docs/GetElementPtr.html - fp2-dev/platform/external/llvm - Gitiles

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 <div class="doc_title">
   The Often Misunderstood GEP Instruction
 </div>

 <ol>
   <li><a href="#intro">Introduction</a></li>
   <li><a href="#questions">The Questions</a>
   <ol>
     <li><a href="#extra_index">Why is the extra 0 index required?</a></li>
     <li><a href="#deref">What is dereferenced by GEP?</a></li>
     <li><a href="#firstptr">Why can you index through the first pointer but not
       subsequent ones?</a></li>
     <li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
     <li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
   </ol></li>
   <li><a href="#summary">Summary</a></li>
 </ol>

 <div class="doc_author">
   <p>Written by: <a href="mailto:rspencer@reidspencer.com">Reid Spencer</a>.</p>
 </div>


 <!-- *********************************************************************** -->
 <div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
 <!-- *********************************************************************** -->
 <div class="doc_text">
   <p>This document seeks to dispel the mystery and confusion surrounding LLVM's
   GetElementPtr (GEP) instruction. Questions about the wiley GEP instruction are
   probably the most frequently occuring questions once a developer gets down to
   coding with LLVM. Here we lay out the sources of confusion and show that the
   GEP instruction is really quite simple.
   </p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section"><a name="questions"><b>The Questions</b></a></div>
 <!-- *********************************************************************** -->
 <div class="doc_text">
   <p>When people are first confronted with the GEP instruction, they tend to
   relate it to known concepts from other programming paradigms, most notably C
   array indexing and field selection. However, GEP is a little different and
   this leads to the following questions; all of which are answered in the
   following sections.</p>
   <ol>
     <li><a href="#firstptr">What is the first index of the GEP instruction?</a>
     </li>
     <li><a href="#extra_index">Why is the extra 0 index required?</a></li>
     <li><a href="#deref">What is dereferenced by GEP?</a></li>
     <li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
     <li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
   </ol>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="firstptr"><b>What is the first index of the GEP instruction?</b></a>
 </div>
 <div class="doc_text">
   <p>Quick answer: The index stepping through the first operand.</p>
   <p>The confusion with the first index usually arises from thinking about
   the GetElementPtr instruction as if it was a C index operator. They aren't the
   same. For example, when we write, in "C":</p>

 <div class="doc_code">
 <pre>
 AType *Foo;
 ...
 X = &amp;Foo-&gt;F;
 </pre>
 </div>

   <p>it is natural to think that there is only one index, the selection of the
   field <tt>F</tt>.  However, in this example, <tt>Foo</tt> is a pointer. That
   pointer must be indexed explicitly in LLVM. C, on the other hand, indexs
   through it transparently.  To arrive at the same address location as the C
   code, you would provide the GEP instruction with two index operands. The
   first operand indexes through the pointer; the second operand indexes the
   field <tt>F</tt> of the structure, just as if you wrote:</p>

 <div class="doc_code">
 <pre>
 X = &amp;Foo[0].F;
 </pre>
 </div>

   <p>Sometimes this question gets rephrased as:</p>
   <blockquote><p><i>Why is it okay to index through the first pointer, but
       subsequent pointers won't be dereferenced?</i></p></blockquote>
   <p>The answer is simply because memory does not have to be accessed to
   perform the computation. The first operand to the GEP instruction must be a
   value of a pointer type. The value of the pointer is provided directly to
   the GEP instruction as an operand without any need for accessing memory. It
   must, therefore be indexed and requires an index operand. Consider this
   example:</p>

 <div class="doc_code">
 <pre>
 struct munger_struct {
   int f1;
   int f2;
 };
 void munge(struct munger_struct *P) {
   P[0].f1 = P[1].f1 + P[2].f2;
 }
 ...
 munger_struct Array[3];
 ...
 munge(Array);
 </pre>
 </div>

   <p>In this "C" example, the front end compiler (llvm-gcc) will generate three
   GEP instructions for the three indices through "P" in the assignment
   statement.  The function argument <tt>P</tt> will be the first operand of each
   of these GEP instructions.  The second operand indexes through that pointer.
   The third operand will be the field offset into the
   <tt>struct munger_struct</tt> type,  for either the <tt>f1</tt> or
   <tt>f2</tt> field. So, in LLVM assembly the <tt>munge</tt> function looks
   like:</p>

