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: + 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; +++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... + // 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<> @@ -297,57 +317,51 @@ file (note that you very rarely have to include this file directly).
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)) { - ... - } -+ 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.
- -Another common example is:
- -- // Loop over all of the phi nodes in a basic block - BasicBlock::iterator BBI = BB->begin(); - for (; PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) - std::cerr << *PN; -+
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.
+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 @@ -359,31 +373,42 @@ 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 +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 "Support/Debug.h" +
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");
...
+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!
$
+$ 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 @@ -401,7 +426,7 @@ program hasn't been started yet, you can always just run it with
...+
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");
...
+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
$
+$ 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 "Support/Debug.h", you don't have to insert the ugly +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 @@ -440,7 +492,7 @@ even if the source lives in multiple files.
The "Support/Statistic.h" file +href="/doxygen/Statistic_8h-source.html">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 @@ -457,27 +509,71 @@ uniform manner with the rest of the passes being executed.
it are as follows:static Statistic<> NumXForms("mypassname", "The # of times I did stuff");
+ 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).
++NumXForms; // I did stuff+
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 ...
+$ opt -stats -mypassname < program.bc > /dev/null +... statistics 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
+ 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 @@ -485,6 +581,56 @@ 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().
+ +// 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";
}
+// 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 @@ -567,18 +722,20 @@ 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&
cerr << *i << "\n";
+// 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: cerr << *blk << "\n";.
- -Note that currently operator<< is implemented for Value*, so -it will print out the contents of the pointer, instead of the pointer value you -might expect. This is a deprecated interface that will be removed in the -future, so it's best not to depend on it. To print out the pointer value for -now, you must cast to void*.
+basic block itself: std::cerr << *blk << "\n";.-
#include "llvm/Support/InstIterator.h"-Easy, isn't it? You can also use InstIterators to fill a +
...
// 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";
+#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;+F, all you would need to do is something like: + +
worklist.insert(inst_begin(F), inst_end(F));
+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.
@@ -624,7 +796,13 @@ 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;
+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 @@ -634,11 +812,19 @@ 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;+
+Instruction* pinst = &*i; ++
is semantically equivalent to
-Instruction* pinst = i;+
+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 @@ -646,7 +832,15 @@ 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";
}
+
+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";
+}
+
+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
+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 +
And the actual code is (remember, because we're writing a FunctionPass, our FunctionPass-derived class simply has to -override the runOnFunction method...):
+override the runOnFunction method): + +
+Function* targetFunc = ...;
+
+class OurFunctionPass : public FunctionPass {
+ public:
+ OurFunctionPass(): callCounter(0) { }
- 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;
};
+ 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;
+};
+
+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";
}
}
+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 @@ -731,7 +970,16 @@ href="/doxygen/classllvm_1_1User.html">User Class and need to know what 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;
...
}
+Instruction* pi = ...;
+
+for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
+ Value* v = *i;
+ // ...
+}
+
+The Core LLVM classes are the primary means of representing the program -being inspected or transformed. The core LLVM classes are defined in -header files in the include/llvm/ directory, and implemented in -the lib/VMCore directory.
- ++This 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. +
#include "llvm/Value.h"
-
-doxygen info: Value Class
+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. +
-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.
++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. +
-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.
++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 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+ + -
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.
++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); -
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 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:
+// NewSTy is potentially invalidated, but StructTy (a PATypeHolder) is +// kept up-to-date +NewSTy = cast<StructType>(StructTy.get()); -Inst->replaceAllUsesWith(ConstVal);-
+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. +
-#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.
++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. +
+ +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.
++PATypeHolder is a form of a "smart pointer" for Type objects. When VMCore +happily goes about nuking types that become isomorphic to existing types, it +automatically updates all PATypeHolder objects to point to the new type. In the +example above, this allows the code to maintain a pointer to the resultant +resolved recursive type, even though the Type*'s are potentially invalidated. +
+ ++PATypeHolder is an extremely light-weight object that uses a lazy union-find +implementation to update pointers. For example the pointer from a Value to its +Type is maintained by PATypeHolder objects. +
The 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.
+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.
-Together, these methods make up the iterator based interface to -the operands of a User.
+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.
-#include "llvm/Instruction.h"
-doxygen info: Instruction Class
-Superclasses: User, Value
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. +
-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.
