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- Added content to use-def/def-use section

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- Added content to subsection "Creating and inserting new Instructions"


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3685 91177308-0d34-0410-b5e6-96231b3b80d8
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Joel Stanley 2002-09-11 22:32:24 +00:00
parent 01040b24e0
commit 753eb71371
1 changed files with 121 additions and 9 deletions
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docs/ProgrammersManual.html
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@ -34,9 +34,12 @@
</ul>
<li><a href="#simplechanges">Making simple changes</a>
<ul>
<li>Creating and inserting new <tt>Instruction</tt>s
<li>Deleting <tt>Instruction</tt>s
<li>Replacing an <tt>Instruction</tt> with another <tt>Value</tt>
<li><a href="#schanges_creating">Creating and inserting new
<tt>Instruction</tt>s</a>
<li><a href="#schanges_deleting">Deleting
<tt>Instruction</tt>s</a>
<li><a href="#schanges_replacing">Replacing an
<tt>Instruction</tt> with another <tt>Value</tt></a>
</ul>
<!--
<li>Working with the Control Flow Graph
@ -269,7 +272,7 @@ dispatch over the instruction type directly.<p>
<dd>The <tt>cast_or_null&lt;&gt;</tt> operator works just like the
<tt>cast&lt;&gt;</tt> operator, except that it allows for a null pointer as an
argument (which it then propogates). This can sometimes be useful, allowing you
argument (which it then propagates). This can sometimes be useful, allowing you
to combine several null checks into one.<p>
@ -277,7 +280,7 @@ to combine several null checks into one.<p>
<dd>The <tt>dyn_cast_or_null&lt;&gt;</tt> operator works just like the
<tt>dyn_cast&lt;&gt;</tt> operator, except that it allows for a null pointer as
an argument (which it then propogates). This can sometimes be useful, allowing
an argument (which it then propagates). This can sometimes be useful, allowing
you to combine several null checks into one.<p>
</dl>
@ -286,7 +289,7 @@ 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
<tt>classof</tt> static methods to the class you are interested casting to.
Describing this is currently outside the scope of this document, but there are
lots of examples in the LLVM sourcebase.<p>
lots of examples in the LLVM source base.<p>
@ -576,7 +579,7 @@ to iterate over all of the values that a particular instruction uses
Instruction* pi = ...;
for(User::op_iterator i = pi-&gt;op_begin(), e = pi-&gt;op_end(); i != e; ++i) {
Value* v = i-&gt;get();
Value* v = *i;
...
}
</pre>
@ -594,6 +597,116 @@ for(User::op_iterator i = pi-&gt;op_begin(), e = pi-&gt;op_end(); i != e; ++i) {
<a name="simplechanges">Making simple changes</a>
</b></font></td></tr></table><ul>
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.
<!--_______________________________________________________________________-->
</ul><h4><a name="schanges_creating"><hr size=0>Creating and inserting
new <tt>Instruction</tt>s</h4><ul>
<i>Instantiating Instructions</i>
<p>Creation of <tt>Instruction</tt>s is straightforward: simply call the
constructor for the kind of instruction to instantiate and provide the
necessary parameters. For example, an <tt>AllocaInst</tt> only
<i>requires</i> a (const-ptr-to) <tt>Type</tt>. Thus:
<pre>AllocaInst* ai = new AllocaInst(Type::IntTy);</pre>
will create an <tt>AllocaInst</tt> instance that represents the
allocation of one integer in the current stack frame, at runtime.
Each <tt>Instruction</tt> subclass is likely to have varying default
parameters which change the semantics of the instruction, so refer to
the <a href="/doxygen/classInstruction.h">doxygen documentation for
the subclass of Instruction</a> that you're interested in
instantiating.</p>
<p><i>Naming values</i></p>
<p>
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 <tt>Name</tt> (default) parameter of the
<tt>Instruction</tt> 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
<tt>AllocaInst</tt> at the first point in the first
<tt>BasicBlock</tt> of some <tt>Function</tt>, and I'm intending to
use it within the same <tt>Function</tt>. I might do:
<pre>AllocaInst* pa = new AllocaInst(Type::IntTy, 0, "indexLoc");</pre>
where <tt>indexLoc</tt> is now the logical name of the instruction's
execution value, which is a pointer to an integer on the runtime
stack.
</p>
<p><i>Inserting instructions</i></p>
<p>
There are essentially two ways to insert an <tt>Instruction</tt> into
an existing sequence of instructions that form a <tt>BasicBlock</tt>:
<ul>
<li>Insertion into an explicit instruction list
<p>Given a <tt>BasicBlock* pb</tt>, an <tt>Instruction* pi</tt> within
that <tt>BasicBlock</tt>, and a newly-created instruction
we wish to insert before <tt>*pi</tt>, we do the following:
<pre>
BasicBlock* pb = ...;
Instruction* pi = ...;
Instruction* newInst = new Instruction(...);
pb->getInstList().insert(pi, newInst); // inserts newInst before pi in pb
</pre>
</p>
<li>Insertion into an implicit instruction list
<p>
<tt>Instruction</tt> instances that are already in
<tt>BasicBlock</tt>s 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 <tt>BasicBlock</tt> by doing:
<pre>
Instruction* pi = ...;
Instruction* newInst = new Instruction(...);
pi->getParent()->getInstList().insert(pi, newInst);
</pre>
In fact, this sequence of steps occurs so frequently that the
<tt>Instruction</tt> class and <tt>Instruction</tt>-derived classes
provide constructors which take (as a default parameter) a pointer to
an <tt>Instruction</tt> which the newly-created <tt>Instruction</tt>
should precede. That is, <tt>Instruction</tt> constructors are
capable of inserting the newly-created instance into the
<tt>BasicBlock</tt> of a provided instruction, immediately before that
instruction. Using an <tt>Instruction</tt> constructor with a
<tt>insertBefore</tt> (default) parameter, the above code becomes:
<pre>
Instruction* pi = ...;
Instruction* newInst = new Instruction(..., pi);
</pre>
which is much cleaner, especially if you're creating a lot of
instructions and adding them to <tt>BasicBlock</tt>s.
</p>
</p>
<!--_______________________________________________________________________-->
</ul><h4><a name="schanges_deleting"><hr size=0>Deleting
<tt>Instruction</tt>s</h4><ul>
<!--_______________________________________________________________________-->
</ul><h4><a name="schanges_replacing"><hr size=0>Replacing an
<tt>Instruction</tt> with another <tt>Value</tt></h4><ul>
<!-- Value::replaceAllUsesWith
User::replaceUsesOfWith
Point out: include/llvm/Transforms/Utils/
@ -602,7 +715,6 @@ for(User::op_iterator i = pi-&gt;op_begin(), e = pi-&gt;op_end(); i != e; ++i) {
-->
<!-- *********************************************************************** -->
</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>
@ -1428,6 +1540,6 @@ 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: Wed Sep 11 15:48:49 CDT 2002
Last modified: Wed Sep 11 17:31:03 CDT 2002
<!-- hhmts end -->
</font></body></html>