 <div class="doc_code">
 <pre>
 void %munge(%struct.munger_struct* %P) {
 entry:
   %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
   %tmp = load i32* %tmp
   %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
   %tmp7 = load i32* %tmp6
   %tmp8 = add i32 %tmp7, %tmp
   %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
   store i32 %tmp8, i32* %tmp9
   ret void
 }
 </pre>
 </div>

   <p>In each case the first operand is the pointer through which the GEP
   instruction starts. The same is true whether the first operand is an
   argument, allocated memory, or a global variable. </p>
   <p>To make this clear, let's consider a more obtuse example:</p>

 <div class="doc_code">
 <pre>
 %MyVar = unintialized global i32
 ...
 %idx1 = getelementptr i32* %MyVar, i64 0
 %idx2 = getelementptr i32* %MyVar, i64 1
 %idx3 = getelementptr i32* %MyVar, i64 2
 </pre>
 </div>

   <p>These GEP instructions are simply making address computations from the
   base address of <tt>MyVar</tt>.  They compute, as follows (using C syntax):
   </p>

 <div class="doc_code">
 <pre>
 idx1 = (char*) &amp;MyVar + 0
 idx2 = (char*) &amp;MyVar + 4
 idx3 = (char*) &amp;MyVar + 8
 </pre>
 </div>

   <p>Since the type <tt>i32</tt> is known to be four bytes long, the indices
   0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
   memory is accessed to make these computations because the address of
   <tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
   <p>The obtuse part of this example is in the cases of <tt>%idx2</tt> and
   <tt>%idx3</tt>. They result in the computation of addresses that point to
   memory past the end of the <tt>%MyVar</tt> global, which is only one
   <tt>i32</tt> long, not three <tt>i32</tt>s long.  While this is legal in LLVM,
   it is inadvisable because any load or store with the pointer that results
   from these GEP instructions would produce undefined results.</p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="extra_index"><b>Why is the extra 0 index required?</b></a>
 </div>
 <!-- *********************************************************************** -->
 <div class="doc_text">
   <p>Quick answer: there are no superfluous indices.</p>
   <p>This question arises most often when the GEP instruction is applied to a
   global variable which is always a pointer type. For example, consider
   this:</p>

 <div class="doc_code">
 <pre>
 %MyStruct = uninitialized global { float*, i32 }
 ...
 %idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1
 </pre>
 </div>

   <p>The GEP above yields an <tt>i32*</tt> by indexing the <tt>i32</tt> typed
   field of the structure <tt>%MyStruct</tt>. When people first look at it, they
   wonder why the <tt>i64 0</tt> index is needed. However, a closer inspection
   of how globals and GEPs work reveals the need. Becoming aware of the following
   facts will dispell the confusion:</p>
   <ol>
     <li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, i32 }</tt>
     but rather <tt>{ float*, i32 }*</tt>. That is, <tt>%MyStruct</tt> is a
     pointer to a structure containing a pointer to a <tt>float</tt> and an
     <tt>i32</tt>.</li>
     <li>Point #1 is evidenced by noticing the type of the first operand of
     the GEP instruction (<tt>%MyStruct</tt>) which is
     <tt>{ float*, i32 }*</tt>.</li>
     <li>The first index, <tt>i64 0</tt> is required to step over the global
     variable <tt>%MyStruct</tt>.  Since the first argument to the GEP
     instruction must always be a value of pointer type, the first index
     steps through that pointer. A value of 0 means 0 elements offset from that
     pointer.</li>
     <li>The second index, <tt>i32 1</tt> selects the second field of the
     structure (the <tt>i32</tt>). </li>
   </ol>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="deref"><b>What is dereferenced by GEP?</b></a>
 </div>
 <div class="doc_text">
   <p>Quick answer: nothing.</p>
   <p>The GetElementPtr instruction dereferences nothing. That is, it doesn't
   access memory in any way. That's what the Load and Store instructions are for.
   GEP is only involved in the computation of addresses. For example, consider
   this:</p>

