Class
+Superclass: Value
The User class is the common base class of all LLVM nodes that may refer to Values. It exposes a list of "Operands" that are all of the Values that the User is referring to. The User class itself is a subclass of -Value.
+Value.
-The operands of a User point directly to the LLVM The operands of a User point directly to the LLVM Value that it refers to. Because LLVM uses Static Single Assignment (SSA) form, there can only be one definition referred to, allowing this direct connection. This connection provides the use-def -information in LLVM.- - -
+
- -These two methods expose the operands of the User in a convenient form -for direct access.
- -
+ +
+ +The User class exposes the operand list in two ways: through +an index access interface and through an iterator based interface.
+ +These two methods expose the operands of the User in a +convenient form for direct access.
Together, these methods make up the iterator based interface to +the operands of a User.
+
| - -The Instruction class - |
#include "llvm/Instruction.h"
+doxygen info: Instruction Class
Superclasses: User, Value
+href="#Value">Value
-The Instruction class is the common base class for all LLVM +The Instruction class is the common base class for all LLVM instructions. It provides only a few methods, but is a very commonly used class. The primary data tracked by the Instruction class itself is the opcode (instruction type) and the parent BasicBlock the Instruction is embedded into. To represent a specific type of instruction, one of many subclasses of -Instruction are used.
+Instruction are used.
-Because the Instruction class subclasses the Because the Instruction class subclasses the User class, its operands can be accessed in the same way as for other Users (with the getOperand()/getNumOperands() and -op_begin()/op_end() methods).- +op_begin()/op_end() methods).
An important file for +the Instruction class is the llvm/Instruction.def file. This +file contains some meta-data about the various different types of instructions +in LLVM. It describes the enum values that are used as opcodes (for example +Instruction::Add and Instruction::ICmp), as well as the +concrete sub-classes of Instruction that implement the instruction (for +example BinaryOperator and CmpInst). Unfortunately, the use of macros in +this file confuses doxygen, so these enum values don't show up correctly in the +doxygen output.
+ +- -Returns the BasicBlock that this -Instruction is embedded into.
- -
- -Returns true if the instruction has side effects, i.e. it is a call, -free, invoke, or store.
- -
- -Returns the opcode for the Instruction.
- - - - - -
| - -The BasicBlock class - |
- - -This class represents a single entry multiple exit section of the code, commonly -known as a basic block by the compiler community. The BasicBlock class -maintains a list of Instructions, which form -the body of the block. Matching the language definition, the last element of -this list of instructions is always a terminator instruction (a subclass of the -TerminatorInst class).
- -In addition to tracking the list of instructions that make up the block, the -BasicBlock class also keeps track of the Function that it is embedded into.
- -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.
- +
+This subclasses represents all two operand instructions whose operands + must be the same type, except for the comparison instructions.
This subclass is the parent of the 12 casting instructions. It provides + common operations on cast instructions.
+This subclass respresents the two comparison instructions, + ICmpInst (integer opreands), and + FCmpInst (floating point operands).
+This subclass is the parent of all terminator instructions (those which + can terminate a block).
+- -The BasicBlock constructor is used to create new basic blocks for -insertion into a function. The constructor simply takes a name for the new -block, and optionally 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.
+
+ +Returns the BasicBlock that +this Instruction is embedded into.
Returns true if the instruction writes to memory, i.e. it is a + call,free,invoke, or store.
Returns the opcode for the Instruction.
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
+
+ +
-+
+
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.
+ +
+| - -The GlobalValue class - |
+
#include "llvm/GlobalValue.h"
+doxygen info: GlobalValue
+Class
+Superclasses: Constant,
+User, Value
+process, GlobalValues know their linkage rules. Specifically, +GlobalValues know whether they have internal or external linkage, as +defined by the LinkageTypes enumeration.
-If a GlobalValue has internal linkage (equivalent to being +If a GlobalValue has internal linkage (equivalent to being static in C), it is not visible to code outside the current translation unit, and does not participate in linking. If it has external linkage, it is visible to external code, and does participate in linking. In addition to linkage information, GlobalValues keep track of which Module they are currently part of.
- -Because GlobalValues are memory objects, they are always referred to by -their address. As such, the Type of a global is -always a pointer to its contents. This is explained in the LLVM Language -Reference Manual.
