312 lines
14 KiB
HTML
312 lines
14 KiB
HTML
<!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">
|
|
<style type="text/css">
|
|
TABLE { text-align: left; border: 1px solid black; border-collapse: collapse; margin: 0 0 0 0; }
|
|
</style>
|
|
</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="#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>
|
|
<pre>
|
|
AType* Foo;
|
|
...
|
|
X = &Foo->F;</pre>
|
|
<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>
|
|
<pre>
|
|
X = &Foo[0].F;</pre>
|
|
<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>
|
|
<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>
|
|
<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>
|
|
<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>
|
|
<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>
|
|
<pre>
|
|
%MyVar = unintialized global i32
|
|
...
|
|
%idx1 = getelementptr i32* %MyVar, i64 0
|
|
%idx2 = getelementptr i32* %MyVar, i64 1
|
|
%idx3 = getelementptr i32* %MyVar, i64 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 = (char*) &MyVar + 0</li>
|
|
<li> idx2 = (char*) &MyVar + 4</li>
|
|
<li> idx3 = (char*) &MyVar + 8</li>
|
|
</ul>
|
|
<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><pre>
|
|
%MyStruct = uninitialized global { float*, i32 }
|
|
...
|
|
%idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1</pre>
|
|
<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>
|
|
<pre>
|
|
%MyVar = uninitialized global { [40 x i32 ]* }
|
|
...
|
|
%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 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 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 i32]* }* %, i64 0, i32 0
|
|
%arr = load [40 x i32]** %idx
|
|
%idx = getelementptr [40 x i32]* %arr, i64 0, i64 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 i32 ] }
|
|
...
|
|
%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 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,
|
|
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>
|
|
<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>
|
|
<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>
|
|
<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>
|
|
<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>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!" /></a>
|
|
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br/>
|
|
Last modified: $Date$
|
|
</address>
|
|
</body>
|
|
</html>
|