+
-#include "llvm/Value.h"
-doxygen info: Value Class
+
+
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).
-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
Instructions and
Functions are
Values.
+
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.
-A particular
Value may be used many times in the LLVM representation
-for a program. For example, an incoming argument to a function (represented
-with an instance of the
Argument class) is "used" by
-every instruction in the function that references the argument. To keep track
-of this relationship, the
Value class keeps a list of all of the
Users that is using it (the
User class is a base class for all nodes in the LLVM
-graph that can refer to
Values). This use list is how LLVM represents
-def-use information in the program, and is accessable through the
use_*
-methods, shown below.
+
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
... statistic output ...
+
+
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.
+
+
+
+
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
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:
- %foo = add int 1, 2
+ // 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";
-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.
+
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";.
-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
-represents this value. Although this may take some getting used to, it
-simplifies the representation and makes it easier to manipulate.
+
+
+
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"
...
// 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";
+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()) 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, since 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)) {
cerr << "F is used in instruction:\n";
cerr << *Inst << "\n";
}
}
-
- Value::use_iterator - Typedef for iterator over the use-list
- Value::use_const_iterator
- - Typedef for const_iterator over the use-list
- unsigned use_size() - Returns the number of users of the value.
- bool use_empty() - Returns true if there are no users.
- use_iterator use_begin()
- - Get an iterator to the start of the use-list.
- use_iterator use_end()
- - Get an iterator to the end of the use-list.
- User *use_back()
- - Returns the last element in the list.
+
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;
...
}
+
+
-These methods are the interface to access the def-use information in LLVM. As with all other iterators in LLVM, the naming conventions follow the conventions defined by the STL.
+
- Type *getType() const
-This method returns the Type of the Value.
+
+
- - bool hasName() const
- std::string getName() const
- void setName(const std::string &Name)
+
+
+
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:
+
+
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.
+
+
+
+
+ - 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.
-This family of methods is used to access and assign a name to a Value,
-be aware of the precaution above.
+
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:
+
-
- void replaceAllUsesWith(Value *V)
+
+ %mylist = type { %mylist*, int }
+
-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:
+
+To build this, use the following LLVM APIs:
+
- Inst->replaceAllUsesWith(ConstVal);
-
+ // 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.
+
+
A particular Value may be used many times in the LLVM representation
+for a program. For example, an incoming argument to a function (represented
+with an instance of the Argument class) is "used" by
+every instruction in the function that references the argument. To keep track
+of this relationship, the Value class keeps a list of all of the Users that is using it (the User class is a base class for all nodes in the LLVM
+graph that can refer to Values). This use list is how LLVM represents
+def-use information in the program, and is accessible through the use_*
+methods, shown below.
+
+
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
-
#include "llvm/User.h"
-doxygen info:
User Class
-Superclass:
Value
+
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
+variable and the operation that produces it. Because of this, any reference to
+the value produced by an instruction (or the value available as an incoming
+argument, for example) is represented as a direct pointer to the instance of
+the class that
+represents this value. Although this may take some getting used to, it
+simplifies the representation and makes it easier to manipulate.
+
+
+
+
+ - Value::use_iterator - Typedef for iterator over the
+use-list
+ Value::use_const_iterator - Typedef for const_iterator over
+the use-list
+ unsigned use_size() - Returns the number of users of the
+value.
+ bool use_empty() - Returns true if there are no users.
+ use_iterator use_begin() - Get an iterator to the start of
+the use-list.
+ use_iterator use_end() - Get an iterator to the end of the
+use-list.
+ User *use_back() - Returns the last
+element in the list.
+ These methods are the interface to access the def-use
+information in LLVM. As with all other iterators in LLVM, the naming
+conventions follow the conventions defined by the STL.
+
+ - Type *getType() const
+
This method returns the Type of the Value.
+
+ - bool hasName() const
+ std::string getName() const
+ void setName(const std::string &Name)
+ This family of methods is used to access and assign a name to a Value,
+be aware of the precaution above.
+
+ - void replaceAllUsesWith(Value *V)
+
+
This method traverses the use list of a Value changing all Users of the current value to refer to
+ "V" instead. For example, if you detect that an instruction always
+ produces a constant value (for example through constant folding), you can
+ replace all uses of the instruction with the constant like this:
+
+ Inst->replaceAllUsesWith(ConstVal);
+
+
+
+
+
+#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.
-
-
-
-
-The User class exposes the operand list in two ways: through an index
-access interface and through an iterator based interface.
-
-
- Value *getOperand(unsigned i)
- unsigned getNumOperands()
+information in LLVM.
-These two methods expose the operands of the User in a convenient form
-for direct access.
+
- User::op_iterator - Typedef for iterator over the operand list
- User::op_const_iterator
- use_iterator op_begin()
- - Get an iterator to the start of the operand list.
- use_iterator op_end()
- - Get an iterator to the end of the operand list.
+
+
+
+
+
+
The User class exposes the operand list in two ways: through
+an index access interface and through an iterator based interface.
+
+
+ - Value *getOperand(unsigned i)
+ unsigned getNumOperands()
+ These two methods expose the operands of the User in a
+convenient form for direct access.
+
+ - User::op_iterator - Typedef for iterator over the operand
+list
+ op_iterator op_begin() - Get an iterator to the start of
+the operand list.
+ op_iterator op_end() - Get an iterator to the end of the
+operand list.
+ Together, these methods make up the iterator based interface to
+the operands of a User.
+
-Together, these methods make up the iterator based interface to the operands of
-a
User.
+
-
-
-
-#include "llvm/Instruction.h"
-doxygen info: 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::SetLE), as well as the
+concrete sub-classes of Instruction that implement the instruction (for
+example BinaryOperator and SetCondInst). Unfortunately, the use of macros in
+this file confuses doxygen, so these enum values don't show up correctly in the
+doxygen output.
+
+
-
-#include "llvm/GlobalValue.h"
-doxygen info: GlobalValue Class
+
+
+
+
+
#include "llvm/GlobalValue.h"
+doxygen info: GlobalValue
+Class
Superclasses: User, Value
+href="#Value">Value
-Global values (
GlobalVariables or
Global values (GlobalVariables or
Functions) are the only LLVM values that are
visible in the bodies of all
Functions.
Because they are visible at global scope, they are also subject to linking with
other globals defined in different translation units. To control the linking
-process,
GlobalValues know their linkage rules. Specifically,
-
GlobalValues know whether they have internal or external linkage.
+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
+int], then the GlobalVariable is a pointer to that array. Although
+the address of the first element of this array and the value of the
+GlobalVariable are the same, they have different types. The
+GlobalVariable's type is [24 x int]. The first element's type
+is int. Because of this, accessing a global value requires you to
+dereference the pointer with GetElementPtrInst first, then its elements
+can be accessed. This is explained in the LLVM
+Language Reference Manual.
+
+
+