 <div class="doc_code">
 <pre>
 %MyVar = uninitialized global { [40 x i32 ]* }
 ...
 %idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17
 </pre>
 </div>

   <p>In this example, we have a global variable, <tt>%MyVar</tt> that is a
   pointer to a structure containing a pointer to an array of 40 ints. The
   GEP instruction seems to be accessing the 18th integer of the structure's
   array of ints. However, this is actually an illegal GEP instruction. It
   won't compile. The reason is that the pointer in the structure <i>must</i>
   be dereferenced in order to index into the array of 40 ints. Since the
   GEP instruction never accesses memory, it is illegal.</p>
   <p>In order to access the 18th integer in the array, you would need to do the
   following:</p>

 <div class="doc_code">
 <pre>
 %idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
 %arr = load [40 x i32]** %idx
 %idx = getelementptr [40 x i32]* %arr, i64 0, i64 17
 </pre>
 </div>

   <p>In this case, we have to load the pointer in the structure with a load
   instruction before we can index into the array. If the example was changed
   to:</p>

 <div class="doc_code">
 <pre>
 %MyVar = uninitialized global { [40 x i32 ] }
 ...
 %idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17
 </pre>
 </div>

   <p>then everything works fine. In this case, the structure does not contain a
   pointer and the GEP instruction can index through the global variable,
   into the first field of the structure and access the 18th <tt>i32</tt> in the
   array there.</p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="lead0"><b>Why don't GEP x,0,0,1 and GEP x,1 alias?</b></a>
 </div>
 <div class="doc_text">
   <p>Quick Answer: They compute different address locations.</p>
   <p>If you look at the first indices in these GEP
   instructions you find that they are different (0 and 1), therefore the address
   computation diverges with that index. Consider this example:</p>

 <div class="doc_code">
 <pre>
 %MyVar = global { [10 x i32 ] }
 %idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
 %idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1
 </pre>
 </div>

   <p>In this example, <tt>idx1</tt> computes the address of the second integer
   in the array that is in the structure in %MyVar, that is <tt>MyVar+4</tt>. The
   type of <tt>idx1</tt> is <tt>i32*</tt>. However, <tt>idx2</tt> computes the
   address of <i>the next</i> structure after <tt>%MyVar</tt>. The type of
   <tt>idx2</tt> is <tt>{ [10 x i32] }*</tt> and its value is equivalent
   to <tt>MyVar + 40</tt> because it indexes past the ten 4-byte integers
   in <tt>MyVar</tt>. Obviously, in such a situation, the pointers don't
   alias.</p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_subsection">
   <a name="trail0"><b>Why do GEP x,1,0,0 and GEP x,1 alias?</b></a>
 </div>
 <div class="doc_text">
   <p>Quick Answer: They compute the same address location.</p>
   <p>These two GEP instructions will compute the same address because indexing
   through the 0th element does not change the address. However, it does change
   the type. Consider this example:</p>

 <div class="doc_code">
 <pre>
 %MyVar = global { [10 x i32 ] }
 %idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
 %idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1
 </pre>
 </div>

   <p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
   its type is <tt>i32*</tt>. The value of <tt>%idx2</tt> is also
   <tt>MyVar+40</tt> but its type is <tt>{ [10 x i32] }*</tt>.</p>
 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section"><a name="summary"><b>Summary</b></a></div>
 <!-- *********************************************************************** -->

 <div class="doc_text">
   <p>In summary, here's some things to always remember about the GetElementPtr
   instruction:</p>
   <ol>
     <li>The GEP instruction never accesses memory, it only provides pointer
     computations.</li>
     <li>The first operand to the GEP instruction is always a pointer and it must
     be indexed.</li>
     <li>There are no superfluous indices for the GEP instruction.</li>
     <li>Trailing zero indices are superfluous for pointer aliasing, but not for
     the types of the pointers.</li>
     <li>Leading zero indices are not superfluous for pointer aliasing nor the
     types of the pointers.</li>
   </ol>
 </div>

