Answer the most frequently asked question, about GEPs. The answer is
sufficiently long that I placed it in a separate file but it links from
the FAQ page. More might need to be added to GetElementPtr.html to
address additional confusion surrounding GEP.
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+ "http://www.w3.org/TR/html4/strict.dtd">
+<html>
+<head>
+ <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+ <title>The Often Misunderstood GEP Instruction</title>
+ <link rel="stylesheet" href="llvm.css" type="text/css">
+</head>
+<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="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>
+</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><pre>
+ %MyStruct = uninitialized global { float*, int }
+ ...
+ %idx = getelementptr { float*, int }* %MyStruct, long 0, ubyte 1</pre>
+ <p>The GEP above yields an <tt>int*</tt> by indexing the <tt>int</tt> typed
+ field of the structure <tt>%MyStruct</tt>. When people first look at it, they
+ wonder why the <tt>long 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*, int }</tt>
+ but rather <tt>{ float*, int }*</tt>. That is, <tt>%MyStruct</tt> is a
+ pointer to a structure containing a pointer to a <tt>float</tt> and an
+ <tt>int</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*, int }*</tt>.</li>
+ <li>The first index, <tt>long 0</tt> is required to dereference the
+ pointer associated with <tt>%MyStruct</tt>.</li>
+ <li>The second index, <tt>ubyte 1</tt> selects the second field of the
+ structure (the <tt>int</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 instruction is for. GEP is
+ only involved in the computation of addresses. For example, consider this:</p>
+ <pre>
+ %MyVar = uninitialized global { [40 x int ]* }
+ ...
+ %idx = getelementptr { [40 x int]* }* %MyVar, long 0, ubyte 0, long 0, long 17</pre>
+ <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 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>
+ <pre>
+ %idx = getelementptr { [40 x int]* }* %, long 0, ubyte 0
+ %arr = load [40 x int]** %idx
+ %idx = getelementptr [40 x int]* %arr, long 0, long 17</pre>
+ <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>
+ <pre>
+ %MyVar = uninitialized global { [40 x int ] }
+ ...
+ %idx = getelementptr { [40 x int] }*, long 0, ubyte 0, long 17</pre>
+ <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 pointer,
+ into the first field of the structure and access the 18th <tt>int</tt> in the
+ array there.</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="firstptr"><b>Why can you index through the first pointer?</b></a>
+</div>
+<div class="doc_text">
+ <p>Quick answer: Because its already present.</p>
+ <p>Having understood the <a href="#deref">previous question</a>, a new
+ question then arises:</p>
+ <blockquote><i>Why is it okay to index through the first pointer, but
+ subsequent pointers won't be dereferenced?</i></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 without any need
+ for accessing memory. It must, therefore be indexed like any other operand.
+ Consider this example:</p>
+ <pre>
+ %MyVar = unintialized global int
+ ...
+ %idx1 = getelementptr int* %MyVar, long 0
+ %idx2 = getelementptr int* %MyVar, long 1
+ %idx3 = getelementptr int* %MyVar, long 2</pre>
+ <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>
+ <ul>
+ <li> idx1 = &MyVar + 0</li>
+ <li> idx2 = &MyVar + 4</li>
+ <li> idx3 = &MyVar = 8</li>
+ </ul>
+ <p>Since the type <tt>int</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>Note that the cases of <tt>%idx2</tt> and <tt>%idx3</tt> are a bit silly.
+ They are computing addresses of something of unknown type (and thus
+ potentially breaking type safety) because <tt>%MyVar</tt> is only one
+ integer long.</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>
+ <pre>
+ %MyVar = global { [10 x int ] }
+ %idx1 = getlementptr { [10 x int ] }* %MyVar, long 0, byte 0, long 1
+ %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</pre>
+ <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>int*</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 int] }*</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="lead0"><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>
+ <pre>
+ %MyVar = global { [10 x int ] }
+ %idx1 = getlementptr { [10 x int ] }* %MyVar, long 1, byte 0, long 0
+ %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</pre>
+ <p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
+ its type is <tt>int*</tt>. The value of <tt>%idx2</tt> is also
+ <tt>MyVar+40</tt> but its type is <tt>{ [10 x int] }*</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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