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Added subsections to 'Basic Inspection and Traversal Routines':

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* Iterating over the BasicBlocks in a Function
* Iterating over the Instructions in a BasicBlock
* Turning an iterator into a class pointer
* Finding call sites: a more complex example


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3606 91177308-0d34-0410-b5e6-96231b3b80d8
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Joel Stanley 2002-09-06 21:55:13 +00:00
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1 changed files with 175 additions and 8 deletions
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docs/ProgrammersManual.html
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@ -24,6 +24,7 @@
in a <tt>BasicBlock</tt></a>
<li><a href="#iterate_convert">Turning an iterator into a class
pointer</a>
<li><a href="#iterate_complex">Finding call sites: a more complex example</a>
</ul>
<li><a href="#simplechanges">Making simple changes</a>
<ul>
@ -59,7 +60,7 @@
</ul>
-->
</ul>
<li><a href="#coreclasses">The Core LLVM Class Heirarchy Reference</a>
<li><a href="#coreclasses">The Core LLVM Class Hierarchy Reference</a>
<ul>
<li><a href="#Value">The <tt>Value</tt> class</a>
<ul>
@ -106,8 +107,8 @@
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->
This document is meant to hi-light some of the important classes and interfaces
available in the LLVM source-base. This manual is not indended to explain what
This document is meant to highlight some of the important classes and interfaces
available in the LLVM source-base. This manual is not intended to explain what
LLVM is, how it works, and what LLVM code looks like. It assumes that you know
the basics of LLVM and are interested in writing transformations or otherwise
analyzing or manipulating the code.<p>
@ -188,9 +189,9 @@ This section describes how to perform some very simple transformations of LLVM
code. This is meant to give examples of common idioms used, showing the
practical side of LLVM transformations.<p>
Because this is a "howto" section, you should also read about the main classes
Because this is a "how-to" section, you should also read about the main classes
that you will be working with. The <a href="#coreclasses">Core LLVM Class
Heirarchy Reference</a> contains details and descriptions of the main classes
Hierarchy Reference</a> contains details and descriptions of the main classes
that you should know about.<p>
<!-- NOTE: this section should be heavy on example code -->
@ -211,23 +212,189 @@ Instruction. Common patterns for all levels. -->
</ul><h4><a name="iterate_function"><hr size=0>Iterating over the
<tt>BasicBlock</tt>s in a <tt>Function</tt> </h4><ul>
It's quite common to have a <tt>Function</tt> instance that you'd like
to transform in some way; in particular, you'd like to manipulate its
<tt>BasicBlock</tt>s. To facilitate this, you'll need to iterate over
all of the <tt>BasicBlock</tt>s that constitute the <tt>Function</tt>.
The following is an example that prints the name of a
<tt>BasicBlock</tt> and the number of <tt>Instruction</tt>s it
contains:
<pre>
// 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";
}
</pre>
Note that i can be used as if it were a pointer for the purposes of
invoking member functions of the <tt>Instruction</tt> class. This is
because the indirection operator is overloaded for the iterator
classes. In the above code, the expression <tt>i->size()</tt> is
exactly equivalent to <tt>(*i).size()</tt> just like you'd expect.
<!-- _______________________________________________________________________ -->
</ul><h4><a name="iterate_basicblock"><hr size=0>Iterating over the
<tt>Instruction</tt>s in a <tt>BasicBlock</tt> </h4><ul>
Just like when dealing with <tt>BasicBlock</tt>s in <tt>Function</tt>s, it's
easy to iterate over the individual instructions that make up
<tt>BasicBlock</tt>s. Here's a code snippet that prints out each instruction in
a <tt>BasicBlock</tt>:
<pre>
// 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 << endl;
}
</pre>
However, this isn't really the best way to print out the contents of a
<tt>BasicBlock</tt>! 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: <tt>cerr << blk <<
endl;</tt>. You might expect this to print out the pointer value of
blk, but operator<< is overloaded for BasicBlock* as well: if you
really want to print the pointer value explicitly, you'll have to
cast.
<!-- _______________________________________________________________________ -->
</ul><h4><a name="iterate_convert"><hr size=0>Turning an iterator into a class
pointer </h4><ul>
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 straightforward.