 <!-- *********************************************************************** -->

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	<title>The Often Misunderstood GEP Instruction</title>
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	<body>

	<div class="doc_title">
	The Often Misunderstood GEP Instruction
	</div>

	<ol>
	<li><a href="#intro">Introduction</a></li>
	<li><a href="#questions">The Questions</a>
	<ol>
	<li><a href="#extra_index">Why is the extra 0 index required?</a></li>
	<li><a href="#deref">What is dereferenced by GEP?</a></li>
	<li><a href="#firstptr">Why can you index through the first pointer but not
	subsequent ones?</a></li>
	<li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
	<li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
	</ol></li>
	<li><a href="#summary">Summary</a></li>
	</ol>

	<div class="doc_author">
	<p>Written by: <a href="mailto:rspencer@reidspencer.com">Reid Spencer</a>.</p>
	</div>


	<!-- *********************************************************************** -->
	<div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
	<!-- *********************************************************************** -->
	<div class="doc_text">
	<p>This document seeks to dispel the mystery and confusion surrounding LLVM's
	GetElementPtr (GEP) instruction. Questions about the wiley GEP instruction are
	probably the most frequently occuring questions once a developer gets down to
	coding with LLVM. Here we lay out the sources of confusion and show that the
	GEP instruction is really quite simple.
	</p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section"><a name="questions"><b>The Questions</b></a></div>
	<!-- *********************************************************************** -->
	<div class="doc_text">
	<p>When people are first confronted with the GEP instruction, they tend to
	relate it to known concepts from other programming paradigms, most notably C
	array indexing and field selection. However, GEP is a little different and
	this leads to the following questions; all of which are answered in the
	following sections.</p>
	<ol>
	<li><a href="#firstptr">What is the first index of the GEP instruction?</a>
	</li>
	<li><a href="#extra_index">Why is the extra 0 index required?</a></li>
	<li><a href="#deref">What is dereferenced by GEP?</a></li>
	<li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
	<li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
	</ol>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="firstptr"><b>What is the first index of the GEP instruction?</b></a>
	</div>
	<div class="doc_text">
	<p>Quick answer: The index stepping through the first operand.</p>
	<p>The confusion with the first index usually arises from thinking about
	the GetElementPtr instruction as if it was a C index operator. They aren't the
	same. For example, when we write, in "C":</p>

	<div class="doc_code">
	<pre>
	AType *Foo;
	...
	X = &Foo->F;
	</pre>
	</div>

	<p>it is natural to think that there is only one index, the selection of the
	field <tt>F</tt>. However, in this example, <tt>Foo</tt> is a pointer. That
	pointer must be indexed explicitly in LLVM. C, on the other hand, indexs
	through it transparently. To arrive at the same address location as the C
	code, you would provide the GEP instruction with two index operands. The
	first operand indexes through the pointer; the second operand indexes the
	field <tt>F</tt> of the structure, just as if you wrote:</p>

	<div class="doc_code">
	<pre>
	X = &Foo[0].F;
	</pre>
	</div>

	<p>Sometimes this question gets rephrased as:</p>
	<blockquote><p><i>Why is it okay to index through the first pointer, but
	subsequent pointers won't be dereferenced?</i></p></blockquote>
	<p>The answer is simply because memory does not have to be accessed to
	perform the computation. The first operand to the GEP instruction must be a
	value of a pointer type. The value of the pointer is provided directly to
	the GEP instruction as an operand without any need for accessing memory. It
	must, therefore be indexed and requires an index operand. Consider this
	example:</p>

	<div class="doc_code">
	<pre>
	struct munger_struct {
	int f1;
	int f2;
	};
	void munge(struct munger_struct *P) {
	P[0].f1 = P[1].f1 + P[2].f2;
	}
	...
	munger_struct Array[3];
	...
	munge(Array);
	</pre>
	</div>

	<p>In this "C" example, the front end compiler (llvm-gcc) will generate three
	GEP instructions for the three indices through "P" in the assignment
	statement. The function argument <tt>P</tt> will be the first operand of each
	of these GEP instructions. The second operand indexes through that pointer.
	The third operand will be the field offset into the
	<tt>struct munger_struct</tt> type, for either the <tt>f1</tt> or
	<tt>f2</tt> field. So, in LLVM assembly the <tt>munge</tt> function looks
	like:</p>