- +href="#Module">Module they are currently part of.
+ +Because GlobalValues are memory objects, they are always referred to +by their address. As such, the Type of a +global is always a pointer to its contents. It is important to remember this +when using the GetElementPtrInst instruction because this pointer must +be dereferenced first. For example, if you have a GlobalVariable (a +subclass of GlobalValue) that is an array of 24 ints, type [24 x +i32], then the GlobalVariable is a pointer to that array. Although +the address of the first element of this array and the value of the +GlobalVariable are the same, they have different types. The +GlobalVariable's type is [24 x i32]. The first element's type +is i32. Because of this, accessing a global value requires you to +dereference the pointer with GetElementPtrInst first, then its elements +can be accessed. This is explained in the LLVM +Language Reference Manual.
+ ++
+
These methods manipulate the linkage characteristics of the GlobalValue.
++
This returns the Module that the +GlobalValue is currently embedded into.
+
+ +
+#include "llvm/Function.h"
doxygen
+info: Function Class
+Superclasses: GlobalValue,
+Constant,
+User,
+Value
| - -The Function class - |
- -The Function class represents a single procedure in LLVM. It is +
The Function class represents a single procedure in LLVM. It is actually one of the more complex classes in the LLVM heirarchy because it must keep track of a large amount of data. The Function class keeps track -of a list of BasicBlocks, a list of formal Arguments, and a SymbolTable.
- -The list of BasicBlocks is the most commonly -used part of Function objects. The list imposes an implicit ordering -of the blocks in the function, which indicate how the code will be layed out by -the backend. Additionally, the first BasicBlocks, a list of formal +Arguments, and a +SymbolTable.
+ +The list of BasicBlocks is the most +commonly used part of Function objects. The list imposes an implicit +ordering of the blocks in the function, which indicate how the code will be +layed out by the backend. Additionally, the first BasicBlock is the implicit entry node for the -Function. It is not legal in LLVM explicitly branch to this initial +Function. It is not legal in LLVM to explicitly branch to this initial block. There are no implicit exit nodes, and in fact there may be multiple exit nodes from a single Function. If the BasicBlock list is empty, this indicates that the Function is actually a function declaration: the actual body of the -function hasn't been linked in yet.
+function hasn't been linked in yet.
-In addition to a list of BasicBlocks, the +In addition to a list of BasicBlocks, the Function class also keeps track of the list of formal Arguments that the function receives. This container manages the lifetime of the Argument nodes, just like the BasicBlock list does for -the BasicBlocks.
+the BasicBlocks.
-The SymbolTable is a very rarely used LLVM -feature that is only used when you have to look up a value by name. Aside from -that, the SymbolTable is used internally to -make sure that there are not conflicts between the names of The SymbolTable is a very rarely used +LLVM feature that is only used when you have to look up a value by name. Aside +from that, the SymbolTable is used +internally to make sure that there are not conflicts between the names of Instructions, BasicBlocks, or Arguments in the function body.+href="#Argument">Arguments in the function body.
+Note that Function is a GlobalValue +and therefore also a Constant. The value of the function +is its address (after linking) which is guaranteed to be constant.
+- -Constructor used when you need to create new Functions to add the the -program. The constructor must specify the type of the function to create and -whether or not it should start out with internal or external linkage.
- -
- -Return whether or not the Function has a body defined. If the function -is "external", it does not have a body, and thus must be resolved by linking -with a function defined in a different translation unit.
- - -
Constructor used when you need to create new Functions to add + the the program. The constructor must specify the type of the function to + create and what type of linkage the function should have. The FunctionType argument + specifies the formal arguments and return value for the function. The same + FunctionType value can be used to + create multiple functions. The Parent argument specifies the Module + in which the function is defined. If this argument is provided, the function + will automatically be inserted into that module's list of + functions.
Return whether or not the Function has a body defined. If the + function is "external", it does not have a body, and thus must be resolved + by linking with a function defined in a different translation unit.
- -These are forwarding methods that make it easy to access the contents of a -Function object's BasicBlock -list.
- -
- -Returns the list of BasicBlocks. This is -neccesary to use when you need to update the list or perform a complex action -that doesn't have a forwarding method.
- - -
- -These are forwarding methods that make it easy to access the contents of a -Function object's Argument list.