Assuming that <tt>i</tt> is a <tt>BasicBlock::iterator</tt> and
<tt>j</tt> is a <tt>BasicBlock::const_iterator</tt>:
<pre>
Instruction& inst = *i; // grab reference to instruction reference
Instruction* pinst = &*i; // grab pointer to instruction reference
const Instruction& inst = *j;
</pre>
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,
<pre>Instruction* pinst = &*i;</pre>
is semantically equivalent to
<pre>Instruction* pinst = i;</pre>
<b>Caveat emptor</b>: The above syntax works <i>only</i> when you're
<i>not</i> working with <tt>dyn_cast</tt>. The template definition of
<tt>dyn_cast</tt> isn't implemented to handle this yet, so you'll
still need the following in order for things to work properly:
<pre>
BasicBlock::iterator bbi = ...;
BranchInst* b = dyn_cast<BranchInst>(&*bbi);
</pre>
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:
<pre>
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 << endl;
}
</pre>
Of course, this example is strictly pedagogical, because it'd be
better to do something like
<pre>if(inst->getNext()) cerr << inst->getNext() << endl;</pre>
<!-- dereferenced iterator = Class &
iterators have converting constructor for 'Class *'
iterators automatically convert to 'Class *' except in dyn_cast<> case
-->
<!--
_______________________________________________________________________
--> </ul><h4><a name="iterate_complex"><hr size=0>Finding call sites:
a slightly more complex example
</h4><ul>
Say that you're writing a FunctionPass and would like to count all the
locations in the entire module (that is, across every <tt>Function</tt>)
where a certain function named foo (that takes an int and returns an
int) is called. As you'll learn later, you may want to use an
<tt>InstVisitor</tt> to accomplish this in a much more straightforward
manner, but this example will allow us to explore how you'd do it if
you didn't have <tt>InstVisitor</tt> around. In pseudocode, this is
what we want to do:
<pre>
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 foo is the function it calls)
increment callCounter
</pre>
And the actual code is (remember, since we're writing a
<tt>FunctionPass</tt> our <tt>FunctionPass</tt>-derived class simply
has to override the <tt>runOnFunction</tt> method...):
<pre>
// Assume callCounter is a private member of the pass class being written,
// and has been initialized in the pass class constructor.
virtual runOnFunction(Function& F) {
// Remember, we assumed that the signature of foo was "int foo(int)";
// the first thing we'll do is grab the pointer to that function (as a
// Function*) so we can use it later when we're examining the
// parameters of a CallInst. All of the code before the call to
// Module::getOrInsertFunction() is in preparation to do symbol-table
// to find the function pointer.
vector<const Type*> params;
params.push_back(Type::IntTy);
const FunctionType* fooType = FunctionType::get(Type::IntTy, params);
Function* foo = F.getParent()->getOrInsertFunction("foo", fooType);
// Start iterating and (as per the pseudocode), increment callCounter.
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>(&*inst)) {
// we know we've encountered a call instruction, so we
// need to determine if it's a call to foo or not
if(callInst->getCalledFunction() == foo)
++callCounter;
}
}
}
}
</pre>
We could then print out the value of callCounter (if we wanted to)
inside the doFinalization method of our FunctionPass.
<!-- ======================================================================= -->
@ -249,7 +416,7 @@ pointer </h4><ul>
<!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
<a name="coreclasses">The Core LLVM Class Heirarchy Reference
<a name="coreclasses">The Core LLVM Class Hierarchy Reference
</b></font></td></tr></table><ul>
<!-- *********************************************************************** -->
@ -286,7 +453,7 @@ of this relationship, the <tt>Value</tt> class keeps a list of all of the <a
href="#User"><tt>User</tt></a>s that is using it (the <a
href="#User"><tt>User</tt></a> class is a base class for all nodes in the LLVM
graph that can refer to <tt>Value</tt>s). This use list is how LLVM represents
def-use information in the program, and is accessable through the <tt>use_</tt>*
def-use information in the program, and is accessible through the <tt>use_</tt>*
methods, shown below.<p>
Because LLVM is a typed representation, every LLVM <tt>Value</tt> is typed, and
@ -1071,6 +1238,6 @@ pointer to the parent Function.
<a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
<!-- Created: Tue Aug 6 15:00:33 CDT 2002 -->
<!-- hhmts start -->
Last modified: Fri Sep 6 13:30:36 CDT 2002
Last modified: Fri Sep 6 16:37:49 EDT 2002
<!-- hhmts end -->
</font></body></html>