	<div class="doc_code">
	<pre>
	void %munge(%struct.munger_struct* %P) {
	entry:
	%tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
	%tmp = load i32* %tmp
	%tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
	%tmp7 = load i32* %tmp6
	%tmp8 = add i32 %tmp7, %tmp
	%tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
	store i32 %tmp8, i32* %tmp9
	ret void
	}
	</pre>
	</div>

	<p>In each case the first operand is the pointer through which the GEP
	instruction starts. The same is true whether the first operand is an
	argument, allocated memory, or a global variable. </p>
	<p>To make this clear, let's consider a more obtuse example:</p>

	<div class="doc_code">
	<pre>
	%MyVar = unintialized global i32
	...
	%idx1 = getelementptr i32* %MyVar, i64 0
	%idx2 = getelementptr i32* %MyVar, i64 1
	%idx3 = getelementptr i32* %MyVar, i64 2
	</pre>
	</div>

	<p>These GEP instructions are simply making address computations from the
	base address of <tt>MyVar</tt>. They compute, as follows (using C syntax):
	</p>

	<div class="doc_code">
	<pre>
	idx1 = (char*) &MyVar + 0
	idx2 = (char*) &MyVar + 4
	idx3 = (char*) &MyVar + 8
	</pre>
	</div>

	<p>Since the type <tt>i32</tt> is known to be four bytes long, the indices
	0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
	memory is accessed to make these computations because the address of
	<tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
	<p>The obtuse part of this example is in the cases of <tt>%idx2</tt> and
	<tt>%idx3</tt>. They result in the computation of addresses that point to
	memory past the end of the <tt>%MyVar</tt> global, which is only one
	<tt>i32</tt> long, not three <tt>i32</tt>s long. While this is legal in LLVM,
	it is inadvisable because any load or store with the pointer that results
	from these GEP instructions would produce undefined results.</p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="extra_index"><b>Why is the extra 0 index required?</b></a>
	</div>
	<!-- *********************************************************************** -->
	<div class="doc_text">
	<p>Quick answer: there are no superfluous indices.</p>
	<p>This question arises most often when the GEP instruction is applied to a
	global variable which is always a pointer type. For example, consider
	this:</p>

	<div class="doc_code">
	<pre>
	%MyStruct = uninitialized global { float*, i32 }
	...
	%idx = getelementptr { float, i32 } %MyStruct, i64 0, i32 1
	</pre>
	</div>

	<p>The GEP above yields an <tt>i32*</tt> by indexing the <tt>i32</tt> typed
	field of the structure <tt>%MyStruct</tt>. When people first look at it, they
	wonder why the <tt>i64 0</tt> index is needed. However, a closer inspection
	of how globals and GEPs work reveals the need. Becoming aware of the following
	facts will dispell the confusion:</p>
	<ol>
	<li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, i32 }</tt>
	but rather <tt>{ float, i32 }</tt>. That is, <tt>%MyStruct</tt> is a
	pointer to a structure containing a pointer to a <tt>float</tt> and an
	<tt>i32</tt>.</li>
	<li>Point #1 is evidenced by noticing the type of the first operand of
	the GEP instruction (<tt>%MyStruct</tt>) which is
	<tt>{ float, i32 }</tt>.</li>
	<li>The first index, <tt>i64 0</tt> is required to step over the global
	variable <tt>%MyStruct</tt>. Since the first argument to the GEP
	instruction must always be a value of pointer type, the first index
	steps through that pointer. A value of 0 means 0 elements offset from that
	pointer.</li>
	<li>The second index, <tt>i32 1</tt> selects the second field of the
	structure (the <tt>i32</tt>). </li>
	</ol>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="deref"><b>What is dereferenced by GEP?</b></a>
	</div>
	<div class="doc_text">
	<p>Quick answer: nothing.</p>
	<p>The GetElementPtr instruction dereferences nothing. That is, it doesn't
	access memory in any way. That's what the Load and Store instructions are for.
	GEP is only involved in the computation of addresses. For example, consider
	this:</p>

