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<title>The Often Misunderstood GEP Instruction</title>
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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>
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<div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
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<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>
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<div class="doc_section"><a name="questions"><b>The Questions</b></a></div>
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<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>
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<div class="doc_subsection">
<a name="extra_index"><b>Why is the extra 0 index required?</b></a>
</div>
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<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>
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<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>
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<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 = &amp;MyVar + 0</li>
<li> idx2 = &amp;MyVar + 4</li>
<li> idx3 = &amp;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>
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<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>
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<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>
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<div class="doc_section"><a name="summary"><b>Summary</b></a></div>
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<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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