-
+ begin(), end() + size(), empty() -Returns the list of Arguments. This is -neccesary to use when you need to update the list or perform a complex action -that doesn't have a forwarding method.
+
These are forwarding methods that make it easy to access the contents of + a Function object's BasicBlock + list.
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.
+
+ 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.
+
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.
+
+
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.
+
+
This traverses the Type of the + Function and returns the return type of the function, or the FunctionType of the actual + function.
- -Return a pointer to the SymbolTable for this -Function or allocate a new SymbolTable if one is not already around. This -should only be used when adding elements to the SymbolTable, so that empty symbol tables are -not left laying around.
+
Return a pointer to the SymbolTable + for this Function.
| - -The GlobalVariable class - |
- -Global variables are represented with the (suprise suprise) -GlobalVariable class. Like functions, GlobalVariables are -also subclasses of GlobalValue, and as such -are always referenced by their address (global values must live in memory, so -their "name" refers to their address). Global variables may have an initial -value (which must be a Constant), and if they -have an initializer, they may be marked as "constant" themselves (indicating -that their contents never change at runtime).
- +
+ +#include "llvm/GlobalVariable.h"
+
+doxygen info: GlobalVariable
+ Class
+Superclasses: GlobalValue,
+Constant,
+User,
+Value
Global variables are represented with the (suprise suprise) +GlobalVariable class. Like functions, GlobalVariables are also +subclasses of GlobalValue, and as such are +always referenced by their address (global values must live in memory, so their +"name" refers to their 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).
++
-Create a new global variable of the specified type. If isConstant is -true then the global variable will be marked as unchanging for the program, and -if isInternal is true the resultant global variable will have internal -linkage. Optionally an initializer and name may be specified for the global variable as well.+
+
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.
+
Returns true if this is a global variable that is known not to + be modified at runtime.
+
+
Returns true if this GlobalVariable has an intializer.
+
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/BasicBlock.h"
+doxygen info: BasicBlock
+Class
+Superclass: Value
This class represents a single entry multiple exit section of the code, +commonly known as a basic block by the compiler community. The +BasicBlock class maintains a list of Instructions, which form the body of the block. +Matching the language definition, the last element of this list of instructions +is always a terminator instruction (a subclass of the TerminatorInst class).
+ +In addition to tracking the list of instructions that make up the block, the +BasicBlock class also keeps track of the Function that it is embedded into.
-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.+
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.
+- -These are forwarding methods that make it easy to access the contents of a -Module object's Function -list.
- -
- -Returns the list of Functions. This is -neccesary to use when you need to update the list or perform a complex action -that doesn't have a forwarding method.
- - -
- -These are forwarding methods that make it easy to access the contents of a -Module object's GlobalVariable -list.
- -
- -Returns the list of GlobalVariables. -This is neccesary to use when you need to update the list or perform a complex -action that doesn't have a forwarding method.
- - - -
- -Return true if the Module has a symbol table allocated to it and if -there is at least one entry in it.
- -
- -Return a pointer to the SymbolTable for this -Module or a null pointer if one has not been allocated (because there -are no named values in the function).
- -
- -Return a pointer to the SymbolTable for this -Module or allocate a new SymbolTable if one is not already around. This -should only be used when adding elements to the SymbolTable, so that empty symbol tables are -not left laying around.
- - - -
- -Look up the specified function in the Module SymbolTable. If it does not exist, return -null.
- - -
- -Look up the specified function in the Module SymbolTable. If it does not exist, add an -external declaration for the function and return it.
- - -
- -If there is at least one entry in the SymbolTable for the specified Type, return it. Otherwise return the empty -string.
- - -
- -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.
- +
+ +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.
| - -The Constant class and subclasses - |
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.
+
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.
Returns a pointer to Function the block is +embedded into, or a null pointer if it is homeless.
| - -The Type class and Derived Types - |
Returns a pointer to the terminator instruction that appears at the end of +the BasicBlock. If there is no terminator instruction, or if the last +instruction in the block is not a terminator, then a null pointer is +returned.
| - -The Argument class - |
This subclass of Value defines the interface for incoming formal +arguments to a function. A Function maintains a list of its formal +arguments. An argument has a pointer to the parent Function.
+
+