	<div class="doc_code">
	<pre>
	%MyVar = uninitialized global { [40 x i32 ]* }
	...
	%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17
	</pre>
	</div>

	<p>In this example, we have a global variable, <tt>%MyVar</tt> that is a
	pointer to a structure containing a pointer to an array of 40 ints. The
	GEP instruction seems to be accessing the 18th integer of the structure's
	array of ints. However, this is actually an illegal GEP instruction. It
	won't compile. The reason is that the pointer in the structure <i>must</i>
	be dereferenced in order to index into the array of 40 ints. Since the
	GEP instruction never accesses memory, it is illegal.</p>
	<p>In order to access the 18th integer in the array, you would need to do the
	following:</p>

	<div class="doc_code">
	<pre>
	%idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
	%arr = load [40 x i32]** %idx
	%idx = getelementptr [40 x i32]* %arr, i64 0, i64 17
	</pre>
	</div>

	<p>In this case, we have to load the pointer in the structure with a load
	instruction before we can index into the array. If the example was changed
	to:</p>

	<div class="doc_code">
	<pre>
	%MyVar = uninitialized global { [40 x i32 ] }
	...
	%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17
	</pre>
	</div>

	<p>then everything works fine. In this case, the structure does not contain a
	pointer and the GEP instruction can index through the global variable,
	into the first field of the structure and access the 18th <tt>i32</tt> in the
	array there.</p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="lead0"><b>Why don't GEP x,0,0,1 and GEP x,1 alias?</b></a>
	</div>
	<div class="doc_text">
	<p>Quick Answer: They compute different address locations.</p>
	<p>If you look at the first indices in these GEP
	instructions you find that they are different (0 and 1), therefore the address
	computation diverges with that index. Consider this example:</p>

	<div class="doc_code">
	<pre>
	%MyVar = global { [10 x i32 ] }
	%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
	%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1
	</pre>
	</div>

	<p>In this example, <tt>idx1</tt> computes the address of the second integer
	in the array that is in the structure in %MyVar, that is <tt>MyVar+4</tt>. The
	type of <tt>idx1</tt> is <tt>i32*</tt>. However, <tt>idx2</tt> computes the
	address of <i>the next</i> structure after <tt>%MyVar</tt>. The type of
	<tt>idx2</tt> is <tt>{ [10 x i32] }*</tt> and its value is equivalent
	to <tt>MyVar + 40</tt> because it indexes past the ten 4-byte integers
	in <tt>MyVar</tt>. Obviously, in such a situation, the pointers don't
	alias.</p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_subsection">
	<a name="trail0"><b>Why do GEP x,1,0,0 and GEP x,1 alias?</b></a>
	</div>
	<div class="doc_text">
	<p>Quick Answer: They compute the same address location.</p>
	<p>These two GEP instructions will compute the same address because indexing
	through the 0th element does not change the address. However, it does change
	the type. Consider this example:</p>

	<div class="doc_code">
	<pre>
	%MyVar = global { [10 x i32 ] }
	%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
	%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1
	</pre>
	</div>

	<p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
	its type is <tt>i32*</tt>. The value of <tt>%idx2</tt> is also
	<tt>MyVar+40</tt> but its type is <tt>{ [10 x i32] }*</tt>.</p>
	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section"><a name="summary"><b>Summary</b></a></div>
	<!-- *********************************************************************** -->

	<div class="doc_text">
	<p>In summary, here's some things to always remember about the GetElementPtr
	instruction:</p>
	<ol>
	<li>The GEP instruction never accesses memory, it only provides pointer
	computations.</li>
	<li>The first operand to the GEP instruction is always a pointer and it must
	be indexed.</li>
	<li>There are no superfluous indices for the GEP instruction.</li>
	<li>Trailing zero indices are superfluous for pointer aliasing, but not for
	the types of the pointers.</li>
	<li>Leading zero indices are not superfluous for pointer aliasing nor the
	types of the pointers.</li>
	</ol>
	</div>

	<!-- *********************************************************************** -->

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