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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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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 = &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 <tt>%MyVar</tt>, 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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