+
+
+
+
+
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).
+
+
+ - isa<>:
+
+ 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).
+ - cast<>:
+
+ 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.
+
+ - dyn_cast<>:
+
+ 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:
+
+
+
+ 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.
+
+
-doxygen info: Value Class
+
- cast_or_null<>:
+
+ 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.
- dyn_cast_or_null<>:
+
+ 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:
+
+
+
+DEBUG(std::cerr << "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:
+
+
+
+DEBUG(std::cerr << "No debug type\n");
+#undef DEBUG_TYPE
+#define DEBUG_TYPE "foo"
+DEBUG(std::cerr << "'foo' debug type\n");
+#undef DEBUG_TYPE
+#define DEBUG_TYPE "bar"
+DEBUG(std::cerr << "'bar' debug type\n");
+#undef DEBUG_TYPE
+#define DEBUG_TYPE ""
+DEBUG(std::cerr << "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 template 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 template 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:
+
+
+
+static Statistic<> NumXForms("mypassname", "The # of times I did stuff");
+
+
The Statistic template can emulate just about any data-type,
+ but if you do not specify a template argument, it defaults to acting like
+ an unsigned int counter (this is usually what you want).
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 ...
+
+
-The
Value class is the most important class in LLVM Source base. It
-represents a typed value that may be used (among other things) as an operand to
-an instruction. There are many different types of
Values, such as
Constants,
Arguments, and even
When running gccas 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 whereever 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 hilight 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().
+
+
+
+
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
+ std::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&
+ std::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: std::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 ptr to a Function instance
+for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
+ std::cerr << *i << "\n";
+
+
+
+
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:
+
+
+
+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()) std::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 pseudocode, 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)) {
+ std::cerr << "F is used in instruction:\n";
+ std::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 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:
+
+
+
+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:
+
+
+ - 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:
+
+
+
+
+
+
Replacing individual instructions
+
+
Including "llvm/Transforms/Utils/BasicBlockUtils.h"
+permits use of two very useful replace functions: ReplaceInstWithValue
+and ReplaceInstWithInst.
+
+
+
+
+ - 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.
+
+
+
+
+
+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.
+
+
+
+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 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*, int }
+
+
+
+
+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*, 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());
+
+// 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*, 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.
+
+
+
+
+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. The SymbolTable class, for example, needs
+move and potentially merge type planes in its representation when a pointer
+changes.
+
+
+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.
+
+
+
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.
+
+
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.
+
+
+
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
+
+
+
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.
+
+
+
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.
+
+
+
+ - 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.
+
+ - 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.
+
+ - 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.
+
+ - 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.
+
+ - type_iterator type_begin():
+ - Get an iterator to the start of the name/Type map.
+
+ - type_const_iterator type_begin() cons:
+ - Get a const_iterator to the start of the name/Type map.
+
+ - 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.
+
+ - 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.
+
+ - 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.
+
+ - 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.
+
+
+
+
+
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.
+
+
+
+
#include "llvm/Value.h"
+
+doxygen info: Value Class
+
+
The Value class is the most important class in the LLVM Source
+base. It represents a typed value that may be used (among other things) as an
+operand to an instruction. There are many different types of Values,
+such as Constants,Arguments. Even Instructions and Functions are Values.
+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
@@ -187,791 +1685,877 @@ of this relationship, the Value class keeps a list of all of the Users that is using it (the User class is a base class for all nodes in the LLVM
graph that can refer to Values). This use list is how LLVM represents
-def-use information in the program, and is accessable through the use_*
-methods, shown below.
+def-use information in the program, and is accessible through the use_*
+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 int 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.
+
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.
+
+