+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.
-Because the Instruction class subclasses the User class, its operands can be accessed in the same -way as for other Users (with the -getOperand()/getNumOperands() and -op_begin()/op_end() methods).
An important file for -the Instruction class is the llvm/Instruction.def file. This -file contains some meta-data about the various different types of instructions -in LLVM. It describes the enum values that are used as opcodes (for example -Instruction::Add and Instruction::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 +
The interface of this class provides three basic types of operations: +
The following functions describe three types of iterators you can obtain +the beginning or end of the sequence for both const and non-const. It is +important to keep track of the different kinds of iterators. There are +three idioms worth pointing out:
+ +| Units | Iterator | Idiom |
|---|---|---|
| Planes Of name/Value maps | PI | +
+for (SymbolTable::plane_const_iterator PI = ST.plane_begin(),
+ PE = ST.plane_end(); PI != PE; ++PI ) {
+ PI->first // This is the Type* of the plane
+ PI->second // This is the SymbolTable::ValueMap of name/Value pairs
+}
+ |
+
| All name/Type Pairs | TI | +
+for (SymbolTable::type_const_iterator TI = ST.type_begin(),
+ TE = ST.type_end(); TI != TE; ++TI ) {
+ TI->first // This is the name of the type
+ TI->second // This is the Type* value associated with the name
+}
+ |
+
| name/Value pairs in a plane | VI | +
+for (SymbolTable::value_const_iterator VI = ST.value_begin(SomeType),
+ VE = ST.value_end(SomeType); VI != VE; ++VI ) {
+ VI->first // This is the name of the Value
+ VI->second // This is the Value* value associated with the name
+}
+ |
+
Using the recommended iterator names and idioms will help you avoid +making mistakes. Of particular note, make sure that whenever you use +value_begin(SomeType) that you always compare the resulting iterator +with value_end(SomeType) not value_end(SomeOtherType) or else you +will loop infinitely.
+ +The 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 ++
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.
+ +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 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.
+ +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.
+ +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.
#include "llvm/Instruction.h"
+doxygen info: Instruction Class
+Superclasses: User, Value
The Instruction class is the common base class for all LLVM +instructions. It provides only a few methods, but is a very commonly used +class. The primary data tracked by the Instruction class itself is the +opcode (instruction type) and the parent BasicBlock the Instruction is embedded +into. To represent a specific type of instruction, one of many subclasses of +Instruction are used.
+ +Because the Instruction class subclasses the User class, its operands can be accessed in the same +way as for other Users (with the +getOperand()/getNumOperands() and +op_begin()/op_end() methods).
An important file for +the Instruction class is the llvm/Instruction.def file. This +file contains some meta-data about the various different types of instructions +in LLVM. It describes the enum values that are used as opcodes (for example +Instruction::Add and Instruction::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.
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.
-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.
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 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.
+ +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.
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.
#include "llvm/GlobalValue.h"
doxygen info: GlobalValue
-Class
-Superclasses: User, Value
Global values (GlobalVariables or Functions) are the only LLVM values that are @@ -1319,15 +2056,17 @@ GlobalValue is currently embedded into.
#include "llvm/Function.h"
doxygen
info: Function Class
-Superclasses: GlobalValue, User, Value
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.
+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 @@ -1393,8 +2132,8 @@ is its address (after linking) which is guaranteed to be constant.
These are forwarding methods that make it easy to access the contents of a Function object's BasicBlock @@ -1406,12 +2145,12 @@ is its address (after linking) which is guaranteed to be constant.
is necessary 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
@@ -1456,20 +2195,22 @@ iterator
href="/doxygen/GlobalVariable_8h-source.html">llvm/GlobalVariable.h"
doxygen info: GlobalVariable
-Class
Superclasses: GlobalValue, 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 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).
- +"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).These are forwarding methods that make it easy to access the contents of a Module object's Function @@ -1574,12 +2315,12 @@ provide a name for it (probably based on the name of the translation unit).
Derived Types
- + + +This subclass of Value defines the interface for incoming formal -arguments to a function. A Function maitanis a list of its formal +arguments to a function. A Function maintains a list of its formal arguments. An argument has a pointer to the parent Function.
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.
- -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: -
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.
- -